Handbook of Molecular Gastronomy: Scientific Foundations, Educational Practices, and Culinary Applications 2020053893, 2020053894, 9781466594784, 9780429168703, 9780367741617

Table of contents :
Cover
Half Title
Title Page
Copyright Page
Table of Contents
About the Editors
List of Contributors
Foreword
Introduction to Molecular Gastronomy and Its Applications
Clarification of What Molecular Gastronomy Is
Science and Cooking
Applications in Schools, Colleges and Universities
Applications to the Culinary Arts
Part I Scientific Foundations
Acids in Foods and Perception of Sourness
Introduction
Influence of Food Matrix Composition on Sourness
Mechanisms Leading to Sour Perception
The Acid Receptor
The Role of Saliva in Sourness Perception
Interactions of Sourness Perception with Other Perceptions
Acceptability of Sourness
Conclusion
References
Anthocyanins in Food
Colour Versatility of Anthocyanins
Colour Stabilization Through Interactions
Anthocyanin Reactions
Colour Changes Induced by Anthocyanin Reactions
Pigment Diffusion and Changes During Cooking
References
Alcoholic Beverages: Production, Trends, Innovations
Introduction
Beer
Production Method and Styles
Home Brewing
Craft Beer
More Trends and Innovations
Wine
Production Method and Styles
Trends and Innovations
Distilled Drinks
Principal Production Method and Styles
Trends and Innovations
Category Blurring
References
Ash in the Kitchen
History
Preparation Method of Leek Ash
Characterization of Leek Ash
“Labmade” Cheese
Meat Maturation
Recipe for Ash-Ripened Danish Duck (by Andreas Rieger)
Ash-Ripened Danish Duck (144 Portions)
Duck Cooking (Results in 8 Portions Per Double Breast/2–3 Portions Per Leg)
Chest
Haunch
Acknowledgements
References
Baking: Laminated Bakery Products
From Basic Ingredients to a Sophisticated Pastry
Ingredient Functionality for Pastry Lift
Pre-dough
Roll-in Fat
Conclusion
References
Baking: Chemical Leaveners
References
Baking: Injera – the Multi-Eyed Flat Bread
Definitions (in Amharic)
Conclusions
Acknowledgments
References
Baking: Viennoiserie – Laminated Pastry Production
A Brief History of the Croissant From the 17th Century
Viennoiserie: A Modern Twist
Production of Laminated Pastry
Dough Stage
Method
Lamination Stage
Dough Stage
Lamination Sequencing 3 4 3
References
Baking: How Does Starch Gelatinization Influence Texture?
Starch and Starch Granules
Starch Gelatinization
Degree of Gelatinization
Gelatinization and Texture: Example of a Protein Gel Modified by Starch
References
Baking: Sourdough Bread
A Mixture With Many Names
Protecting the Heritage of Sourdough
Artisan Over Convenience
Sourdough Fermentation – It Is All About the Microbes
Lactic Acid Bacteria
Yeasts
Sourdough Fermentation Process
Traditional Sourdough Fermentation with a Touch of Back-Slopping
Stability of the Ecosystem
Bringing the Dough Together
References
Barbecue: The Chemistry Behind Cooking on a Barbecue
Composition of Wood, Charcoal and Meat
Flavour From the Burning Wood
Flavour From the Cooked Food
Harmful Substances Created through Barbecuing
Conclusion
References
Bioactivity and Its Measurement
Compound Release (CR)
In Situ Quantitative Nmr
Classification of Bioactivities
Modelling Saccharides
References
Browning: The Glycation and Maillard Reactions – Major Non-Enzymatic Browning Reactions in Food
The Basic Chemical Pathways of the Maillard Reaction
Initial Phase
Propagation Phase
Termination Phase
Yield of Different Maillard Reaction Products
What Is Behind the Browning of Butter?
Acrylamide, the Dark Side of Glycation
Concluding Remarks and Perspectives
References
Canning: Appert and Food Canning
Appertization: The Art of Preserving Animal and Vegetal Substances
The Botulinum Cook
Thermobacteriology: Modelling the Thermal Processing of Foods
References
Capillarity in Action
Diffusion Versus Capillarity
Sometimes, There Is No Entrance; Sometimes, There Is One
The Mathematics Behind the Phenomenon
The Example of “Shitao”
References
Champagne Tasting From a Scientific Perspective
Champagne Cork Popping Revisited
What Is the Best Way to Pour Champagne?
How Many Bubbles in Your Glass of Bubbly?
Flute Versus Coupe: And the Winner Is …
Flow Patterns in Champagne Glasses Increase the Perception of Aromas
Unravelling Two-Dimensional Vortices at the Champagne Surface
Bursting Bubbles Provide An Aromatic Boost to Champagne
References
Chantillys: The Cousins of Whipped Cream
Chocolate Chantilly
Other Chantillys
References
Cheese: Hot Culinary Uses of Cheese
Cheese
Using the Emulsifying Property of Cheeses: The Fondue
Heated Cheese
Physico-Chemical Mechanisms Within the Fondue
Practical Considerations
Non-Enzymatic Browning of Cheeses: the Gratin
Physico-Chemical Mechanisms of Cheese Browning
Practical Considerations
The Spreadability of Cheeses
Physico-Chemical Mechanisms of Cheese Spreading
Practical Considerations
Cheese Stretching: Pizzas and “Aligot”
Physico-Chemical Mechanisms of Stretching
Practical Considerations: Aligot
Conclusions
References
Chocolate: Chocolates From Around the World, Simple Physics, complex Flavour
Hierarchy of Chocolate Production
The Science Behind Chocolate
The Forced, Non- Equilibrium Phase β (V)
Thermodynamics of Chocolate
Differential Scanning Calorimetry of Chocolate
Effect of Cocoa Percentage On Chocolate
Effect of Sugar Replacers On Thermal Properties of Chocolate
Salmiakki or “Salty Liquorice Chocolates”
Conclusions
Acknowledgements
References
Chocolate: Oral Processing of Chocolate – Successive Interplay of Sensory and Physicochemical Parameters
Fatty Phase
Taste: Bitter-Sweet
Phenols: Bitterness, Astringency and Structure Formation
Colloidal Precipitation in the Mouth
Conclusions
References
Coffee Preparation – From Roasted Beans to Beverage
Grinding
Influence of Particle Size
Different Grinding Methods
Parameters Influencing the Grinding Process
Extraction and Characteristics of the Resulting Beverage
Chemical Composition of the Extract
Impact of the Coffee Preparation Method
Impact of the Brewing Method
Impact of Coffee/water Ratio
Organoleptic Properties of the Resulting Beverage
Conclusions
References
Colour: Natural Pigments in Foods and Their Technical Uses
Anthocyanins
Betalains
Carotenes and Carotenoids
Chlorophylls and their Derivatives
Neutralisation of Acids
High-Temperature Short-Time Processing
Enzymatic Conversion of Chlorophylls to Chlorophyllides
Transformation of Chlorophylls Into More Stable Metallo-Complexes
Conclusions
References
Cooking
A Table for Innovation
In More Depth: Some Chemistry of Cooking
References
Cooking: Culinary Precisions and Robustness of Recipes
Testing Culinary Precisions
Reasons for Culinary Precisions: The Robustness Assumption
Comparative Molecular Gastronomy
References
Cryogenics in the Kitchen
A Short History of How Cryogens Came into Restaurants
How Do Cryogens Work?
Why Are Cryogens Useful in Food Preparation?
Industrial Use of Cryogens in Food processing
Freezing Fresh Foods
Producing Powders and Ground Spices
Flavour Extraction in Wine and Oil Production (Cryomaceration)
Ice Cream
Use of Cryogens in Restaurants
Cryo-Grinding (Cryo-Powders)
Cryo-Grating
Cryo-Slicing
Cryo-Shattering
Cryo-Searing
Cryo-Shaping
Cryo-Carbonation
Cryo-Peeling
Cryo-Freeze Drying
On Show – in the Dining Room
Cryo-Poaching
Liquid Nitrogen Ice Creams
Fogging Odours
Liquid Nitrogen Cocktails
Safely Storing, Handling and Using Cryogens
Training
Equipment
Risk Assessment
References
Dairy: Milk Gels – a Gastrophysics View
Introduction
Native Milk, a Simplified Picture
Yogurt and Its Production
Yogurt From Raw and Boiled Milk
Rheology Basics
Rheological Measurements of Yogurts
Colostrum, a Brief Insight
Creaming of Colostrum
Gel Formation From Colostrum
Conclusions
References
Dairy: Culinary Uses of Milk, Butter and Ice Cream
The Complexity of Milk
The Production of Butter
The Texture of Butter
The Science of Ice Cream
Structural Considerations for Butter and Ice Cream
Conclusion
References
Dairy: Ginger Milk Curd
Fool-Proof Ginger Milk Curd
Mechanism of Gelling
Ideas for Further Experimentation
References
Dehydration
Introduction
Moisture in Foods
Moisture in Air
Hygroscopicity of Foods
The Engineering Approach
Methods for Dehydrating Foods
Dried Products
References
Dispersed System Formalism
A Simple Case for Training: How to Describe a Raw Egg
The Objects for the Formal Description
Mixing Orders of Magnitude for An Overall Description
Use of Dsf for Scientific Explorations and for Innovation
Conclusions and Perspectives
References
Distillation: The Behaviour of Volatile Compounds during Distillation of Hydro-Alcoholic Solutions and during ...
Characterization of Volatility
Partial Pressures for the Liquid–vapour Equilibrium
Absolute and Relative Volatility
Distillation of a Hydro-Alcoholic Solution
Distillation of Volatile Compounds in a Hydro-Alcoholic Solution
Behaviour of Volatile Compounds in Boiling Water
Conclusion
References
Eggs: Let Us Have an Egg
A Code for Innovation
Some Examples
Conclusion
References
Emulsions: Emulsified Systems in Food
Principles Behind Emulsions and Foams
Types of Emulsifiers in Food Emulsions and Foams
Examples of Emulsified Systems in Food
Milk
“Plant-Based Emulsions” (Non-Dairy)
Butter and Margarine
Spreads, Dips, Dressings and Sauces
Sausages
Anise-Flavoured Liquors and Spirits (Microemulsions)
Beverages
Foams in Food Systems
Beer Foams
Ice Creams
Pacojet Ice Cream
Conclusion
References
Emulsions and Foams: Ostwald Ripening and Disproportionation in Practice
Emulsions and Foams
Let Us Remember That Metastable Systems are Not Thermodynamically Stable
Destabilization Mechanisms
Changes in Emulsions
Changes in Foams
How Does This Apply?
Application to Emulsions
Application to Foams
Conclusions and Perspectives
References
Emulsions: Lecithin
Lecithin
Composition of Lecithin
Phospholipids
Glycolipids
Triglycerides
Other Components
Applications of Lecithin
Emulsions – Types
Labelling of Lecithin
References
Emulsions: Emulsions and Surfactants in the Kitchen
Oil Does Not Mix With Water; Surface Tension
Surfactants
HLB Values
Making Colloidal Systems
Why Are Micelles Depicted As Spherical?
Micellar Concentration
Surfactants in Emulsions
Conclusions
References
Essential Oils
The Odour of Plants Is a Result of Their Metabolism
How to Capture Smell? A Chronological Point of View
From the Point of View of the Parameters Linked to/Depending Upon the Nature of Molecules: Partition Coefficients
From the Point of View of the Manufacturing Processes
Primary Extracts: Volatile Solvent Extraction and Essential Oils
Essential Oil Production Plants
Tinctures, Soluble Oils, Alcoholates
Secondary (Re-Processed) Extracts
Industrial Constraints
Expected Yields As a Function of Time and Energy
References
Essential Oils: How to Safely Use Essential Oils
Brief History of Safety Concerns
North American Safety Approach by Congeneric Groups
European Safety Approach by Substances With a Particular Concern
Applications
Conclusion
References
Evaporation
Ordering Systems
Food Liquids: O, S
Hydrophobic Solutions: O
Aqueous Solutions
Aqueous Solutions at Room Temperatures or in the Mouth
Aqueous Solutions Being Heated
Stock and Reductions
Steam Extraction
More Complex Systems Having a Continuous Liquid Phase
Water Activity
Evaporation From Emulsions
Flavour Release for More Complex Systems
Crust Formation
Water Vaporization and Frying
Conclusion
References
Expansion
Early Experiments
Testing Culinary Precisions
Other Expansions of This Kind
One by One?
References
Fats and Oils: Physicochemical Properties of Edible Oils and Fats
How Are Dietary Lipids Designed to Achieve a Certain Functionality?
Hydrogenation
Fractionation
Interesterification
References
Fats and Oils: From Fat Droplets in Plant Seeds to Novel Foods
Oleosomes Meet the Culinary World: Spreads and Sauces
Fat Mayonnaise-Type Spread
Mustard Spread
Vinaigrette
Béchamel Sauce
Liquid Creamers
References
Fats and Oils: Oxidation of Dietary Lipids
The Autoxidation Mechanism
Final Oxidation Products
How to Avoid Or Limit Oxidation
Conclusion
References
Fats and Oils: Extra Virgin Olive Oil in Cooking – Molecular Keys for Traditional and Modern Mediterranean Gastronomy
What Is Evoo?
EVOO in ‘mediterranean Molecular Cuisine’
EVOO in Fish Canning and Cooking
Tomato–EVOO Interactions
Marinating Meat in Evoo Before Roasting
Cooking in Evoo (Deep-Frying,-Pan-Frying, Sautéing, Sofrito)
EVOO and Milk Proteins
Conclusions
Acknowledgements
References
Fermentation: Kimchi
References
Fermentation: Fermenting Flavours With Yeast
Yeast
Fermentation
Flavour and Aroma Properties of Yeast
Yeast in the Food and Beverage Industry
References
Fermentation: A Short Scientific and Culinary Overview of Kefir
Introduction
Origin, Production and Consumption
Scientific Investigations
Culinary Uses
Acknowledgements
References
Filtration Membranes for Food Processing and Fractionation
Introduction
The Basics of Filtration Membranes
From Frontal to Tangential Filtration
Driving Force, Permeate Flux and Retention
Intrinsic Limits and Challenges: Concentration Polarization and Fouling
Applications
Overview of the Interest in Filtration Membranes in Food Processing
Purification
Concentration/Extraction
Integrated Separation Processes
Wastewater Treatment and Valuation
Conclusion
References
Food Matrices and the Matrix Effect in the Kitchen
Introduction
What Is a Food Matrix?
Classifying Food Matrices by the Dispersed System Formalism
Solid and Liquid Matrices (S)
Emulsions (L1/L2) and Suspension (S/L) Matrices
Gel Matrices (X/D3(S), XxD3(S))
Porous Matrices
More Complex Matrices
FM and Cooking
Texture
Flavour Release
Nutrition
Food Analysis
Conclusions
References
Food Pairing: Is It Really About Science?
Refutation of the Food Pairing Theory: A Critical Look at the Literature
Conclusion
References
Freeze-Drying
The Principle of Freeze-Drying
Freeze-Drying Equipment
Sublimation and Condensation in freeze-Drying
Freezing of Water in Foods
Raoult’s Law
Ice Formation and Melting
Freeze-Drying of Food Materials
Heat and Mass Transfer
Freeze-Drying Temperature
Challenges
Packaging
Manufacturing Costs
Freeze-Dried Foods
Freeze-Dried Coffee
Freeze-Dried Berries, Fruits and Vegetables
Herbs and Spices
Meat and Seafood
Ready Meals
Novelties
Conclusions
References
Foams: Pickering Edible Oil Foam – Toward New Food Products
Introduction
Formation and Properties of Pickering Foams
Foam Stabilization by Particles
Instability Mechanisms in Pickering Foams
Key Differences Between Aqueous Pickering Foams and Pickering Edible Oil Foams
Formation and Properties of Pickering Edible Oil Foams
Properties and Structure of the Crystalline Particles
Strategy and Process to Produce Edible Oil Foams
Foamability and Oil Foam Stability
Structure and Rheological Properties of Edible Oil Foams
Conclusion
References
Frying
Introduction
Oil Uptake
Chemical, Physical and Microstructural Changes
Conclusions
Acknowledgements
References
Gastrophysics: A New Scientific Approach to Eating
Introduction
Molecular Gastronomy
Neurogastronomy
Gastrophysics
Conclusions
References
Gels
Using the Dispersed System Formalism for Describing Gels
Simple Gels As Described Using Dsf Operators
More Complex Gels
More Than Two Phases: Dynagels/statgels
References
Heat Transfer in Culinary Sciences
Introduction
Heat Transfer Modes
Internal Energy Variation
Heat Balance Equations
Basic Applications
Refrigerating a Jelly
Cooking Potatoes in Boiling Water
Introduction to Coupled Phenomena
Baking Bread
Freezing Ice Cream
References
Hydrocolloid Usages As Gelling and Emulsifying Agents for Culinary and Industrial Applications
Introduction
Use of Hydrocolloids As Gelling Agents
Agar-Agar
Chemical Composition/Structure
The Gelation of Agar-Agar
External Effectors and Synergies
Applications of Agar-Agar
Carrageenans (Iota and Kappa)
Chemical Composition/structure
The Gelation of Carrageenans
External Effectors and Synergies
Applications of Carrageenan Gels
Sodium Alginate
Chemical Composition/structure
Alginate Solutions
The Gelation of Alginate
External Effectors and Synergies
Acidity
Calcium
Salt
Applications of Alginate
Gellan Gum
Chemical Composition and Structure
Hydration of Gellan Gum
The Gelation of Gellan Gum
Low Acyl
High Acyl
Gelling Mechanism
External Effectors and Synergies
Properties of Gellan Gum Gels
Gellan Gum Fluid Gels
Applications of Gellan Gums
Hydrocolloids as Emulsifiers
Exudate Gums
Gum Arabic
Chemical Composition/Structure
Emulsifying Properties
Gum Ghatti
Chemical Composition/Structure
Emulsifying Properties
Other Polysaccharide Hydrocolloids
(Modified) Starch
Chemical Composition/structure
Emulsifying Properties
Cellulose Derivatives
Chemical Composition/structure
Emulsifying Properties
Pectins
Chemical Composition/structure
Emulsifying Properties
Protein Hydrocolloids and Their Interaction With Polysaccharides
Physico-Chemical Parameters of Interaction
Functional Properties
References
Imaging Foodstuffs and Products of Culinary Transformations
Imaging of Emulsions: Mayonnaise, Butter, and Whipped Cream
Imaging of Cheese
Imaging of Dried Vegetables: Crunchy Japanese Tsukemono
Imaging Squid
Conclusion
Acknowledgments
References
Meat: Meat Tenderness and the Impact of Cooking
Muscle Structure and Transformation of Muscle Into Meat During Ageing
Evolution of Meat Structure During Cooking
Is It Possible to Totally Control the Tenderness of Cooked Meat?
Conclusion
References
Meat: Heat Transfer in Meat
Cooking
Thawing and Freezing
Browning
References
Meat: Reduction of Nitrate and Nitrite Salts in Meat Products – What Are the Consequences and Possible Solutions?
Introduction
Do Nitrate-Rich Plant Extracts Offer a Viable Alternative?
Role of Nitrates/Nitrites in Colour
Colour Formation
Alternatives
Addition of Natural Colourants
Addition of Bacteria
Role of Nitrates/Nitrites in Flavour
Flavour Development, Antioxidant Role
Alternatives to Nitrates and Nitrites
Role of Nitrates/Nitrites in Microbiological qualities
Growth, Survival and Activity of Starters
Growth and Survival of Spoilage Bacteria
Inhibition of Pathogenic Bacteria
Clostridium Botulinum
Salmonella
Listeria
Alternatives
Plant Extracts
Lactic Acid Bacteria
Conclusion
References
Microwave Heating and Modern Cuisine
Introduction
Principle of Operation of a Microwave Oven
Microwave Heating of Food
References
Mineral Ions and Cooking
Involvement of Mineral Ions in Food in Taste Perception and Appreciation
Dairy Products
Meat Products
Tomatoes
Drinking Water
Mineral Ions May Affect the Colour of Food
Crude Food Products
During Cooking
Mineral Ions Affect the Texture and Structure of Food
Vegetables
Emulsions
Conclusion
References
Osmosis in the Kitchen
Osmosis and Semipermeable Membranes
Where Are These Semipermeable Membranes?
Other Culinary Processes With Osmosis
Mechanisms
Applications
Conclusion
References
Pasta: Durum Wheat Proteins – a Key Macronutrient for Pasta Qualities
Pasta Quality Is Driven by Both Raw Material and Process
Durum Wheat Components
Pasta-Making Process
Pasta Qualities
Gluten Proteins Control Pasta Qualities
Protein Contribution to Quality Throughout pasta-Making Process
Protein and Pasta Qualities
Conclusion
References
Pasteurization in the Kitchen
Introduction
Pasteurization Processes Used in the food Industry
Low-Temperature, Short-Time Pasteurization
High-Temperature, Short-Time Pasteurization
Steam Pasteurization
Sous-Vide Cooking
Pathogens of Concern for Thermal Pasteurization
Physical-Chemical Modifications of Foods During Pasteurization
Meat
Protein Modifications
Lipid Modification
Fruits and Vegetables
Saccharide Modification During Pasteurization
Colour and Texture
Antioxidant Activity
Phenolic Compounds
Carotenoids
Vitamins
Novel Pasteurization Methods
References
Plating: The Science of Plating
Introduction
Assessing Orientation Preferences for the Plating of Food
Pointing Angularity Away From the Diner
Ascending to the Right: Preferential Orientation for Linear Elements
Orientation Preference for the Horizontal/Vertical
Conclusions
References
Proteins and Proteases
Introduction
Amino Acids
Protein Structure
Tertiary Structure
Quaternary Structure
Protein Denaturation
Proteases and Proteins
Enzymes and Cheese
Fruit Enzymes
Marinades and Meat
Conclusions
References
Puddings: The Secret of the Rice Pudding
Rice Pudding
Reference
Roasting
The Particular Example of Coffee
Some Modifications of Coffee Seeds during Roasting
Other Seeds
Conclusions
References
Salt: When Should Salt Be Added to Meat Being Grilled?
Juice Extraction by Salt
How Much Salt Goes Into Meat During Grilling?
References
Sauces
How to Use the New Classification?
References
Sauces: Hollandaise Sauce
Hollandaise Sauce History
Mother and Daughter Sauces
A Warm Emulsion
Mixing of the Immiscible
Ratios
The Art of Mixing
Stabilizers and Emulsifiers
The Droplets’ Life in Solitude
Hollandaise Ingredients
The Problematic Butter
Hollandaise Preparation
Summary of Success Points for Emulsions and Warm Butter Sauces
Hollandaise Sauce Recipe
Base:
White Wine and Vinegar Reduction:
Flavorings:
References
Sauces and Purées: The Underside of Applesauce
Apple Fruit: An Organized Structure From Macroscopic to Microscopic Scale
From Apple Fruit to Applesauce: thermo-Mechanical Treatment
Applesauce: A Soft Plant Particle Suspension
Which Parameters Influence the Texture of Apple Puree?
Particle Concentration and Serum Viscosity
Particle Morphology
Intrinsic Parameters
Variety and Cultivars
Maturity/Loss of Adhesion
Thermo-Mechanical Treatment
Link Between Perceived Texture (Sensory) and Measured Texture (Instrumental)
Conclusions and Perspectives
References
Seaweeds: Phycogastronomy – the Culinary Science of Seaweeds
Sensory Perception of Seaweeds
Visual Appearance and Aesthetics
Auditory Sensation
Aroma and Odour
Texture (Mouthfeel)
Taste
Umami and Dashi
Acknowledgements
References
Size Reduction
Introduction
Cutting
Grinding
Emulsions and Suspensions
Processes and Equipment
Measuring Particle Size
Particle Size Distributions
References
Smoked Foods
Introduction
Smoking Methods
Odorants in Smoke
Different Smoking Materials
Taste
Texture
Health Aspects
Applications of Smoke
References
Sous Vide Cooking
Packaging
Cooking Sous Vide
Meat, Poultry, and Fish
Doneness, Appearance, and Texture
Tender Cuts
Tough Cuts
Storing and Reheating
Finishing for Service
References
Spherification
Introduction
The Science Behind the Technique
The Future
References
Squid: Gastrophysics of Squid – From Gastronomy to Science and Back Again
Introduction
The Challenge of Tough Muscular Fibres
Texture of Squid
Umami Taste of Squid
Some New Squid Products of Gastronomical value
Acknowledgements
References
Sugars: Soft Caramel and Sucre à la Crème – an Undergraduate Experiment about Sugar Crystallization
Introduction
Historical and Cultural Background
Physical Chemistry of the Confections
Conclusion
Acknowledgements
References
Sugars: Sugar (and Its Substitutes) in Pastries
Sweet Taste
Colour and Flavour Profile
Texture Properties
Preservation
Experimentation
References
Sugars: Erythritol–sucrose Mixtures Out of Equilibrium – Exciting Thermodynamics in the Mouth
Experimental Observation of Glass Transitions and Crystallization
Glass Transitions
Crystallization
Melting
Inspiration for the Kitchen
Ice Blossoms
Hot Syrup
Conclusion
References
Sugars: Intramolecular Dehydration of Hexoses
Arithmetic of Hexose Dehydration
Inspection of Systematic Possibilities of Hexose Dehydration
Most Commonly Observed Hexose Dehydration Products and Properties
Conclusion
References
Taste and Sound
Food-Intrinsic Sounds
Food-Extrinsic Sounds
Sonic Ingredients
Technology for “sonic Seasoning”
Crossmodal Correspondences Between Taste and Sound
Multisensory Gastromusical Art
Conclusion
References
Temporal Domination of Sensation: When Building Dishes, let’s Take Temporality Into Account
Origin of Tds: From Intensity to Dominance
Pairing Tds With Liking, Wanting and Satiation
Temporal Dominance of Emotions (TDE)
No Dominance and Dual-TDS
Using TDS-Liking for Descriptive and Hedonic Evaluation of Food Pairing
A Need for TDS Data Analysis Refinements
Last But Not Least: What Is Dominance?
References
Texture: The Physics of Mouthfeel – Spreadable Food and Inulin Particle Gels
Introduction
Liver Sausages: The Role of Proteins and Fats
Inulin: Selectively Crystallisable Molecules
Fractal Particle Gels and “Simulated Melting”
Conclusions
References
Texture: How Texture Makes Flavour
Interplay Between Mouthfeel and Other Sensory Perceptions
Taste → Touch
Touch → Taste
Temperature → Taste
Taste → Irritation
Irritation → Taste
Smell → Touch
Touch → Smell
Smell → Temperature
Temperature → Smell
Smell → Irritation
Irritation → Smell
Touch → Temperature
Temperature → Touch
Touch → Astringency
Temperature → Irritation
Irritation → Temperature
Texture
Texture and Food Culture
Texture and the Recognition of Food
Food Complaints Often Relate to Mouthfeel
Modifying Texture
The Challenge in Designing Texture
Acknowledgements
References
Texture: Tsukemono – the Art and Science of Preparing Crunchy Vegetables
Preparation of Tsukemono
Salt, Salts, and Texture
Drying and Texture
Flavour
Aesthetics and Health
Acknowledgements
References
Thickeners: Cellulose and Its Derivatives
Carboxymethylcellulose
Structure
Solubility
Viscosity
Hydration
Rheological Behaviour
Compatibility With Other Ingredients
Applications
Methylcellulose and Hydoxypropylmethylcellulose
Structure
Dispersion/Solubility
Rheological Behaviour
Gelation Characteristics
Effect of Other Ingredients On Gelling Temperatures
Mechanism of Gelation
Applications
Hydroxypropylcellulose
Ethylcellulose
References
3D Printing of Food
Introduction and History
Principle of 3D Printing of Food
Possible End-Use Scenarios
Food Applications
Chocolate
Pasta
Confectionery
Meals
Specialist Nutritional Foods
Fruit
Culinary Applications
Bakeries
Restaurants and Chefs Serving 3D-Printed Foods
3D Printing and Fine Dining
Collaborative Projects and Applications
Applications of Molecular Gastronomy
Specialised Stores and Personalised Products
Limitations
Future Speculations
Conclusions
References
Umami: The Molecular Science of Umami Synergy
Molecular Mechanism of Umami Synergy
Umami in the Kitchen
Umami and Healthy Eating
Acknowledgements
References
Part II Educational Practices
The Right Words for Improving Communication in Food Science, Food Technology, and Between Food Science and Technology ...
About Flavour (and Flavor)
About Colloids
Emulsions and Emulsified Systems
Gels
Foams and Aerated Systems
About Chemistry in General
Should We Speak of Maillard Reactions? Probably Not
Conclusion
References
Experimental Flavour Workshops
Developments Since 2000 in Primary Schools
Departing From “instinct”
Protocols and Their Instructional Support
Instructional Efforts
An Example Activity: The Whipped Egg White Contest
The Instructional Objectives
Experimental Sheet
Material for a Class of 30 Children:
Protocol
Instructional Commentary
Possible Extensions
Conclusion
References
Teaching Argumentation and Inquiry Through Culinary Claims
Introduction
Background
Argumentation, Inquiry and Socio-Scientific Issues in Education
Toulmin’s Argumentation Pattern (tap) and Education
The “kitchen Stories” Project
Practical Approach in the “kitchen Stories” Project
First Phase (2–3 Weeks’ Duration, Part-Time)
Step 1 – Collect and Document Culinary Claims
Step 2 – Analyse and Construct Plausible Arguments for a Few Selected Claims
Second Phase (2–3 Weeks’ Duration, Part-Time)
Step 3 – Test/Experiment Analysed Claims
Step 4 – Record and Publish Results, Documentation of cultural Heritage
Case Example: You Can’t Make Jelly Containing Fresh Kiwi
First Phase (3 Weeks)
Second Phase (3 Weeks)
Results and Discussion
Research Questions, Observations, Preliminary Results and Discussion
Culinary Claims, Tap and Argumentation Instruction
Promoting Minds-On As Well As Minds-On-Hands-On Practical Work
Epistemic Status: Source Awareness, Sourcing Skills and Critical Thinking
Kitchen Stories and Declarative Knowledge
Conclusions
Outlook
References
Cooking and Science Workshops: The “Soft of the World”, Gelling Agents
Gelling Agents: Agar-Agar and Gelatine
More Gelling Agents: Sodium Alginate
References
Culinary Sciences for the Enhancement of the Public Understanding of Science
Science in Formal Educational Settings
Science Outreach on Informal Platforms
Some Examples Using the Science of Food and Taste in Outreach Activities
Acknowledgments
References
“Science and Cooking Activities” for Secondary School Students
Objectives and Methods
Workshop 17: How Do We Use Sugar Syrup to Create Multilayered Cocktails?
Workshop 19: Where Does the Color of Green Beans Come From After Cooking?
References
How to Reduce Oil in French Fries: A Student Experiment
Protocol
Results
References
An Educational Satellite Project Around the Scientific Elucidation of Culinary Precisions in Lebanon and in the Middle East
Introduction
Research Activities
Social Outreach
Teaching Structures
School Level
Bachelor Degree Level
Master Degree Level
Continuing Education and Knowledge Transfer
Regional Expansion
Conclusion
Acknowledgements
References
Bon Appétit, Marie Curie! A Stanford University Introductory science of Cooking Course
Introduction and Overview
Details of the Class Sessions
Session 1: Taste and Flavor
Session 2: Why Cook Food?
Section 3: The Chemistry of Cooking
Session 4: Fermentation
Session 5: Sauces as Complex Mixtures
Session 6: Modernist
Session 7: Chocolate
Teaching the Course On Stanford Campus
Teaching in Paris
Teaching During a Global Pandemic
Conclusions and Future Directions
References
Molecular Gastronomy in Science Education and Science Communication at the National University of Singapore
The Project: Reaching Out With Molecular Gastronomy in Singapore
Science Education and Science Communication
An Example of a Typical Workshop
People and Planning
People
Roles, Tasks and Teaching Material
Choice of Topics Related to Molecular Gastronomy
Benefits and Impacts
Teaching Science to Workshop Attendees
Empowering Undergraduates
Plans for the Future
Acknowledgements
References
Molecular Gastronomy: A Universal Portal to the Molecular Sciences
Acids and Bases
Oxidoreduction
Glycation/Maillard Reaction
Enzyme Kinetics
Elasticity
References
Heat Transfer in the Kitchen – Exercises
Cooking Pasta
Freezing an Apple Sauce
Refrigerating a Soup Tray
Ionic Diffusion in Spherified Calcium Alginate Gels: A Laboratory Experiment
Introduction
Sources and Chemistry of Alginic Acid and sodium Alginate
Uses of Sodium Alginate
Medicine and Dentistry
Innovative Materials
Art
Molecular Cooking
Physical Properties of Calcium Alginate
Experimental Protocol
Preparation of Sodium Alginate Solution
Jellification and Conservation Bath
Preparation of Acidic Cacl2 Solutions and Measurements of Turning Times and Diameter of Spheres
Results
Digital Spectrophotometric Analysis
Conclusions
Supplementary Material
Acknowledgements
References
Simple Calculations Based On Cooking
For Some of Us, Calculation Is a Need That Can Become Fun After Realizing That It Is Easy
Simple Knowledge
How to Calculate
Which Tool?
A First Summary, a “Backbone” for Calculation
The Quality of Calculation: Validation
A First Example: How Many Bubbles Form in a Whipped Egg White?
The Question
Analysis of the Question
The Data Are Introduced
Qualitative Model
Quantitative Model
Solving
Expressing the Results
Finding Digital Data
Introduction of Data Into the Formal Solution
Discussion
Conclusions and Perspectives
A Second Example: Why Do Oil Droplets Cream in Mayonnaise Sauce?
The Question: In Natural Language, With the Right Words, in a Concise and Simple Way (subject, Verb, Complement)
Analysis of the Question
The Data Are Introduced
Qualitative Model
Quantitative Model
Solving
Looking for a Solving Strategy
Implementing the Strategy
Validation
Expressing the Results
The Formal Result Found Is Written Down Again
Finding Digital Values
Introduction of Data in the Formal Solution
Discussion
Why Does Stokes Drag Relate to R and Not r2?
Conclusions and Perspectives
A List of Questions That Students Can Use For Developing Their Problem-Solving Skills
About Gases
About Liquids
About Foams
About Emulsions
About Gels
About Suspensions
More Complex Systems
Conclusions
References
Teaching and Cooking With Culinary Teachers
The Monthly Inrae-Agroparistech Seminars On Molecular Gastronomy
Many Questions
In Practice
References
Part III Culinary Applications
New Greek Cuisine
Greek Salad (Granita)
Cassoulet
Dolmas
Soft Boiled Egg
Orange Explosion
A Few Words by the Chef(s)
What Is Your Professional Background?
How Would You Define Your Cooking Style?
The Recipe(s) You Are the Proudest Of?
What Ingredients Inspire You More Particularly?
How to Be Creative and Keep “fashion” in Cooking?
What Do You Do Now That You Did Not Do Ten Years Ago?
What Would You Like to Be Able to Do in 10 Years That You Are Not Able to Do Now?
Do You Collaborate With Scientists? Other People (artists, Designers, Engineers, …)?
What Is Your Strategy to Innovate/create new Dishes?
3D Printed Note by Note Recipe: Soya Lobster Prototype
Recipe
Cooking (with) Olive Oil
Crunchy Evoo
EVOO Powder
Jellified Evoo Vinaigrette
EVOO Cold Mousse
EVOO Hot Mousse
EVOO Encapsulation
EVOO Ice Cream
EVOO Sponge Cake
Frozen Evoo Soufflé
Cooking for the Elderly
Culinary Constructivism and Note by Note Cooking
Debyes
Debye Raspberry/basil
Debye Capers/anchovies
Debye Chocolate/orange
Diracs
Dirac Compact
Fibrous Dirac 1
Another Fibrous Dirac
The “Chick Corea”
The Whole Work
First for Culinary Constructivism
Culinary Precisions, Knowledge and Gourmandise
Note by Note Cooking
Conclusions and Perspectives
Decantation
An Old Process for Cooking and Alchemy
The Old Dynamics
Decanting in the Kitchen
Other Applications of This System
References
Note by Note Recipes for a Press Conference and Tasting Organized at ITHQ, 2012
Bubbles
Cloud
Tart
Flama
Ultra
Annex: Soaking the Surimi
Using Liquid Nitrogen to Prepare Ice Creams in the Restaurant
The Brandy Sorbet by André Daguin
Fruit Sorbet by Noël Gutrin
Crêpes Mademoiselle by Philippe Labbé
A Note by Note Traditional Chinese Dinner Created and Served in Singapore
The Note by Note Menu, With Comments From the Chefs
First Dish: Do, Re, Mi Come Together in a Mini Dim Sum Bamboo Steamer
Second Dish: Sea of Change
Third Dish: Longevity
Fourth Dish: Lioness Head
Fifth Dish: Save the Shark
Sixth Dish: Hidden Heart
Seventh Dish: Last Kiss
The Recipes
Do – Pot Sticker, Note by Note
Dough:
Meat Base:
Daikon Fibre:
Marinade + Meat Base:
Dumplings:
Method, Dough:
Method, “meat” Base:
Filling Preparation:
Method, Dumplings:
Re-Salted Egg Crisp, Note by Note
Dough:
Salted Egg Seasoning:
Method, Dough:
Mi – Bao Char Siu, Note by Note
Dough:
“Meat” Base:
Daikon Fibre:
Char Siu Sauce:
Base Meat + Sauce:
Method, Dough:
Method, “Meat” Base:
Method, Char Siu Sauce:
Method, Filling:
Filling + Dough:
Sea of Change – Oyster Pancake, Note by Note
Batter:
Oyster Seasoning:
Chilli Sauce Note:
Method, Batter:
Method, Seasoning:
Method, Sauce:
Longevity – Noodle, Note by Note
Dough:
Chicken Jelly:
Directions:
Lioness Head, Carrot Cake, Note Leek, Note by Note
“Meat” Base:
Daikon Fibre:
Note by Note Crumb:
Meat Balls:
Broth and Sauce:
Daikon Cake:
Note Leek:
Method, “Meat” Base:
Method, Crumb:
Save the Shark, Note by Note
Soup:
Fin:
Method, Soup:
Method, Fin:
Hidden Heart, Note by Note
Coconut Agar Gel:
Method:
Last Kiss – Lollipop, Note by Note
Filling:
Method:
Greek Diracs
Introduction
The Menu
Amuse-Bouche Note by Note
Entrée Note by Note
First Course
Sherbet Note by Note
Main Course
Dessert Note by Note
Mignardise Note by Note
The Recipes
Evocation Sausage-Tempura, Evocation Sausage-Tempura,-Mustard-Wasabi
Dirac Sausage-Tempura:
Garlic Jelly:
Dough for Frying:
Note by Note Flour (portion):
Note by Note Bread:
Mustard Foam:
Evocation Potato and Tomato Chips, Evocation Cucumber, French Bread and Garlic, Evocation Milk and Cheese
Evocation Potato:
Evocation Tomato:
Evocation Milk and Cheese:
Emulsion Cucumber:
Evocation Mediterranean Seafoods
Dirac Shrimp Bass:
Sablé Dough (for the Shrimp):
Feta Evocation:
Tomato Sauce:
Ravioli Evocation Earth
Ravioli Dough:
Beetroot Jelly:
Cheese Foam:
Garlic Transparent Gel:
Foam Caramel – Mushroom – Beetroot:
Beetroot, Mushroom and Caramel Juice:
N-Fruit, Imaginary Fruit, Fruity and Sour, Kernel Evocation Caramel, Vanilla, Earth Evocation Coconut, Milk, Vanilla and Smoke
N-Fruit Mousse:
Meringue:
Honey Caramel Caviar (14 G / Portion):
Cigarette:
N-fruit Skin:
Vanilla Earth:
Sponge Black:
An Eclipse Dish
Modern Swiss Cooking
Sparkling Meringue / Double Cream of Gruyère
Double Cream Mixture
Spicy Cigar
Sparkling Meringues
Chocolate Sorbet
Dressage
Crisp Fondue “Vacherin Fribourgeois”
Ingredients
Recipe
Finishing
How Do Eggs Coagulate?
Uncooking Eggs
Using This Knowledge to Make New Eggs
References
Vegetable Salad
Ingredients
For the Broth
For the Vegetables
For the Dressing
Methods
For the Broth
For the Vegetables
For the Dressing
Filtration
Modern Techniques
Future Developments?
References
Waiter! There Is Garlic in My Meringue!
References
Lobster and Juniper
Lobster
Juniper Gel
Grapefruit Caramel
Lobster Sabayon
Juniper Oil
Juniper Cream
Molecular Cooking
Tools
Ingredients
Methods and Techniques
Centrifugation
Cryo-Techniques Using Dry Ice and Liquid Nitrogen
Distillation
Drying
Filtration
Foaming
Gelling
Sous Vide Cooking
Smoking
Spherification
Conclusion
References
Note by Note Cooking and Note by Note Cuisine
What Is Note by Note Cooking?
A Short Prehistory and a Birth
Why Is Nbn Different From Molecular Cooking?
Why Is It Important?
A Short History
A Comment On the Name
First Lectures
Hong Kong
Pioneers
Acceleration: Book and Courses
International Contests
Products
Creating An Nbn Dish
Recent Developments
References
Spherification
Some Technical Information
Sodium Alginate
Instructions for Use
The Role of Calcium
General Principles
Recipes
Basic Spherification Recipe
Reverse Frozen Spherification
Cryogenic Cooking – Cryo-Diffusion for Making Frozen Spheres Which Are Then Gelled in a Sodium Alginate Bath
A Recipe With Cranberries
References
The Raspberry Pear Viennoiserie
The Four Stages of Materials Preparation
Raspberry Pear Preparation
The Crème Patissiere
Stage # 2:
Preparation and Cutting of the Pastry
References
Molecular Mixology: Welcome Coffee, a Cocktail With Ten Layers
More Layers Than Usually Done: The Welcome Coffee
A Cocktail for Restaurants
Ingredients for Six People
Preparation
Recipe 1 – Gelled Strawberry Juice
Recipe 2 – Foie Gras Cream
Recipe 3 – Custard With Pistachio
Recipe 4 – Olive Oil With Citrus
Recipe 5 – Strawberry Mousse
Finishing
References
Cube of “Chicken-Carrot” with Chips of “Chicken-Carrot”-“Basil-Lemon”
Sphere of “Chicken-Carrot”
Gelatine Cube “pork Stock”
“Rind With Kiwi Seeds” Layer
“Basil-Lemon” Chips
“Tomato Granité” Juice
Some of the Easiest Note by Note Recipes Served at Senses Restaurant
Recipe 1. NbN “Cucumber” and Buttermilk Spheres (Figure 132.1)
For the Coating
Recipe 2. NbN “Nuts” Air Bubbles (Figure 132.2)
Recipe 3. NbN “Goat Cheese” Frozen Pearls
Recipe 4. NbN “Beetroot” and Nbn “Gorgonzola” Rocks
Part 1
Part 2
Recipe 5. Nbn Bread Grissini (Figure 132.3)
Recipe 6. Nbn “potato” Tuiles (Figure 132.4)
Recipe 7. Smoked Bell Pepper Nbn Ketchup (Figure 132.5)
Recipe 8. Orange Blossom Sorbet
The Forest Floor
Mushroom Meringue Recipe
Bacon Soil Recipe
Horseradish, Basil and Pea Sponge Recipe
Basil and Matcha Tea Leaves Recipes
A Note by Note Macaron
Note by Note Cooking Can Teach a Lot
The Recipe for the Macaron
Note by Note Cooking
Introduction
The Base Chord
The Middle Chord
The Top Chord
How to Work to Create Note by Note Flavours?
References
Note by Note Sushis
Consistency of Rice “Note by Note”
“Note by Note” Covering Sheet
Slowly Cooked Lamb Neck With Fermented Flour Pancakes, Sunchoke Puree and Beer Glaze
For the Lamb
For the Sunchoke Puree
For the Fermented Flour Pancake
For the Confied Sunchokes
For the Frosting of Black Beer
A Few Words by the Chef
What Is Your Professional Background?
How Would You Define Your Cooking Style?
The Recipe(s) You Are the Proudest Of?
What Ingredients Inspire You More Particularly?
How to Be Creative and Keep “fashion” in Cooking?
Index

Citation preview

Handbook of Molecular Gastronomy Scientific Foundations, Educational Practices, and Culinary Applications

Handbook of Molecular Gastronomy Scientific Foundations, Educational Practices, and Culinary Applications

Edited by Róisín M. Burke, Alan L. Kelly, Christophe Lavelle and Hervé This vo Kientza

First edition published 2021 by CRC Press 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487-​2742 and by CRC Press 2 Park Square, Milton Park, Abingdon, Oxon OX14 4RN © 2021 Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, LLC The right of Róisín M.  Burke, Alan L.  Kelly, Christophe Lavelle and Hervé This vo Kientza to be identified as the authors of the editorial material, and of the authors for their individual chapters, has been asserted in accordance with sections 77 and 78 of the Copyright, Designs and Patents Act 1988. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, access www. copyright.com or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-​750-​8400. For works that are not available on CCC please contact [email protected] Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging‑in‑Publication Data Names: Burke, Róisín, editor. Title: Handbook of molecular gastronomy: scientific foundations and culinary applications / edited by Róisín Burke, Alan Kelly, Christophe Lavelle, and Hervé This vo Kientza. Description: First edition. | Boca Raton: CRC Press, 2021. | Includes bibliographical references and index. Identifiers: LCCN 2020053893 (print) | LCCN 2020053894 (ebook) | ISBN 9781466594784 (hardback) | ISBN 9780429168703 (ebook) Subjects: LCSH: Molecular gastronomy–Handbooks, manuals, etc. Classification: LCC TX651 .H335 2021 (print) | LCC TX651 (ebook) | DDC 641.01/3--dc23 LC record available at https://lccn.loc.gov/2020053893 LC ebook record available at https://lccn.loc.gov/2020053894 ISBN: 978-​1-​4665-​9478-​4  (hbk) ISBN: 978-0-367-74161-7 (pbk) ISBN: 978-​0-​429-​16870-​3  (ebk) Typeset in Times by Newgen Publishing UK

Contents About the Editors.................................................................................................................................................................................. xiii Contributors............................................................................................................................................................................................xv Foreword...............................................................................................................................................................................................xxv Introduction to Molecular Gastronomy and Its Applications.............................................................................................................1

Part I  Scientific Foundations...................................................................................................................5 Acids in Foods and Perception of Sourness..........................................................................................................................................7 Christian Salles Anthocyanins in Food...........................................................................................................................................................................13 Véronique Cheynier Alcoholic Beverages: Production, Trends, Innovations.....................................................................................................................19 Konstantin Bellut, Kieran M. Lynch and Elke K. Arendt Ash in the Kitchen.................................................................................................................................................................................25 Marta Ghebremedhin, Christine Schreiber, Bhagyashri L. Joshi, Andreas Rieger and Thomas A. Vilgis Baking: Laminated Bakery Products..................................................................................................................................................35 Roxane Detry, Christophe Blecker and Sabine Danthine Baking: Chemical Leaveners...............................................................................................................................................................41 Linda A. Luck Baking: Injera –​the Multi-​Eyed Flat Bread......................................................................................................................................43 Mahelet Girma, Sumaya M. Abdullahi and Benjamin L. Stottrup Baking: Viennoiserie –​Laminated Pastry Production......................................................................................................................47 James A. Griffin Baking: How Does Starch Gelatinization Influence Texture?...........................................................................................................53 Anaïs Lavoisier Baking: Sourdough Bread....................................................................................................................................................................57 Mark Traynor and Imran Ahmad Barbecue: The Chemistry behind Cooking on a Barbecue...............................................................................................................63 Florent Allais Bioactivity and Its Measurement.........................................................................................................................................................71 Hervé This vo Kientza Browning: The Glycation and Maillard Reactions –​Major Non-​Enzymatic Browning Reactions in Food................................81 Frédéric J. Tessier Canning: Appert and Food Canning...................................................................................................................................................87 Jean-​Christophe Augustin Capillarity in Action.............................................................................................................................................................................91 Hervé This vo Kientza

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Champagne Tasting from a Scientific Perspective.............................................................................................................................97 Gérard Liger-​Belair, Clara Cilindre, Daniel Cordier, Guillaume Polidori, Fabien Beaumont and Thomas Séon Chantillys: The Cousins of Whipped Cream....................................................................................................................................105 Hervé This vo Kientza Cheese: Hot Culinary Uses of Cheese...............................................................................................................................................107 Sébastien Roustel and John A. Hannon Chocolate: Chocolates from around the World, Simple Physics, Complex Flavour....................................................................121 Bhagyashri L. Joshi, Sarah Gindra and Thomas A. Vilgis Chocolate: Oral Processing of Chocolate – Successive Interplay of Sensory and Physicochemical Parameters.......................131 Thomas A. Vilgis Coffee Preparation –​from Roasted Beans to Beverage..................................................................................................................139 Laura Febvay and Hervé This vo Kientza Colour: Natural Pigments in Foods and Their Technical Uses.......................................................................................................151 Juan Valverde Cooking................................................................................................................................................................................................157 Hervé This vo Kientza Cooking: Culinary Precisions and Robustness of Recipes..............................................................................................................163 Hervé This vo Kientza Cryogenics in the Kitchen..................................................................................................................................................................171 Peter Barham Dairy: Milk Gels –​a Gastrophysics View.........................................................................................................................................181 Judith Hege, Marta Ghebremedhin, Bhagyashri L. Joshi, Christine Schreiber, H.-​C. Gill and Thomas A. Vilgis Dairy: Culinary Uses of Milk, Butter and Ice Cream.....................................................................................................................191 Alan L. Kelly and David S. Waldron Dairy: Ginger Milk Curd...................................................................................................................................................................199 Martin Lersch Dehydration.........................................................................................................................................................................................203 José M. Aguilera Dispersed System Formalism.............................................................................................................................................................207 Hervé This vo Kientza Distillation: The Behaviour of Volatile Compounds during Distillation of Hydro-​Alcoholic Solutions and during Hydro-​Distillation...................................................................................................................................................................213 Martine Esteban-​Decloux Eggs: Let Us Have an Egg..................................................................................................................................................................221 Hervé This vo Kientza Emulsions: Emulsified Systems in Food...........................................................................................................................................227 Markus Ketomäki, Trivikram Nallamilli, Christine Schreiber and Thomas A. Vilgis Emulsions and Foams: Ostwald Ripening and Disproportionation in Practice...........................................................................241 Hervé This vo Kientza Emulsions: Lecithin............................................................................................................................................................................249 Elzbieta Kozakiewicz and Daniel Cossuta

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Emulsions: Emulsions and Surfactants in the Kitchen...................................................................................................................257 Hervé This vo Kientza Essential Oils.......................................................................................................................................................................................265 Eric Angelini and Laure Dziuba Essential Oils: How to Safely Use Essential Oils..............................................................................................................................275 Eric Angelini and Laure Dziuba Evaporation.........................................................................................................................................................................................281 Hervé This vo Kientza Expansion............................................................................................................................................................................................291 Hervé This vo Kientza Fats and Oils: Physicochemical Properties of Edible Oils and Fats...............................................................................................295 Sabine Danthine Fats and Oils: From Fat Droplets in Plant Seeds to Novel Foods...................................................................................................299 Juan C. Zambrano, Behic Mert and Thomas A. Vilgis Fats and Oils: Oxidation of Dietary Lipids......................................................................................................................................305 Luc Eveleigh Fats and Oils: Extra Virgin Olive Oil in Cooking –​Molecular Keys for Traditional and Modern Mediterranean Gastronomy................................................................................................................................................311 Raffaele Sacchi Fermentation: Kimchi........................................................................................................................................................................321 Weon-​Sun  Shin Fermentation: Fermenting Flavours with Yeast...............................................................................................................................327 Angela M. Coral Medina and John P. Morrissey Fermentation: A Short Scientific and Culinary Overview of Kefir................................................................................................331 Christophe Lavelle and Jean-​Baptiste Boulé Filtration Membranes for Food Processing and Fractionation......................................................................................................335 Marie-​Laure Lameloise Food Matrices and the Matrix Effect in the Kitchen.......................................................................................................................343 José M. Aguilera and Hervé This vo Kientza Food Pairing: Is It Really about Science?.........................................................................................................................................347 Hervé This vo Kientza and Christophe Lavelle Freeze-​Drying......................................................................................................................................................................................349 Yrjö H. Roos Foams: Pickering Edible Oil Foam –​Toward New Food Products................................................................................................357 Anne-​Laure  Fameau Frying...................................................................................................................................................................................................365 Franco Pedreschi Gastrophysics: A New Scientific Approach to Eating......................................................................................................................371 Charles Spence Gels.......................................................................................................................................................................................................375 Hervé This vo Kientza

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Heat Transfer in Culinary Sciences...................................................................................................................................................381 Denis Flick Hydrocolloid Usages as Gelling and Emulsifying Agents for Culinary and Industrial Applications..........................................385 Rachel Edwards-​Stuart and Reine Barbar Imaging Foodstuffs and Products of Culinary Transformations....................................................................................................409 Mathias Porsmose Clausen, Morten Christensen and Ole G. Mouritsen Meat: Meat Tenderness and the Impact of Cooking........................................................................................................................415 Jean-​François Hocquette and Alain Kondjoyan Meat: Heat Transfer in Meat.............................................................................................................................................................419 Douglas Baldwin Meat: Reduction of Nitrate and Nitrite Salts in Meat Products –​What Are the Consequences and Possible Solutions?.......................................................................................................................................................................423 Régine Talon and Sabine Leroy Microwave Heating and Modern Cuisine.........................................................................................................................................429 Alan L. Kelly and Hervé This vo Kientza Mineral Ions and Cooking.................................................................................................................................................................433 Christian Salles Osmosis in the Kitchen.......................................................................................................................................................................441 Hervé This vo Kientza Pasta: Durum Wheat Proteins –​a Key Macronutrient for Pasta Qualities..................................................................................447 Coline Martin, Marie-​Hélène Morel and Bernard Cuq Pasteurization in the Kitchen.............................................................................................................................................................451 Gabriela Precup and Dan-​Cristian Vodnar Plating: The Science of Plating..........................................................................................................................................................459 Charles Spence Proteins and Proteases........................................................................................................................................................................463 Linda A. Luck and Alan L. Kelly Puddings: The Secret of the Rice Pudding.......................................................................................................................................471 Martin Lersch Roasting...............................................................................................................................................................................................473 Laura Febvay and Hervé This vo Kientza Salt: When Should Salt Be Added to Meat Being Grilled?.............................................................................................................491 Hervé This vo Kientza, Marie-​Paule Pardo and Rolande Ollitrault Sauces...................................................................................................................................................................................................495 Hervé This vo Kientza Sauces: Hollandaise Sauce.................................................................................................................................................................499 Guro Helgesdotter Rognså Sauces and Purées: The Underside of Applesauce...........................................................................................................................505 Cassandre Leverrier Seaweeds: Phycogastronomy –​the Culinary Science of Seaweeds.................................................................................................517 Ole G. Mouritsen

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Size Reduction.....................................................................................................................................................................................523 José M. Aguilera Smoked Foods......................................................................................................................................................................................527 Jane K. Parker and Alice Pontin Sous Vide Cooking..............................................................................................................................................................................531 Douglas Baldwin Spherification......................................................................................................................................................................................537 Linda A. Luck Squid: Gastrophysics of Squid –​from Gastronomy to Science and Back Again..........................................................................541 Ole G. Mouritsen, Charlotte Vinther Schmidt, Peter Lionet Faxholm and Mathias Porsmose Clausen Sugars: Soft Caramel and Sucre à la Crème –​an Undergraduate Experiment about Sugar Crystallization...........................545 Irem Altan, Patrick Charbonneau and Justine de Valicourt Sugars: Sugar (and Its Substitutes) in Pastries................................................................................................................................549 Anne Cazor and Ramon Morató Sugars: Erythritol–​Sucrose Mixtures out of Equilibrium –​Exciting Thermodynamics in the Mouth......................................557 Hannah M. Hartge, Birgitta I. Zielbauer and Thomas A. Vilgis Sugars: Intramolecular Dehydration of Hexoses.............................................................................................................................563 Marie-​Charlotte Belhomme, Stéphanie Castex and Arnaud Haudrechy Taste and Sound..................................................................................................................................................................................569 Bruno A. Mesz Temporal Domination of Sensation: When Building Dishes, Let’s Take Temporality into Account...........................................575 Pascal Schlich Texture: The Physics of Mouthfeel –​Spreadable Food and Inulin Particle Gels.........................................................................581 Thomas A. Vilgis Texture: How Texture Makes Flavour..............................................................................................................................................585 Ole G. Mouritsen Texture: Tsukemono –​the Art and Science of Preparing Crunchy Vegetables............................................................................593 Ole G. Mouritsen Thickeners: Cellulose and Its Derivatives........................................................................................................................................599 Rachel Edwards-​Stuart 3D Printing of Food............................................................................................................................................................................605 Megan M. Ross, Róisín M. Burke and Alan L. Kelly Umami: The Molecular Science of Umami Synergy........................................................................................................................619 Ole G. Mouritsen

Part II   Educational Practices.............................................................................................................625 The Right Words for Improving Communication in Food Science, Food Technology, and between Food Science and Technology and a Broader Audience............................................................................................................................627 Hervé This vo Kientza Experimental Flavour Workshops.....................................................................................................................................................635 Hervé This vo Kientza

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Teaching Argumentation and Inquiry through Culinary Claims...................................................................................................643 Erik Fooladi Cooking and Science Workshops: The “Soft of the World”, Gelling Agents.................................................................................651 Pere Castells Culinary Sciences for the Enhancement of the Public Understanding of Science........................................................................655 Ole G. Mouritsen “Science and Cooking Activities” for Secondary School Students.................................................................................................659 Marie-​Claude Feore, Laure Fort, Marie-​Blanche Mauhourat and Hervé This vo Kientza How to Reduce Oil in French Fries: A Student Experiment...........................................................................................................663 Hervé This vo Kientza An Educational Satellite Project around the Scientific Elucidation of Culinary Precisions in Lebanon and in the Middle East...............................................................................................................................................................................665 Reine Barbar, Jean-​Marie Malbec, Christophe Lavelle and Hervé This vo Kientza Bon Appétit, Marie Curie! A Stanford University Introductory Science of Cooking Course.....................................................673 Markus W. Covert and Imanol Arrieta-​Ibarra Molecular Gastronomy in Science Education and Science Communication at the National University of Singapore.............679 Linda Sellou and Lau Shi Yun Molecular Gastronomy: A Universal Portal to the Molecular Sciences........................................................................................683 Patricia B. O’Hara Heat Transfer in the Kitchen –​Exercises.........................................................................................................................................687 Manuel Combes Ionic Diffusion in Spherified Calcium Alginate Gels: A Laboratory Experiment........................................................................689 Lorenzo Soprani, Lara Querciagrossa, Silvia Cristofaro, Luca Muccioli, Silvia Orlandi, Elena Strocchi, Alberto Arcioni and Roberto Berardi Simple Calculations Based on Cooking............................................................................................................................................703 Hervé This vo Kientza Teaching and Cooking with Culinary Teachers...............................................................................................................................717 Christophe Lavelle The Monthly INRAE-​AgroParisTech Seminars on Molecular Gastronomy................................................................................721 Hervé This vo Kientza

Part III   Culinary Applications...........................................................................................................725 New Greek Cuisine.............................................................................................................................................................................727 Georgianna Hiliadaki and Nikos Roussos 3D Printed Note by Note Recipe: Soya Lobster Prototype.............................................................................................................735 Róisín M. Burke Cooking (with) Olive Oil....................................................................................................................................................................737 Christophe Lavelle Cooking for the Elderly......................................................................................................................................................................741 Christophe Lavelle

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Culinary Constructivism and Note by Note Cooking......................................................................................................................743 Pierre Gagnaire Decantation..........................................................................................................................................................................................751 Hervé This vo Kientza Note by Note Recipes for a Press Conference and Tasting Organized at ITHQ, 2012.................................................................755 Erik Ayala-​Bribiesca and Ismael Osorio Using Liquid Nitrogen to Prepare Ice Creams in the Restaurant..................................................................................................759 Christophe Lavelle and Hervé This vo Kientza with chefs André Daguin, Noël Gutrin and Philippe Labbé A Note by Note Traditional Chinese Dinner Created and Served in Singapore...........................................................................763 Kelly Lee, Aaron Goh, Tony Choo, Nicolas Vergnole, Gn Ying Wei and Tais Berenstein Greek Diracs........................................................................................................................................................................................771 Makis Kalossakas and Nicolas Nikolakopoulos An Eclipse Dish...................................................................................................................................................................................775 Hervé This vo Kientza Modern Swiss Cooking.......................................................................................................................................................................777 Denis Martin How Do Eggs Coagulate?...................................................................................................................................................................779 Hervé This vo Kientza Vegetable Salad...................................................................................................................................................................................785 Jean Chauvel Filtration..............................................................................................................................................................................................789 Hervé This vo Kientza Waiter! There Is Garlic in My Meringue!........................................................................................................................................793 César Vega Lobster and Juniper...........................................................................................................................................................................797 David Toutain Molecular Cooking..............................................................................................................................................................................801 Róisín M. Burke and Pauline Danaher Note by Note Cooking and Note by Note Cuisine............................................................................................................................809 Hervé This vo Kientza and Róisín M. Burke Spherification......................................................................................................................................................................................819 Sasa Hasic The Raspberry Pear Viennoiserie......................................................................................................................................................825 James A. Griffin Molecular Mixology: Welcome Coffee, a Cocktail with Ten Layers..............................................................................................827 Hervé This vo Kientza and Pierre Gagnaire Cube of “Chicken-​Carrot” with Chips of “Basil-​Lemon”..............................................................................................................829 Pasquale Altomonte and Dao Nguyen Some of the Easiest Note by Note Recipes Served at Senses Restaurant.......................................................................................831 Andrea Camastra

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The Forest Floor..................................................................................................................................................................................837 Sophie Dalton A Note by Note Macaron....................................................................................................................................................................841 Julien Binz Note by Note Cooking.........................................................................................................................................................................843 Michael Pontif Note by Note Sushis............................................................................................................................................................................847 Guillaume Siegler Slowly Cooked Lamb Neck with Fermented Flour Pancakes, Sunchoke Puree and Beer Glaze................................................849 Alex Tsionitis Index.....................................................................................................................................................................................................851

About the Editors Róisín M. Burke is a senior lecturer in the School of Culinary Arts and Food Technology, College of Arts and Tourism, Technological University Dublin, City Campus, Ireland. She obtained her PhD from University College Dublin and subsequently carried out postdoctoral research at the Agricultural University in Wageningen, The Netherlands. Róisín lectures and conducts research, specializing in Culinary Science and Food Product Development. In the last 14  years, she has developed Molecular Gastronomy as a subject discipline in The School of Culinary Arts and Food Technology, TU Dublin. She has published widely in international peer-​reviewed journals and has joined a number of editorial teams. For many years, Róisín has lectured to international students, and she is the TU Dublin co-​ ordinator of the Erasmus+ MSc programme in Food Innovation and Product Design (FIPDes). She has given guest lectures in Ireland and abroad. Alan L. Kelly is a professor in the School of Food and Nutritional Sciences at University College Cork in Ireland. His teaching interests include food processing and preservation, dairy product technology and new food product development, as well as regularly giving courses on effective scientific communication. He leads an active research group on the chemistry and processing of milk and dairy products, has published over 250 research papers, review articles and book chapters, and has supervised over 40 MSc and PhD students to completion. He has been an editor of the International Dairy Journal since 2005 and has acted as an external examiner in universities and reviewed for journals and funding agencies around the world. In July 2009, he received the Danisco International Dairy Science award from the American Dairy Science Association for his contributions to research in dairy science and technology. In recent years, he has become very interested in the interface between the worlds of food and culinary sciences, and has organized several workshops and seminars on this topic and molecular gastronomy. In 2019, he published a book entitled Molecules, Microbes and Meals: The Surprising Science of Food (Oxford University Press), and in 2020, he published How Scientists Communicate: Dispatches from the Frontiers of Knowledge (Oxford University Press), both of which are aimed at a general audience.

Christophe Lavelle is a research scientist at the French National Centre for Scientific Research, working at the National Museum of Natural History and Sorbonne University in Paris. He is an expert in biophysics, epigenetics and food science and teaches in many universities and professional schools (including Sorbonne University, Le Cordon Bleu and Basque Culinary Center). He is frequently invited to conferences for general public or professional audiences, and is also responsible for the scientific training of cookery teachers at the national level. He is the author of more than 60 research papers and 12 books on food, including Toute la chimie qu’il faut savoir pour devenir un chef! (Flammarion, 2017)  and Je mange donc je suis. Petit dictionnaire curieux de l’alimentation (Museum National d’Histoire Naturelle, 2019). He is a member of several scientific and food societies (including the French Biophysical Society, the American Biophysical Society, the Disciples d’Escoffier Society and the Association for the Study of Food and Society). Hervé This vo Kientza is a physical chemist, one of the two creators of Molecular and Physical Gastronomy and a promoter of “molecular cooking” (he devised the name in 1999). He is currently working for the French National Research Institute for Agriculture, Food and the Environment, is a professor at AgroParisTech, and is the director of the AgroParisTech-​INRAE International Centre for Molecular and Physical Gastronomy. As such, he organizes the International Workshops on Molecular and Physical Gastronomy. He is also the head of the Educational Committee of the Institute for Advanced Studies in Gastronomy and a member of several academies: the French Academy of Agriculture, the Belgian Academy of Sciences, the Academy of Alsace and the Academy Stanislas. He is doing scientific research (molecular and physical gastronomy) in the Inrae-​AgroParisTech Group of Molecular Gastronomy. At the same time, he is lecturing extensively worldwide for the promotion of molecular gastronomy, stimulating the creation of groups for research and education in universities and research centres, but also of “note by note cooking”, an application of molecular gastronomy that he proposed as early as 1994. He has published many books and chapters of books, made TV programmes and radio shows, and has several blogs, but he is also the author of more than 150 scientific articles.

xiii

Contributors Sumaya M. Abdullahi Augsburg University, Department of Physics, 2211 Riverside Ave, Minneapolis, MN 55454, United States

Alberto Arcioni Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale del Risorgimento 4, Bologna I-​40136, Italy

José M. Aguilera Department of Chemical Engineering and Bioprocesses, Pontificia Universidad Católica de Chile, Avenida Libertador Bernardo O’Higgins 340, Santiago, Chile

Elke K. Arendt School of Food and Nutritional Sciences and APC Microbiome Ireland, University College Cork, T12 YN60, Ireland

Imran Ahmad Chaplin School of Hospitality and Tourism Management, Florida International University, North Miami, Florida, United States of America

Imanol Arrieta-​Ibarra Department of Management Science and Engineering, Stanford University, Stanford, CA, United States of America

Florent Allais URD Agro-​Biotechnologies Industrielles, CEBB, AgroParisTech, 51110, Pomacle, France Irem Altan Department of Chemistry, Duke University, Durham, North Carolina, United States Pasquale Altomonte The Kitchen Lab, 23 chemin de la Tour, Meyrin-​Village, Geneva, Switzerland Eric Angelini V. MANE FILS, Route de Grasse, 06620, Le Bar-​sur-​Loup, France

Jean-​Christophe Augustin Ecole Nationale Vétérinaire d’Alfort, 7 avenue du Général de Gaulle, F-​94704 Maisons-​Alfort, France Erik Ayala-​Bribiesca Food Science and Technology Consultant and Professor at Cégep de St-​Hyacinthe, Quebec, Canada Douglas Baldwin Breville Pty Ltd 19400, South Western Ave, Torrance, California 90501, United States Reine Barbar UMR IATE, Univ. Montpellier, INRAE, Institut Agro, 2 Place Pierre Viala, F-​34060 Montpellier Cedex, France and Department of Agriculture and Food Engineering, School of Engineering, Holy Spirit University of Kaslik, P.O. Box 446 Jounieh, Mount Lebanon, Lebanon xv

xvi Peter Barham H. H. Wills Physics Laboratory, University of Bristol, Bristol, BS8 1TL, United Kingdom Fabien Beaumont Laboratoire de Thermomécanique (GRESPI), Université de Reims Champagne-​Ardenne, Reims 51100, France

List of Contributors Róisín M. Burke School of Culinary Arts and Food Technology, College of Arts and Tourism, Technological University Dublin, City Campus, Dublin 1, Ireland Andrea Camastra Restaurant Senses, Bielanska 12, Warsaw 00-​085, Poland

Marie-​Charlotte Belhomme Institut de Chimie Moléculaire de Reims, UMR CNRS 7312, SFR Condorcet FR CNRS 3417, Université de Reims, BP 1039, F-​51687 REIMS Cedex, France

Pere Castells Science and Cooking World Congress, Av. de la Torre Blanca 57, Sant Cugat des Vallès, Barcelona, Spain

Konstantin Bellut School of Food and Nutritional Sciences, University College, Cork, T12 YN60, Ireland

Stéphanie Castex Institut de Chimie Moléculaire de Reims, UMR CNRS 7312, SFR Condorcet FR CNRS 3417, Université de Reims, BP 1039, F-​51687 REIMS Cedex, France

Roberto Berardi (deceased, 2020) Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna Tais Berenstein Chef at the At-​Sunrice GlobalChef Academy Julien Binz Restaurant Julien Binz, Ammerschwihr, Alsace, France Christophe Blecker Food Science and Formulation, Gembloux Agro-​Bio  Tech, University of Liège, Avenue de la faculté d’Agronomie 2B, 5030 Gembloux, Belgium Jean-​Baptiste  Boulé Genome Structure and Instability, CNRS UMR7196 /​INSERM U1154, National Museum of Natural History, Sorbonne University, Paris 75005, France

Anne Cazor 28 Tain Seng A, Scinnoy, 6 Rue d’Estienne, D’Orves, 92110 Clichy, France Patrick Charbonneau Departments of Chemistry and Physics, Duke University, Durham, North Carolina, United States Jean Chauvel Restaurant Jean Chauvel, 33 avenue du Général Leclerc, Boulogne-​Billancourt, France Véronique Cheynier SPO, INRAE, Université de Montpellier, Institut Agro-Montpellier SupAgro, 2 Place Viala, 34060 Montpellier, France

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List of Contributors Tony Choo Chef at the At-​Sunrice GlobalChef Academy, 28 Tai Seng Street, Level 5, Singapore 534106, Singapore

Daniel Cossuta Bunge Zrt., Katalin Kővári Innovation Centre, Illatos út 38. DÜP II. Building G/​3rd floor, 1097 Budapest, Hungary

Morten Christensen Taste for Life, University College Lillebælt, c/​o Kold College, 55 Landbrugsvej, DK-​5260 Odense S, Denmark

Markus W. Covert Department of Bioengineering, Stanford University, Stanford CA, United States

Clara Cilindre Equipe Effervescence, Champagne et Applications (GSMA), UMR CNRS 7331, Université de Reims Champagne-​Ardenne, Reims, France Mathias Porsmose Clausen Department of Green Technology, SDU Biotechnology, University of Southern Denmark, 55 Campusvej, DK-​5230 Odense M, Denmark Manuel Combes Teacher of Physics in Prep Class, La Pérouse-Kerichen, Brest, France and Research Associate in Laboratoire Géosciences Océan (UMR 6538), Brest, France Angela M. Coral Medina School of Microbiology, University College Cork, T12 YN60, Ireland Daniel Cordier Equipe Effervescence, Champagne et Applications (GSMA), UMR CNRS 7331, Université de Reims Champagne-​Ardenne, Reims, France

Silvia Cristofaro Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale del Risorgimento 4, Bologna I-​40136, Italy Bernard Cuq L’Institut Agro – Montpellier SupAgro, UMR IATE, 2 place Viala 34060 Montpellier Cedex, France André Daguin (deceased, 2019) Chef Sophie Dalton School of Culinary Arts and Food Technology, College of Arts and Tourism, Technological University Dublin, City Campus, Ireland Pauline Danaher School of Culinary Arts and Food Technology, City Campus, Technological University Dublin, Ireland Sabine Danthine Food Science and Formulation, Gembloux Agro-​Bio  Tech, University of Liège, Avenue de la faculté d’Agronomie 2B, 5030 Gembloux, Belgium Roxane Detry Food Science and Formulation, Gembloux Agro-​Bio  Tech, University of Liège, Avenue de la faculté d’Agronomie 2B, 5030 Gembloux, Belgium

xviii Justine de Valicourt Vestjyllands højskole, Ringkøbing, Denmark and Les Colibris –​Entreprise permacole, St-​Jean-​de-​Matha, Québec, Canada Laure Dziuba Consultant chromatographic analysis of fragrances and related compounds, 1955 chemin des vergers, 06620 Le Bar-​sur-​Loup, France Rachel Edwards-​Stuart Culinary Science Department, Westminster Kingsway College, 76 Vincent Square, London SW1P 2PD, United Kingdom Martine Esteban-​Decloux Unité Mixte de Recherche Ingénierie Procédés Aliments, AgroParisTech, INRAE, Université Paris-​Saclay, F-​91300  Massy, France Luc Eveleigh Sayfood (UMR 0782), INRAE, AgroParisTech, Université Paris-​Saclay, 91300, Massy, France Anne-​Laure  Fameau Research & Innovation, International Physical-​Chemistry Department, L’Oréal, Saint-​Ouen  93400, France Peter Lionet Faxholm Department of Food Science, Taste for Life, Design and Consumer Behavior, University of Copenhagen, 26 Rolighedsvej, DK-​1958 Frederiksberg C, Denmark

List of Contributors Laura Febvay Aerial, 250 Rue Laurent Fries, Parc d’innovation, 67412 Illkirch, France Marie-​Claude  Feore Group of Molecular Gastronomy, INRAE-​AgroParisTech, International Centre for Molecular Gastronomy, F-​75005  Paris, France and UMR 0782 SayFood, AgroParisTech (INRAE), Université Paris-​Saclay, F-​91300  Massy, France Denis Flick AgroParisTech, 16 rue Claude Bernard, Paris, France Erik Fooladi Department of Science and Mathematics, Faculty of Humanities and Education, Volda University College, P.O. Box 500, Volda, Norway Laure Fort Group of Molecular Gastronomy, INRAE-​AgroParisTech, International Centre for Molecular Gastronomy, F-​75005  Paris and UMR 0782 SayFood, AgroParisTech (INRAE), Université Paris-​Saclay, F-​91300  Massy, France Pierre Gagnaire Restaurants Pierre Gagnaire: Bordeaux, Chatelaillon, Courchevel, Danang, Dubai, Hong Kong, Las Vegas, London, Nîmes, Paris, Shanghai, Seoul, Tokyo Marta Ghebremedhin Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

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List of Contributors H.-​C.  Gill Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

Judith Hege Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

Sarah Gindra Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

Georgianna Hiliadaki Funky Gourmet Restaurant Athens Greece

Mahelet Girma Augsburg University, Department of Physics, 2211 Riverside Ave, Minneapolis, MN 55454, United States Aaron Goh Chef at the At-​Sunrice GlobalChef Academy, 28 Tai Seng Street, Level 5, Singapore 534106, Singapore James A. Griffin School of Culinary Arts and Food Technology, College of Arts and Tourism, Technological University Dublin, City Campus, Dublin 1, Ireland Noël Gutrin Retired; former Head Food & Beverage at Le Futuroscope, Chasseneuil-​du-​Poitou, France John A. Hannon (deceased, 2021) Glanbia Ireland DAC Hannah M. Hartge Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Sasa Hasic R&D Chef, Croatia Arnaud Haudrechy Institut de Chimie Moléculaire de Reims, UMR CNRS 7312, SFR Condorcet FR CNRS 3417, Université de Reims, BP 1039, F-​51687 REIMS Cedex, France

Jean-​François Hocquette Clermont University, INRAE, VetAgro Sup, UMR 1213 Herbivores, Theix, 63122 Saint-​Genès Champanelle, France Bhagyashri L. Joshi Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Makis Kalossakas Chef teacher at the Institut Le Monde, 45 Thessalonikis Str, Moschato, Athens 18346, Greece Alan L. Kelly School of Food and Nutritional Sciences, University College, Cork, T12 YN60, Ireland Markus Ketomäki Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Alain Kondjoyan INRAE, Unité Qualité des Produits Animaux, Theix, 63122 Saint-​Genès Champanelle, France Elzbieta Kozakiewicz Market Innovation and Technology Engineer, Koninklijke Bunge B.V., 221 rue de la loi, 1040 Bruxelles, Belgium

xx Philippe Labbé Former chef of La Tour d’Argent, Paris, France Marie-​Laure Lameloise UMR SayFood, AgroParisTech, INRAE, Université Paris-​Saclay, Massy 91300, France Christophe Lavelle National Museum of Natural History /​Sorbonne University, CNRS UMR7196 /​INSERM U1154, 43 rue Cuvier, Paris 75005, France and Institut National Supérieur du Professorat et de l’Education (INSPE), Toulouse University and Cergy-​Pontoise University, France Anaïs Lavoisier UMR SayFood (Paris-Saclay Food and Bioproduct Engineering Research Unit), INRAE, AgroParisTech, Université Paris-Saclay, F-91300 Massy, France Kelly Lee Chef at the At-​Sunrice GlobalChef Academy, 28 Tai Seng Street, Level 5, Singapore 534106, Singapore Sabine Leroy Université Clermont Auvergne, INRAE, MEDIS, Clermont-​Ferrand, France Martin Lersch https://khymos.org Cassandre Leverrier UMR SayFood, AgroParisTech, INRAE, Université Paris-​Saclay, Massy 91300, France

List of Contributors Gérard Liger-​Belair Equipe Effervescence, Champagne et Applications (GSMA), UMR CNRS 7331, Université de Reims Champagne-​Ardenne, Reims, France Linda A. Luck Professor of Chemistry-​Emeritus, State University of New York at Plattsburgh, Plattsburgh, 12901 New York, United States Kieran M. Lynch School of Food and Nutritional Sciences, University College, Cork, T12 YN60, Ireland Jean-​Marie  Malbec Conseiller pédagogique sur la zone MOPI, Lycée Bonaparte, Doha, Qatar Coline Martin INRAE, UMR IATE, 2 place Viala, 34060 Montpellier cedex, France Denis Martin Restaurant Denis Martin, Rue du Château 2, 1800 Vevey, Switzerland Ramon Morató Creative Director for Cacao Barry brand Marie-​Blanche Mauhourat Education nationale, Inspection générale, 110 rue de Grenelle, Paris 75007, France Behic Mert Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

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List of Contributors Bruno Mesz Universidad Nacional de Tres de Febrero (UNTREF), Instituto de Investigación en Arte y Cultura (IIAC), Sáenz Peña, Argentina Ramon Morató Creative Director for Cacao Barry brand Marie-​Hélène  Morel INRAE, UMR IATE, 2 place Viala, 34060 Montpellier cedex, France. John P. Morrissey School of Microbiology, University College Cork, T12 YN60, Ireland Ole G. Mouritsen Department of Food Science, Taste for Life, Design and Consumer Behavior, University of Copenhagen, 26 Rolighedsvej, DK-​1958 Frederiksberg C, Denmark Luca Muccioli Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale del Risorgimento 4, Bologna I-​40136, Italy Trivikram Nallamilli Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Dao Nguyen The Kitchen Lab, 23 chemin de la Tour, Meyrin-​Village, Geneva, Switzerland Nicolas Nikolakopoulos Pastry Chef at the Institut Le Monde, 45 Thessalonikis Str, Moschato, Athens 18346, Greece

Patricia B. O’Hara Amherst College, Amherst, MA 01002, United States Rolande Ollitrault Crêperie Ti Joos, Rue Delambre, 75014 Paris, France and Group of Molecular Gastronomy, INRAE-​AgroParisTech, International Centre for Molecular Gastronomy, F-​75005  Paris, France Silvia Orlandi Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale del Risorgimento 4, Bologna I-​40136, Italy Ismael Osorio Freelance chef and cook at the ITHQ, Montreal, Quebec, Canada Marie-​Paule  Pardo Crêperie Ti Joos, Rue Delambre, 75014 Paris, France, and Group of Molecular Gastronomy, INRAE-​AgroParisTech, International Centre for Molecular Gastronomy, F-​75005  Paris, France Jane K. Parker Department of Food and Nutritional Sciences, University of Reading, United Kingdom Franco Pedreschi Departamento de Ingeniería Química y Bioprocesos, Pontificia Universidad Católica de Chile, P.O. Box 306, Santiago 6904411, Chile Guillaume Polidori Laboratoire de Thermomécanique (GRESPI), Université de Reims Champagne-​Ardenne, Reims 51100, France

xxii Michael Pontif Igemusu Inc., Paris, France Alice Pontin Department of Food and Nutritional Sciences, University of Reading, United Kingdom Gabriela Precup Faculty of Food Science and Technology, Institute of Life Sciences, University of Agricultural Sciences and Veterinary Medicine Cluj-​Napoca, Calea Mănăştur  3–​5, 400372 Cluj-​Napoca, Romania Lara Querciagrossa Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale del Risorgimento 4, Bologna I-​40136, Italy Andreas Rieger Restaurant einsunternull, Hannoversche Str. 1, 10115 Berlin, Germany Guro Helgesdotter Rognså Nofima Processing Technology, Måltidets Hus, Richard Johnsens gt. 4, Po box 8034, 4068 Stavanger, Norway Yrjö H. Roos School of Food and Nutritional Sciences, University College Cork, Cork, T12 YN60, Ireland Megan M. Ross School of Food and Nutritional Sciences, University College Cork, Cork, T12 YN60, Ireland Nikos Roussos Funky Gourmet Restaurant, Athens, Greece

List of Contributors Sébastien Roustel Chr-​Hansen, Cheese application, Boge Allé  10-​12, 2970 Horsholm, Denmark Raffaele Sacchi Department of Agricultural Sciences, Unit of Food Science and Technology, University of Naples Federico II, Via Università 100, I-​80055 Portici (Napoli), Italy Christian Salles CSGA (Centre des Sciences du Goût et de l’Alimentation), AgroSup Dijon, CNRS, INRAE, Université de Bourgogne Franche-​Comté, F-​21000  Dijon, France Pascal Schlich Centre des Sciences du Goût et de l’Alimentation, AgroSup Dijon, CNRS, INRAE, Université Bourgogne Franche-Comté, F-​21000  Dijon, France and CNRS, INRAE, ChemoSens Facility, PROBE Infrastructure, F-21000 Dijon, France Charlotte Vinther Schmidt Department of Food Science, Taste for Life, Design and Consumer Behavior, University of Copenhagen, 26 Rolighedsvej, DK-​1958 Frederiksberg C, Denmark Christine Schreiber Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Linda Sellou National University of Singapore, 21 Lower Kent Ridge road, Singapore 119077, Singapore

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List of Contributors Thomas Séon Institut Jean Le Rond d’Alembert, UMR CNRS 7190, Sorbonne Universités, Paris, France

Régine Talon Université Clermont Auvergne, INRAE, MEDIS, Clermont-​Ferrand, France

Lau Shi Yun National University of Singapore, 21 Lower Kent Ridge road, Singapore 119077, Singapore

Frédéric J. Tessier Université de Lille, Inserm U1167, F-​59000  Lille, France

Weon-​Sun  Shin Laboratory of Food Chemistry & Molecular Gastronomy, Department of Food & Nutrition, Hanyang University, Republic of Korea

Hervé This vo Kientza Group of Molecular Gastronomy, INRAE-​AgroParisTech International Centre for Molecular Gastronomy, F-​75005  Paris, France and UMR 0782 SayFood, AgroParisTech (INRAE), Université Paris-​Saclay, F-​91300  Massy, France

Guillaume Siegler Chef-​teacher Le Cordon Bleu Paris, 13–​15 Quai André Citroën, Paris, France Lorenzo Soprani Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale del Risorgimento 4, Bologna I-​40136, Italy Charles Spence Head of the Crossmodal Research Laboratory, Department of Experimental Psychology, Anna Watts Building, University of Oxford, Oxford OX2 6GG, United Kingdom Benjamin L. Stottrup Augsburg University, Department of Physics, 2211 Riverside Ave, Minneapolis, MN 55454, United States Elena Strocchi Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale del Risorgimento 4, Bologna I-​40136, Italy

David Toutain Restaurant David Toutain, 29 rue Surcouf, Paris 75007, France Mark Traynor Department of Nutrition, Dietetics, and Hospitality Management, College of Human Sciences, Auburn University, Auburn, Alabama, United States Alex Tsionitis CTC Restaurant, Athens, Greece Juan Valverde Business Development and Innovation Manager, O’Reilly Institute, Trinity College Dublin, Dublin, Ireland César Vega Barry Callebaut Americas, 600 W Chicago Avenue, Chicago, IL 60654, United States

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List of Contributors

Nicolas Vergnole Chef at the At-​Sunrice GlobalChef Academy, 28 Tai Seng Street, Level 5, Singapore 534106, Singapore

Gn Ying Wei Chef at the At-​Sunrice GlobalChef Academy, 28 Tai Seng Street, Level 5, Singapore 534106, Singapore

Thomas A. Vilgis Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

Juan C. Zambrano Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

Dan-​Cristian  Vodnar Faculty of Food Science and Technology, Institute of Life Sciences, University of Agricultural Sciences and Veterinary Medicine Cluj-​Napoca, Calea Mănăştur  3–​5, 400372 Cluj-​Napoca, Romania

Birgitta I. Zielbauer Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

David S. Waldron School of Food and Nutritional Sciences, University College, Cork, T12 YN60, Ireland

Foreword One of the most exciting areas of research and experimentation in the food and culinary areas in recent years is the emerging discipline of molecular and physical gastronomy (in short, “molecular gastronomy”), the scientific discipline dedicated to the study of phenomena that occur during the preparation and consumption of dishes. Molecular gastronomy considers the chemistry, biology and physics of food, along with the physiology of food consumption. The objective of this book is to provide a comprehensive overview of this field, based on contributions from some of the main scientists in their areas, but also with contributions by chefs for the application part. In this last part, the book also addresses some of the most popular techniques of molecular cooking, a cooking style associated with molecular gastronomy and characterized mainly by the use of new tools and methods, often imported from chemistry laboratories, as well as cultural and artistic aspects of food preparation. The newer cooking trend called “note by note cuisine” is also explored. Considering that “gastronomy is the knowledge and understanding of all that relates to man, as he eats” (JA Brillat-​ Savarin, Physiology of taste, 1825), it may be proposed that molecular gastronomy is the chemical, physical and biological part of this knowledge, i.e., the scientific activity that relates to culinary phenomena. As natural sciences have technical applications, molecular gastronomy can lead to better and/​or new ways of cooking. This book aims to fulfil the need for a comprehensive reference in molecular gastronomy along with a practical guide to molecular cooking and more recent applications such as note

by note cuisine. Indeed, many books already exist for a general audience, either addressing food science in a “light” way and/​ or dealing with modern cooking techniques and recipes, but no book exists yet that encompasses the whole molecular gastronomy field, providing a strong interdisciplinary background in the physics, biology and chemistry of food and food preparation, along with discussions on creativity and the art of cooking. We hope that such a new resource will be very useful for food scientists and chefs, as well as students studying food science or food technology and all lay people interested in gastronomy. It should be noted that the chapters in each part are organized in what appeared to be the most logical structure, and are hence alphabetical in Part I and in a logical sequence in Parts II and III. The book is not intended to be read in sequence but to be used as a reference work, which can be consulted under specific headings or dipped into at will. Finally, we as editors wish to thank all the authors, coming from so many different fields, for their enthusiasm for the project and their patience with the process of editing and producing this very significant end product, and to thank Stephen Zollo and colleagues at CRC Press/​Taylor and Francis for their support for this project, guidance through the publication process, and great patience in awaiting the end result. We also thank Nicola Howcroft at Newgen Publishing UK for support during the production stages. We would also like to thank Ciara Tobin of the School of Food and Nutritional Sciences at University College Cork for significant and invaluable assistance in the preparation of this final manuscript, and Dylan Kelly for his assistance with editorial work in the process.

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Introduction to Molecular Gastronomy and Its Applications Hervé This vo Kientza, Christophe Lavelle, Róisín Burke and Alan Kelly Molecular Gastronomy was originally defined as the scientific discipline that has the goal of “looking for mechanisms of phenomena occurring during culinary processes” (Announcement poster for the International Workshop on Molecular and Physical Gastronomy, Ettore Majorana Centre for Scientific Culture, Erice, Italy, 1992). The initial name “molecular and physical gastronomy” was later shortened to “molecular gastronomy”, but the scientific programme did not change, except that it was progressively made clearer when it was increasingly recognized that technology and education were not scientific activities but rather, applications of science. In spite of this quite clear definition by its creators, “molecular gastronomy” has been widely referred to in the media and in culinary circles using different definitions and terminology. These include, for example, “Molecular Cooking/​Cuisine”, “Modernist Cuisine” and “Scientific Cooking”, to name but a few. Thus, some confusion has remained, because “gastronomy” is often confused with “haute cuisine” or “fine dining”, and also because innovative chefs were interested in the possible applications of molecular gastronomy and hence, indeed, doing molecular cooking!

Clarification of What Molecular Gastronomy Is Even after years of efforts to explain the differences, two main sources of confusion remain: 1. between molecular gastronomy, on the one hand, and food science and technology, on the other; 2. between molecular gastronomy, molecular cooking and molecular cuisine. In terms of the first of these, it should be observed that “molecular gastronomy” is certainly not intended to include all food sciences, but only part of this field. Indeed, the emergence of molecular gastronomy was due in part to the fact that, in the 1980s, food scientists did not study real culinary processes and dishes prepared daily in homes or restaurants or other culinary enterprises, but mainly industrial processes and food ingredients. There were no scientific investigations into the physical and chemical transformations that occurred during the preparation of coq au vin, Irish stew or in fact, most dishes. For example, it should be noted that cooking using wine was never considered by science, in spite of the fact that wine is used in about half of all the recipes for French traditional sauces!

Of course, it could be imagined that molecular gastronomy would disappear when its duty was done, i.e., promoting the scientific study of dishes, but at the same time, there is a strong argument for keeping it in the field of food science and technology, because this is not the same as the science of food ingredients or the sciences of processes. The work is not finished, and the field remains a source of discoveries and technical innovation through technological work, particularly because the newly created interface lies between so many disciplines. Molecular Gastronomy was first proposed as a scientific activity, but it is true that at the time when it was introduced, there was also a side goal of modernizing culinary processes, in particular with tools from labs (with a basis in chemistry and physics). The expression “molecular cooking” was introduced to describe the new culinary techniques using these tools. This led progressively to the introduction of a new culinary style, called “molecular cuisine”, based on these new tools. Two examples of this are dishes made using siphons and meat cooked at a low temperature; these are the obvious hallmark of chefs practising molecular cooking or molecular cuisine, which was, in fact, the original intention. Let us insist on the slight difference: molecular cooking is the name for cooking with “modern” culinary tools, especially tools that were not found in kitchens in the 1970s; in particular, these include tools coming from laboratories, such as siphons, thermal circulators, liquid nitrogen, ultrasonic probes and rotary evaporators. On the other hand, molecular cuisine is the style of cooking based on using such new tools, in particular for new applications. All this is well illustrated in Part III of this Handbook. Let us finish by observing that the “modern tools” of the 1980s are no longer modern today, and young chefs cannot even imagine a world without low-​temperature cooking or sous-​vide, just as chefs at the end of the 20th century could hardly have imagined how to cook when gas wasn’t widely available. It has even been said that molecular cooking and molecular cuisine are already no longer current concepts (because “note by note cooking” is a drastic modernization), but the technical transformation of culinary activity is certainly not complete. Only today have companies begun to sell ultrasonic probes for making emulsions, for example.

Science and Cooking For “science”, one should first observe that this word has various meanings, such as knowledge in general, or “natural sciences”. However, when someone discusses the issue of precision and 1

2 rigour, the general meaning of knowledge is not specified, and instead, the natural sciences are emphasized. The goal of science is to discover mechanisms and phenomena, using a method that classically goes through (1)  identification of a phenomenon; (2) quantitative characterization of the phenomenon; (3) grouping the data in equations (“laws”); (4) producing theories or models by grouping these laws and introducing new notions or concepts; (5) drawing testable theoretical conclusions from the theory; and (6) experimentally testing these theoretical conclusions. This has nothing to do with making food per se, and it is clear as well that cooking has nothing to do with the natural sciences; the goals and methods are both completely different, and they will always remain separate. In another point about the relationship between science and cooking, there has been some discussion about “science in the kitchen”, in particular in relation to a quotation by the French chefs Auguste Escoffier, Phileas Gilbert and Emile Fetu, who predicted in their famous Guide Culinaire (1907) that cooking would become “scientific”. Let’s first observe that cooking is an activity mixing a social component, a technical component and art (see Hervé This and Pierre Gagnaire’s discussion in Cooking, a quintessential art, University of California Press, 2010). Indeed, there is also an element of human relationships and sociology in the act of producing food for others or of eating food that was prepared by others. In terms of technique, it is clear that cutting, heating and mixing are all technical activities, but one important component is “art”, i.e., the activity of producing “beautiful” objects. Here, the beauty is in the food to be eaten, not only to be looked at, which leads to the conclusion that “good” means “beautiful to eat”. The fact is that there will never be any sciences “in” the kitchen. On the other hand, one can have applications of natural sciences in the kitchen (e.g., molecular cooking or note by note cooking), and also, one can use culinary phenomena for scientific objects, which is indeed the main principle of molecular gastronomy. As shown in the description of the cycle of the natural scientific method, there is no end to the process, as theories are always insufficient and can always be improved. In contrast to molecular cooking, which will end when all the “new” hardware has become old, molecular gastronomy will continue to evolve. Today, as molecular cooking is less trendy than it used to be (low-​temperature cooking and making foam with siphons are “classical” techniques nowadays), molecular gastronomy, in contrast, is developing regularly in universities around the world, with new laboratories, new research groups and new educational curricula. In this way, new aspects of molecular gastronomy are generated, and the discipline continues to evolve. It is a very exciting time … which is also why this Handbook is timely! This book is divided into three parts. The first one includes scientific information about phenomena that occur during culinary activities, and this is, as already said, exactly what molecular gastronomy is. The second and third parts then show educational and technical and culinary applications, respectively. In the case of “education”, groups all over the world have been using results from molecular gastronomy in order to promote scientific studies. Regarding technical applications, many new

Introduction to Molecular Gastronomy and Its Applications techniques have been introduced after research in molecular gastronomy, examples of which will also be given.

Applications in Schools, Colleges and Universities We have said that molecular gastronomy is a scientific activity, and this is true. As for any scientific field, there are applications in both directions: educational and technical. These applications are developing simultaneously, and even more since cooking has become more popular, with TV shows in all countries. Indeed, cooking, which was traditionally an activity for women, is now routinely done by men. One can observe that in developed countries such as France (and probably also elsewhere), in the 1950s, culinary lessons were given only to girls at school. For this country in particular, so called “Ateliers expérimentaux du goût”, i.e., activities mixing natural sciences and cooking (plus art), were introduced in all primary schools in 2001. Today, “science and cooking” activities are performed in colleges, high schools and even universities focusing on the sciences. In specialized colleges, such as culinary schools, more and more molecular gastronomy activities are being performed, and today the new French teachers for culinary schools are all being educated in molecular gastronomy with a view to experimenting in their classes, conducting science and cooking activities, or activities in which the culinary teacher is working alongside science teachers. For chefs, there are many initiatives for continuous education; for example, in France, every month for 18  years, seminars of molecular gastronomy have been run in Paris. Such seminars also exist in many other places, with various names. The general public has through organized workshops and other activities and through the media, many opportunities to enjoy the delights that applying science in the kitchen can bring. However, this is not new; as early as the very beginning of the 20th century, the microbiologist Edouard de Pomiane was a star, in particular with Radio-Cuisine, the first French programme on cooking, writing best-​sellers about what was not called molecular gastronomy. For all these educational applications, in spite of their great success, one could make the same criticism as for science popularization in general, i.e., that a discourse is given, avoiding calculations, and this is a difficulty, because in this way, the receiver of the popularized discourse cannot evaluate the validity of this information but has to trust the source. Indeed, the receiver remains at the surface, and probably the only way to circumvent this difficulty is through experimenting. This is what is proposed in many educational activities mentioned here, and, for sure, testing “culinary precisions” is a good way to invite everybody to share the excitement of science by taking the very first step and making it useful. It is no use trying to investigate phenomena that do not exist!

Applications to the Culinary Arts We have explained that the applications of science are not science itself, and it is true that the fruit is not the tree. However, it is

Introduction to Molecular Gastronomy and Its Applications also true that molecular gastronomy has had many technical applications. We also explain that molecular cooking was the name given (in 1999 only) to the use of “new” tools, but this cannot be considered as a direct application of molecular gastronomy, because no new knowledge of the mechanisms of cooking discovered by molecular gastronomy was needed for this. For example, the proposal to use tools from chemistry laboratories in the kitchen was put forward just when one of us began his studies of testing culinary precisions, well before the official creation of molecular gastronomy. However, this is not to say that nothing in molecular cooking was a result of molecular gastronomy; on the contrary. For example, the proposal of cooking eggs at 65 °C was a result of theoretical explorations of egg coagulation, and this is only one example among many. At some time before the first International Workshop on Molecular and Physical Gastronomy (1992), the lawyer and

3 gastronome Jean-​Anthelme Brillat-​Savarin commented that “the discovery of a new dish does more for the happiness of mankind than the discovery of a new star”. It was a goal, and a good one, and this is why the third part of this book is devoted to innovation in the culinary art. This is the place for molecular cooking, for molecular cuisine and for “note by note cooking”, the new way of cooking in which the ingredients are pure compounds. Also, without delay, let’s say that after about two decades of development of this “synthetic cooking” (as for synthetic music), a style begins to appear, so that “note by note cuisine” has to be discussed along with note by note cooking. Finally, all this is very exciting, because there is a wide family of initiatives all over the world, and this Handbook intends to show some of them. The book cannot be a comprehensive description, because there are too many things to cover, but it should give the reader a good idea of what has been done in recent decades. Celebrate Knowledge! Celebrate Enlightened Gourmandise!

Part I

Scientific Foundations

Acids in Foods and Perception of Sourness Christian Salles CSGA (Centre des Sciences du Goût et de l’Alimentation), AgroSup Dijon, CNRS, INRAE, Université de Bourgogne Franche-​Comté, F-​21000 Dijon, France

Introduction Sourness is mainly caused by the detection of protons. This perception may act as a warning signalling the presence of a high concentration of acid dangerous for the body, of unripe fruits, or of spoiled foods contaminated by microorganisms. At low concentrations, acidic stimuli evoke a sour taste, while at high concentrations, the trigeminal lingual system is activated, leading to a sensation of irritation. A low level of sour taste is attractive to humans and animals in some foods such as citruses, while it is aversive when present at a high level. Numerous organic and mineral acids are responsible for sourness. Citric acid, malic acid, and tartaric acid are generally found in vegetal products (e.g., fruits and vegetables), while lactic acid is found in animal products (e.g., dairy products and meat). Compounds other than organic and mineral acids can also be responsible for a sour note; amino acids with a lateral acid function, such as L-​glutamic and L-​aspartic acids, have an acid taste, while their sodium salts are responsible for umami taste (Kato et  al., 1989). Peptides with a sour taste generally contain acidic amino acids such as aspartic and glutamic acid. A dipeptide with a sour taste contains at least one acidic amino acid linked to another acidic, neutral, or aromatic amino acid (Kirimura et al., 1969). In this chapter, I  will give a non-​exhaustive overview of the main characteristics of sour compounds from a physico-​chemical and sensory point of view.

Influence of Food Matrix Composition on Sourness The release of acids in the mouth during eating and the perception of sourness may be influenced by the food matrix composition and structure. For example, fat, acid, and salt were found to influence the temporal perception of firmness, saltiness, and sourness of cheese analogues (Stampanoni and Noble, 1991). Increasing salt and acid contents were both found to increase the perception of firmness, and a higher fat content resulted in softer but sourer cheese analogues. Whatever the fat level, analogue cheeses with high acid and high salt levels had higher sourness intensity and the longest sourness perception. The time to reach

maximum saltiness and sourness intensity was shortest for low-​ salt and low-​acid analogue cheeses. The kinetic release of non-volatile compounds (leucine, phenylalanine, glutamic acid, citric acid, lactic acid, propanoic acid, sodium, potassium, calcium, magnesium, chloride, and phosphates) during the eating of a model cheese and the relationships to some oral (salivary and masticatory) parameters have also been studied (Pionnier et  al., 2004a). The maximum concentration in saliva varied significantly according to the compound. However, there was no significant effect of the compound on the time to reach maximum concentration. A long time to reach maximum concentration and total quantity of released compounds could be related to high chewing time and low saliva flow rates, low chewing rates, low masticatory performances, and low swallowing rates. The time to reach the maximal intensity of the sour attribute was positively related to the time to reach the maximal concentration of citric acid and to oral parameters (Pionnier et al., 2004b). Significant differences among several Camembert cheeses concerning bitterness and saltiness were reported, but no difference was observed for sourness (Engel et  al., 2001a). During ripening, a decrease of the perceived sour note was observed in all portions of the cheese (rind, under-​rind, and centre). The sour note in food products may be partially due to H3O+ concentration, which progressively varies with the consumption of lactic acid by microorganisms. However, pH and sourness may not be correlated, and H3O+ concentration was not sufficient to fully explain the sour taste; other chemical species, such as sodium chloride, may act on this taste characteristic. The effect of pH and interactions between sourness and other perceptions are detailed later. The evolution of sourness in the cheese portions may be explained by the migration phenomena of taste-​active compounds such as molecules that change the pH (acids, and phosphates in particular) or those that are responsible for some enhancing effect (Engel et al., 2001c). The fatty acid residue compositions of oil species affect taste perception. For oil-​in-​water emulsions with basic taste substances and oil species, Koriyama et al. (2002) reported that the type of oil extended retention of perception, and did not affect sweetness or saltiness, but suppressed sourness and bitterness. The degree of sourness was dependent on oil species, in particular the fatty 7

8 acid residue composition. Thus, oils to be added to food should be carefully selected by the manufacturer because of their effects on taste perception as well as on texture and aroma (retronasal odour). An effect of viscosity on perceived intensity of sour taste was also reported (Sediva et  al., 2004). The sourness intensity decreases with increasing solution viscosity. However, the differences depend on the concentration of thickener agents and the acid used.

Mechanisms Leading to Sour Perception The Acid Receptor The molecular mechanisms of detection of sourness are not well known yet. A wide range of cell types, receptors, and even receptor-​independent mechanisms have been proposed to mediate acid detection in the tongue. It has been proposed that sour taste is mediated by a unique cell type, independent of all other taste qualities (Huang et al., 2006). In the tongue, polycystic kidney disease 2-​ like 1 (PKD2L1) ion channel, coexpressed with PKD1L3, has been demonstrated to be necessary for the detection of sour compounds. Therefore, sour, salty, sweet, bitter, and umami tastes are each mediated by a unique cell type, independent of all other taste qualities. Salty and sour tastes directly activate ion channels while bitter, sweet, and umami tastes are elicited by G-​ protein-​ coupled receptors (Briand and Salles, 2016). Recently, it has been shown that the Otop gene family encodes a family of ion channels that are unrelated structurally to previously identified ion channels and are highly selective for protons (Tu et al., 2018). Particular proteins were reported to be able to convert sourness to sweetness. Miraculin, a tetramer and dimer of a 25  kDa protein, was found to transform a sour taste into a sweet taste (Kurihara and Beidler, 1968). Both the tetramer and the dimer have taste-​modifying properties. Miraculin has been estimated to be as much as 400,000 times sweeter than sucrose on a molar basis. The mechanisms involved in the sweetness of miraculin are unique; at neutral pH, miraculin interacts with the sweet taste receptor T1R2/​T1R3, but it does not activate the receptor and partially suppresses the response to other sweeteners. At acidic pH, miraculin changes its conformation and activates the sweet taste receptor (Briand and Salles, 2016). Another protein named neoculin (previously named curculin) elicits a sweet taste and also exhibits sweet taste-​modifying properties able to convert sourness to sweetness. In addition, it is noteworthy that peptides generated from pork loin during cooking were capable of strongly suppressing sourness (Okumura et  al., 2004). The proposed mechanism of sourness suppression by the peptide was an inhibition of the binding of sour taste substances at the proton-​ sensitive ion channel level.

The Role of Saliva in Sourness Perception Sourness is related to the proton concentration and thus pH, as sourness intensity decreases with increasing pH. However, pH does not fully explain the intensity of acid solutions. It has been

Christian Salles shown that sourness perception is fully dependent on titratable acidity: Norris et  al. (1984) showed that binary acid mixtures of equal pH and titratable acid differed significantly in sourness intensity and in saliva-​inducing capacity. They observed significant differences in both maximum intensity of perceived sourness and parotid flow as a function of the specific anionic composition (i.e., citric, tartaric, or fumaric) of the samples, since they had equal potential (titratable) hydrogens and equal free hydrogens (pH). For example, in tartaric–​fumaric acid mixtures varying in pH (3.0–​3.75) at a constant titratable acidity and varying in titratable acidity at a constant pH, sourness intensity and salivary flow rate increased with acidity and decreased with pH. A large contrast between subjects with high and low salivary flow rate and perceived intensity of sourness was also reported (Norris et al., 1984). In response to variation in pH and total acid, the high-​flow subjects demonstrated marked alteration in flow but little change in sourness perception, while low-​flow subjects responded at a lower absolute level of acids but showed marked changes in sourness and little change in salivary flow rate. Another study reported that changes in solution pH were related to individual salivary flow rates. Greater increases in expectorated solution pH were observed for individuals with higher flow rates (Christensen et  al., 1987). Moreover, on measuring taste threshold and suprathreshold responses to different volumes of acids, those authors demonstrated that individuals with high salivary flow rates were less sensitive to the taste of acids and that large volumes of acid were more easily perceived. It was suggested that dilution by saliva with different pH is not the correct mechanism and also that adaptation of taste receptors to differing concentrations of free hydrogen ions was unlikely, because the sour threshold results were opposite to those predicted by an adaptation model. This was interpreted as greater flow rate adding a greater quantity of saliva to taste solutions and consequently adding a greater total amount of salivary buffers as bicarbonate concentration increased (Christensen et al., 1987). Comparing low-​ flow judges and high-​ flow judges, it was found that salivary flow rate did not affect temporal responses for sourness, sweetness, or fruity flavour (Bonnans and Noble, 1995). Large differences in sweetness and small, insignificant differences in sourness were produced at a constant acid concentration by increasing sweetener levels. However, salivary flow rate was significantly correlated only with sourness ratings. Changes in perceived sourness intensity influenced salivary flow independently of acid concentration, suggesting that salivary response is not only due to stimuli concentration and may be mediated by the cognitively processed taste response (Bonnans and Noble, 1995). Saliva has been suggested to act as a buffering system affecting the way we perceive the sourness of acids. Moreover, Bonnans and Noble (1995) observed that the maximum intensity of sourness and salivary flow rate decreased as the level of sweeteners was raised at constant acid concentration. This suggested that salivary flow rate is mediated by cognitively processed taste response and not only the concentration of stimuli. Salivary flow rate was found to increase when the pH of different beverages decreased (Guinard et al., 1998).

9

Acids: Perception of Sourness The chemical properties of the sour molecules, such as pKa, number of carboxyl groups, hydrophobicity, and salivary flow rate, influence sour temporal perception. High-​ flow subjects and low-​flow subjects were submitted to a continuous stimulus delivery flow rate of acid solutions, and the time–​intensity perception and the pH on the tongue surface were continuously measured (Lugaz et al., 2005). The results showed that the saliva of high-​flow subjects decreased the acidity of the acid solution more efficiently than the saliva of low-​flow subjects. However, high-​flow subjects exhibited higher perceived intensity for acid solutions than low-​flow subjects. The saliva of high-​flow subjects can modify the pH of an acid solution more efficiently than the saliva of low-​flow subjects, thanks either to a dilution effect or to a difference in buffering capacity of saliva between high-​flow subjects and low-​ flow subjects. Moreover, by comparing the effect of different weak acids, the authors reported that titratable acidity rather than pH has to be considered to explain sour perception. They reported also that the difference in sensitivity between high-​ flow subjects and low-​ flow subjects might be due to a higher permeability of epithelial tissue to hydrophobic molecules (Lugaz et  al., 2005). Acid stimuli also influence the composition of saliva. Annexin A1 and calgranulin A, anti-​ inflammatory compounds involved in inflammation, were over-​ represented after umami, bitter, and sour stimulations (Neyraud et al., 2006).

Interactions of Sourness Perception with Other Perceptions In mixtures, the various tastes are known to interact through binary taste–​taste interactions (Breslin, 1996; Keast and Breslin, 2002; Figure  1.1). The physiological mechanisms involved in these interactions, while not well understood yet, occur at the taste receptor cell level. However, the integration of taste signal at the central level cannot be excluded. Concerning sourness, it was reported that sour and salty mixtures symmetrically affect the intensity of each other, with enhancement at low concentrations and suppression or no effect at high concentrations. At low concentration, sourness had variable effects on sweetness, while at higher concentrations of mixed of sour and sweet compounds, sweetness and sourness were mutually suppressed. Mixtures of sour and bitter compounds enhanced each other at low concentration; at moderate intensity, bitterness was suppressed and sourness enhanced, while at high concentration, sourness was suppressed, and the effect on bitterness was variable (Keast and Breslin, 2002). These taste–​taste interactions are functional in complex media such as real foods. For example, in goat cheeses, sourness is mainly due to a synergistic effect involving sodium chloride, phosphates, and lactic acid (Engel et al., 2000). Omission tests using model goat cheese extracts showed that the sourness of lactic acid was enhanced by sodium chloride but lowered by the presence of phosphates. The enhancement of sourness due to lactic acid by sodium chloride was also reported in Camembert cheese (Engel et al., 2001b). In tomato juices, the omission of all the sugars led to an almost total disappearance of sweetness and

FIGURE 1.1  Binary taste interactions involving sourness perception: Low (a), medium (b), high (c) intensity/​concentration. Enhanced (+); suppressed (-​); variable (v), nil effect (0). (Adapted from Keast and Breslin, 2002)

a significant increase in sourness and saltiness, which had been partly masked by the sugars (Salles et al., 2003). Thus, sourness is mainly due to citric and malic acids but modulated by sugars. Such interactions between sourness and sweetness perceptions were also reported in champagne (Martin, 2002). Interaction between sourness and saltiness was also reported in rice vinegar. Both detection and recognition sourness thresholds were decreased when vinegar ingredients (salt) were added, inducing sourness (Hatae et al., 2009). These authors studied the interaction of saltiness and sourness at the threshold level. They measured the detection and recognition thresholds for salt solution to which vinegar of subthreshold concentration was added, and for vinegar solutions to which salt of subthreshold concentration was added. When vinegar at a half concentration of the detection threshold of each panellist was added to the salt solution, both the detection and the recognition threshold of salt were reduced significantly. This phenomenon was more pronounced with rice black vinegar than with rice vinegar. In contrast, when

10 NaCl solution was added to vinegar at half of the concentration level of the detection threshold for each panellist, no intensifying or weakening effect on the detection and recognition threshold was observed. Thus, this saltiness–​ sourness interaction was found to be asymmetrical, as no effect of sourness was observed on saltiness with the same model. Moreover, this result shows the potential of vinegar to partially substitute salt to give a satisfactory salty taste in dishes. Aroma was found to interact with sourness perception through crossmodal perceptive interactions. It has been shown that no physicochemical mechanism was involved in the odour–​taste interactions (Pfeiffer et al., 2006). For example, the sourness of a citric acid solution can be lowered by caramel aroma (Stevenson et al., 1999), while an increase of the fruitiness of a water solution flavoured with orange was observed when sourness and sweetness were perceived (Bonnans and Noble, 1993). The influence of the concentration of sucrose and citric acid on the flavour of drinks and sherbets containing orange and lemon aromas was studied (Stampanoni, 1993); there was a positive effect of citric acid and sucrose on all the rated descriptors, fruitiness, freshness, juiciness, and global impact, but not on peely note. In the case of orange sherbets, sourness was correlated with freshness and peeliness, while in the case of lemon sherbets, sourness was correlated with freshness and juiciness. Citric acid and sucrose were able to contribute to retronasal aroma intensity of a citral solution for some panellists (Kuo et al., 1993). Lemon and strawberry odour were found to enhance the sourness of citric acid (Frank, 2003).

Acceptability of Sourness Sour taste is related to the presence of organic acids, but the association between nutrient content and taste varies among foods that have diverse taste profiles (Liem and Russell, 2019). Among these, nutrient-​rich foods such as fruits were mostly classified as sweet/​sour in different studies. There are several biological underpinnings to the sense of taste. At birth, humans can already distinguish between sweet, sour, and bitter tastes, as shown by distinct facial expressions of newborns when exposed to tasting substances. As judged by facial expression, their sucking responses to and to some extent, intake of these tastants follow the positive (e.g., sweet taste) or negative (e.g., bitter and to some extent, sour taste) facial expressions (Steiner et al., 2001). Strong sour and bitter solutions are both disliked by newborns, but the facial response to sour solutions (e.g., lip pursing) is remarkably different from the facial response to bitter solutions (Desor et  al., 1975; Steiner et  al., 2001). The combination of lip pursing and sucking, seen typically in response to tasting sour substances, contributes to stimulation of salivary flow in the oral cavity. In adults, it has been suggested that the increased flow and buffering capacity of saliva neutralize sour-​tasting substances, as described previously. Infants’ ingestive responses to sour taste do not indicate a clear rejection; no difference in the infants’ ingestion has been observed in response to water and water with added citric acid (Cowart et al., 1990). Some infants and young children showing

Christian Salles a preference for high concentrations of citric acid in a sugar solution were found to be more likely to consume fruit than others (Blossfeld et  al., 2007; Liem et  al., 2006; Liem and Mennella, 2003). This suggests that sour taste preferences directly influence food consumption. Thus, dietary learning and experience are important for the acceptance and intake of sour fruits (Liem and Mennella, 2002). Children fed on formula containing sour-​ tasting protein hydrolysates tended to have higher acceptance and intake of sour foods and higher levels of citrate in orange juice (Liem and Mennella, 2002). Similarly, children tend to have a higher preference for sourness during childhood (Liem and Mennella, 2003). A perception study of sourness and food behavioural data for apple juice and fruit drinks with different dry matter levels showed that children on average had a high preference for versions of beverages perceived as less sour (Kildegaard et al., 2011); however, a minor segment of children with high liking and wanting for the apple juice perceived as most sour was observed. To date, the application of sourness intensity has been little studied as a stimulus in adults in relation to its impact on food intake or the onset of satiation. Moreover, similarly to bitterness, many sour foods tend to have low energy density and are unlikely to contribute significantly to energy intake (Forde, 2016). It has been reported that sourness of citric, malic, tartaric, and acetic acid solutions could be related to an interaction of the titratable acidity and the pH of the solution. Sourness also influenced the overall acceptability of imitation fruit beverages prepared with these acids (Coseteng et  al., 1989). Pre-​stirred flavoured commercial yogurts were evaluated for sweetness and sourness rather than for overall liking. The results showed that overall consumer liking was significantly correlated with sweetness intensity, sweetness–​sourness ratio, and the cumulative impact of sweetness and sourness for strawberry and raspberry yogurt. Generally, it was found that the sweeter the yogurt, the higher the acceptance of these fruit-​flavoured yogurts by consumers (Barnes et al., 1991).

Conclusion The sour note found in many foods is caused by the presence of protons and by substances that lead to their production by hydrolysis. The sour compounds are, in particular, organic acids, the structure, size, polarity, or pKa of which influence the intensity of the sour note. There is a great variety of these, because they come from the major metabolic pathways; however, some of them are predominant in certain foods. Due to their physicochemical properties, they are sensitive to the buffering capacity of saliva. This acid perception is modulated by the presence of other stimuli responsible for bitterness, sweetness, and saltiness. For example, in cheeses, the perception of the acid note can be influenced by the presence of sodium chloride. The perception of particular aromas can also modulate acid perception through perceptual interactions. Overall, there is less work on sour perception than on bitter, salty, and sweet perceptions. Indeed, the masking of bitterness and the strengthening of saltiness and sweetness are of definite economic interest, either to

Acids: Perception of Sourness improve the taste quality of food, drink, or medicines, in the case of bitterness, or for public health reasons, in the case of saltiness and sweetness. However, perception of sourness is important in some cases, as it brings a certain balance with other perceptions, thus impacting appreciation by consumers. This sour perception is appreciated in certain contexts but is often rejected if it is too intense. The mechanisms at play in the perception of acid at the peripheral level are still little known; they deserve deeper study to better control this perception.

REFERENCES Barnes DL, Harper SJ, Bodyfelt FW, McDaniel MR. 1991. Prediction of consumer acceptability of yogurt by sensory and analytical measures of sweetness and sourness. Journal of Dairy Science, 74, 3746–​3754. Blossfeld I, Collins A, Boland S, Baixauli R, Kiely M, Delahunty C. 2007. Relationships between acceptance of sour taste and fruit intakes in 18-​month-​old infants. British Journal of Nutrition, 98, 1084–​1091. Bonnans S, Noble AC. 1993. Effect of sweetener type and of sweetener and acid levels on temporal perception of sweetness, sourness and fruitiness. Chemical Senses, 18, 278–​283. Bonnans SR, Noble AC. 1995. Interaction of salivary flow with temporal perception of sweetness, sourness, and fruitiness. Physiology & Behavior, 57, 569–​574. Breslin PAS. 1996. Interactions among salty, sour and bitter compounds. Trends in Food Science and Technology, 7, 390–​399. Briand L, Salles C. 2016. Taste perception and integration. In: Etievant P, Guichard E, Salles C, Voilley A (eds.), Flavour: From food to behaviors, wellbeing and health. Elsevier Ltd, Duxford, UK, 101–​119. Christensen CM, Brand JG, Malamud D. 1987. Salivary changes in solution pH –​a source of individual differences in sour taste perception. Physiology & Behavior, 40, 221–​227. Coseteng MY, Mclellan MR, Downing DL. 1989. Influence of titratable acidity and pH on intensity of sourness of citric, malic, tartaric, lactic and acetic acids solutions and on the overall acceptability of imitation apple juice. Canadian Institute of Food Science and Technology Journal-​Journal de L’Institut Canadien de Science et Technologie Alimentaires, 22,  46–​51. Cowart B, Beauchamp G, Mcbride R, Macfie H. 1990. Early development of taste perception. In: McBride R and MacFie H (eds.), Psychological Basis of Sensory Evaluation. Elsevier Applied Science, London, New York. Desor JA, Maller O, Andrews K. 1975. Ingestive responses of human newborns to salty, sour, and bitter stimuli. Journal of Comparative and Physiological Psychology, 89, 966–​970. Engel E, Nicklaus S, Septier C, Salles C, Le Quéré JL. 2000. Taste active compounds in a goat cheese water-​ soluble extract. 2.  Determination of the relative impact of water-​ soluble extract components on its taste using omission tests. Journal of Agricultural and Food Chemistry, 48, 4260–​4267. Engel E, Nicklaus S, Septier C, Salles C, Le Quéré JL. 2001a. Evolution of the taste of a bitter Camembert cheese during ripening: Characterization of a matrix effect. Journal of Agricultural and Food Chemistry, 49, 2930–​2939. Engel E, Septier C, Leconte N, Salles C, Le Quéré JL. 2001b. Determination of taste-​active compounds of a bitter Camembert cheese by omission tests. Journal of Dairy Research, 68, 675–​688. Engel E, Tournier C, Salles C, Le Quéré JL. 2001c. Evolution of the composition of a selected bitter Camembert cheese during ripening: Release and migration of taste-​active compounds. Journal of Agricultural and Food Chemistry, 49, 2940–​2947.

11 Forde CG. 2016. Flavour perception and satiation. In: Etievant P, Guichard E, Salles C, Voilley A. (eds.), Flavour: From Food to Behaviors, Wellbeing and Health. Elsevier Ltd, Duxford, UK, 251–​276. Frank RA. 2003. Response context affects judgments of flavour components in foods and beverages. Food Quality and Preference, 14, 139–​145. Guinard JX, Zoumas-​Morse C, Walchak C. 1998. Relation between parotid saliva flow and composition and the perception of gustatory and trigeminal stimuli in foods. Physiology & Behaviour, 63, 109–​118. Hatae K, Takeutchi F, Sakamoto M, Ogasawara Y, Akano H. 2009. Saltiness and acidity: Detection and recognition thresholds and their interaction near the threshold. Journal of Food Science, 74, S147–​S153. Huang AL, Chen X, Hoon MA, Chandrashekar J, Guo W, Trankner D, Ryba NJP, Zuker CS. 2006. The cells and logic for mammalian sour taste detection. Nature, 442, 934–​938. Kato H, Rhue MR, Nishimura T. 1989. Role of free amino acids and peptides in food taste. In: Teranishi R, Buttery RG, Shahidi F (eds.), Flavour Chemistry Trends and Developments. American Chemical Society, Washington, DC, 158–​175. Keast RSJ, Breslin PAS. 2002. An overview of binary taste-​taste interactions. Food Quality and Preference, 14, 111–​124. Kildegaard H, Tonning E, Thybo AK. 2011. Preference, liking and wanting for beverages in children aged 9–​14  years: Role of sourness perception, chemical composition and background variables. Food Quality and Preference, 22, 620–​627. Kirimura J, Shimizu A, Kimizuka A, Ninomiya T, Katsuya N. 1969. Contribution of peptides and amino acids to taste of foodstuffs. Journal of Agricultural and Food Chemistry, 17, 689–​695. Koriyama T, Wongso S, Watanabe K, Abe H. 2002. Fatty acid compositions of oil species affect the 5 basic taste perceptions. Journal of Food Science, 67, 868–​873. Kuo YL, Pangborn RM, Noble AC. 1993. Temporal patterns of nasal, oral, and retronasal perception of citral and vanillin and interaction of these odourants with selected tastants. International Journal of Food Science and Technology, 28, 127–​137. Kurihara K, Beidler LM. 1968. Taste-​modifying protein from miracle fruit. Science, 161, 1241–​1243. Liem DG, Bogers RP, Dagnelie PC, De Graaf C. 2006. Fruit consumption of boys (8–​11 years) is related to preferences for sour taste. Appetite, 46,  93–​96. Liem DG, Mennella JA. 2002. Sweet and sour preferences during childhood: Role of early experiences. Developmental Psychobiology, 41, 388–​395. Liem DG, Mennella JA. 2003. Heightened sour preferences during childhood. Chemical Senses, 28, 173–​180. Liem DG, Russell CG. 2019. The influence of taste liking on the consumption of nutrient rich and nutrient poor foods. Frontiers in Nutrition, 6, 174. Lugaz O, Pillias AM, Boireau-​Ducept N, Faurion, A. 2005. Time-​ intensity evaluation of acid taste in subjects with saliva high flow and low flow rates for acids of various chemical properties. Chemical Senses, 30, 89–​103. Martin N. 2002. Sweet/​sour balance in champagne wine and dependence on taste/​odour interactions. Food Quality and Preference, 13, 295–​305. Neyraud E, Sayd T, Morzel M, Dransfield E. 2006. Proteomic analysis of human whole and parotid salivas following stimulation by different tastes. Journal of Proteome Research, 5, 2474–​2480. Norris MB, Noble AC, Pangborn RM. 1984. Human-​saliva and taste responses to acids varying in anions, titratable acidity, and pH. Physiology & Behavior, 32, 237–​244.

12 Okumura T, Yamada R, Nishimura T. 2004. Sourness-​suppressing peptides in cooked pork loins. Bioscience, Biotechnology, and Biochemistry, 68, 1657–​1662. Pfeiffer JC, Hort J, Hollowood TA, Taylor AJ. 2006. Taste-​aroma interactions in a ternary system: A model of fruitiness perception in sucrose/​acid solutions. Perception & Psychophysics, 68, 216–​227. Pionnier E, Chabanet C, Mioche L, Taylor AJ, Le Queré JL, Salles C. 2004a. In vivo nonvolatile release during eating of a model cheese: Relationships with oral parameters. Journal of Agricultural and Food Chemistry, 52, 565–​571. Pionnier E, Nicklaus S, Chabanet C, Mioche L, Taylor AJ, Le Quéré JL, Salles C. 2004b. Flavour perception of a model cheese: Relationships with oral and physico-​ chemical parameters. Food Quality and Preference, 15, 843–​852. Salles C, Nicklaus S, Septier C. 2003. Determination and gustatory properties of taste-​active compounds in tomato juice. Food Chemistry, 81, 395–​402. Sediva A, Panovska ZK, Pokorny J. 2004. Effect of viscosity on the perceived intensity of acid taste. Czech Journal of Food Sciences, 22, 143–​149.

Christian Salles Stampanoni CR. 1993. The “quantitative flavour profiling” technique. Perfumer & Flavourist, 18,  19–​24. Stampanoni CR, Noble AC. 1991. The influence of fat, acid, and salt on the perception of selected taste and texture attributes of cheese analogs: A scalar study. Journal of Texture Studies, 22, 367–​380. Steiner JE, Glaser D, Hawilo ME, Berridge KC. 2001. Comparative expression of hedonic impact: Affective reactions to taste by human infants and other primates. Neuroscience and Biobehavioral Reviews, 25,  53–​74. Stevenson RJ, Prescott J, Boakes RA. 1999. Confusing tastes and smell: How odours can influence the perception of sweet and sour tastes. Chemical Senses, 24, 627–​635. Tu YH, Cooper AJ, Teng B, Chang RB, Artiga DJ, Turner HN, Mulhall EM, Ye W, Smith AD, Liman ER. 2018. An evolutionarily conserved gene family encodes proton-​selective ion channels. Science, 359, 1047–​1050.

Anthocyanins in Food Véronique Cheynier SPO, INRAE, University of Montpellier, Institut Agro-Montpellier SupAgro, 2, Place Viala, 34060 Montpellier, France

Anthocyanins are a major group of natural pigments that confer red, purple, blue, and even black colour on plants. These compounds, belonging to the polyphenol family, are essential components of the human diet, contributing to the attractiveness of fruits, vegetables, and beverages such as wine, and to the health benefits associated with the consumption of plant-​derived products. Anthocyanins are also widely exploited by the food industry as natural colourants and putative bioactives. Anthocyanins are found in numerous fruits (Wu and Prior, 2005a) as well as in some vegetables, nuts (Wu and Prior, 2005b), and cereals (Wu and Prior, 2005b; Zhu, 2018). Most anthocyanins in foods are based on six aglycones called anthocyanidins, usually glycosylated in the 3-​position (Figure 2.1). Known exceptions are 3-​deoxyanthocyanidins (e.g., luteolinidin and apigeninidin), found in some plant sources such as sorghum, which lack the hydroxyl group at the C-​3 position (Wu and Prior 2005b), and cyanidin 4′-​ O-​ glucoside derivatives, identified in red onions (Fossen et al., 2003). The large anthocyanin diversity is due to variations in the nature of the anthocyanidin skeleton, i.e., number and positions of hydroxyl and methoxyl groups (Figure  2.1) and in the glycosylation and acylation patterns, i.e., nature, number, and positions of sugar and acyl substituents (Andersen and Jordheim, 2006). Conjugation with sugars and acids can result in very complex structures, e.g., in red cabbage (Wu and Prior, 2005b). Some examples of anthocyanin structures are presented in Figure 2.1. Moreover, further complexity and diversity result from anthocyanin reactions taking place during processing, cooking, and storage of plant-​derived foods and beverages.

Colour Versatility of Anthocyanins Anthocyanins are usually described as red flavylium cations. The colour of these pigments shifts from orange to purple as the number of phenolic hydroxyl groups increases (from pelargonidin to delphinidin). Their stability is increased by methylation of the phenolic groups, by the lack or glycosylation of the hydroxyl group at the C-​3 position, and by acylation of the sugar, especially with phenolic acids. However, the flavylium cation form is prevalent only under highly acidic conditions. When dissolved in water, it readily

converts to other forms through hydration and proton transfer reactions (Figure  2.2) (Trouillas et  al., 2016). Thus, as the pH increases, flavylium cations are deprotonated to blue quinonoidal bases or hydrated to colourless water adducts (called hemiketals or hemiacetals). The 2-​water adduct (the most abundant) is in fast equilibrium with its open chain isomer (cis-​chalcone, CE), which is in slow equilibrium with the trans-​isomer (Trouillas et al., 2016). These equilibria are characterized by thermodynamic constants, K (hydration constant), relating the flavylium cation and the mixture of hydrated forms (hemiketals and chalcones), and K (deprotonation constant), relating the flavylium cation with the mixture of tautomeric neutral quinonoidal bases, which can be determined by spectrophotometry. Most common anthocyanins (except highly acylated ones) have pKh values (i.e., the pH value at which the anthocyanin is evenly distributed between the flavylium and hydrated forms) in the range 2–​3, meaning that they are present mostly in colourless hydrated forms in mildly acidic solutions such as fruit juices and wine, unless some pigment-​stabilizing mechanism is involved. Like water, sulfite reversibly adds onto the flavylium cation to form a colourless product, which is converted back to the flavylium under acidic conditions. This explains why fumigation with sulphur dioxide, used to control post-​harvest diseases and prevent browning on fruits such as litchi, results in significant bleaching.

Colour Stabilization through Interactions Colour stabilization is provided by molecular interactions of the anthocyanin chromophore with other compounds, referred to as copigments. A copigment can be an identical anthocyanin molecule (self-​ association), one of its aromatic acyl group substituents (intramolecular copigmentation), or another molecule (intermolecular copigmentation). The major driving force is π–π  stacking between the planar pigmented forms (flavylium cation or quinonoidal base) and another planar molecule, forming complexes from which water is excluded (Brouillard et  al., 1989; Trouillas et  al., 2016). Under conditions where hydrated forms predominate, these interactions result in displacement of the hydration equilibrium toward the pigmented forms and hence in colour enhancement (hyperchromic effect) and often

13

14

Véronique Cheynier

FIGURE 2.1  Chemical structure of anthocyanidins (left) and examples of anthocyanins (right); malvidin 3-​glucoside and cyanidin 3-​(sinapoyl)diglucoside 5-​glucoside are particularly abundant in grape and red cabbage, respectively.

a small bathochromic shift (shift toward higher wavelengths, from red to purple). Conversely, addition of copigments has little effect on anthocyanins which are present in foods mostly in pigmented forms such as 3-​deoxyanthocyanins (Awika, 2008), or anthocyanins acylated with phenolic acids, which are already stabilized by intramolecular copigmentation (Yoshida et  al., 1991). Moreover, metal ions bind with anthocyanins, showing ortho di-​or tri-​hydroxy substitution, and may contribute to stabilization of copigmentation complexes and the expression of blue colours (Trouillas et al., 2016).

Anthocyanin Reactions Anthocyanins are highly reactive, yielding numerous products with extremely diverse colour properties. Discolouration and browning can be observed following bruising or cutting of plant tissues containing anthocyanins, due to enzymatic reactions (e.g., oxidation, catalysed by polyphenoloxidases and peroxidases, or loss of sugar and acyl substituents, catalysed by glycosidases and hydrolases, respectively). Anthocyanin reactions can also occur chemically, especially during heating under acidic conditions or as a result of oxygen exposure during food preparation and storage. Anthocyanidins may also be generated by acid-​catalysed cleavage of condensed tannins (syn. proanthocyanidins), another major group of phenolic compounds, resulting in the appearance of red colouration. Anthocyanin reactions have been particularly explored in wine, as they are responsible for the colour changes from red to purple

and then to tawny observed during ageing of red and rosé wines. Several reaction mechanisms involving other food components such as flavanols (i.e., catechins and proanthocyanidins, also called condensed tannins), hydroxycinnamic acids, or aldehydes (e.g., acetaldehyde, which can form in planta, during fermentation, or through oxidation of ethanol, furfural, and hydroxymethylfurfural (HMF) formed from sugars during heating) have been established in this context and later shown to occur in fruits and other food products. Anthocyanins can react both as electrophiles (i.e., able to react with electron-​ rich species), because of the positive charge of the flavylium cation, and as nucleophiles, in their hemiketal form, showing an electron excess, in the C6 and C8 positions. This ambivalence enables a variety of reactions. The nucleophilic hemiketal adds to electrophiles such as flavylium cations, carbocations formed by acid-​ catalysed cleavage of tannins, and o-​quinones resulting from enzymatic oxidation of hydroxycinnamic acids, yielding anthocyanin dimers, tannin–​ anthocyanin adducts, or hydroxycinnamic acid–​ anthocyanin adducts, respectively. Anthocyanins in their hemiketal form can also undergo condensation with aldehydes (R-​ CHO) such as acetaldehyde, glyoxylic acid, furfural, hydroxymethyl furfural, or vanillin, yielding R-​ methine anthocyanin oligomers or R-​ methine flavanol–​anthocyanin adducts, if flavanols (monomers or condensed tannins) are also involved. In all these products, the anthocyanin units are initially in the hemiketal form but can dehydrate to the flavylium cation. The flavylium cation undergoes nucleophilic addition of flavanols, yielding colourless flavenes, which can oxidize to the

15

Anthocyanins in Food R3 '

R3'

OH

OH O

HO

+

O

HO

R5'

R5 '

OR

HO

O

R'

OH

OH HO

OH

O

HO

HO

O

+

R3'

R5' OR

O

O

OH

O

neutral quinoinoid base

+

R5 ' OR

R5'

OH

OH HO

methyl-methine tannin-anthocyanin adduct

OR

Ka - H+

R3'

OR OH

R5'

OH

OH

Kh

HO

+ H2O / - H+

OH

HO HO

O OR

OH HO

Tannin-anthocyanin adduct

R3' OH

O

OH

OH

R5 '

R5' OR O

cis-chalcone

OR

OH

cis-chalcone

OH

R3

ring cleavage

OH

OH OH

OH

R5' OR

OH

R3'

O

O

OH

+

hemiketal

OH

HO

OH R3'

OH

R5 '

OH

pyranoanthocyanin dimer

O

R3'

flavylium cation

pyranoanthocyanin dimer

OH O

R5'

O

R3'

O

O

OH +

R3'

portisin

HO

R3'

CH3

pyranoanthocyanin pyranoanthocyanin

OH

HO

O

O OH

HO

OH

OR

O

OH

OH

+

HO

R3' HO

OH

OH

OR

A-type anthocyanin-tannin adduct

R5' OH

O

HO

OH

R5 O

OH

O OH

trans-chalcone

FIGURE 2.2  Structural transformations of anthocyanins in solution. (Colours of structures correspond approximately to typical colours.)

flavylium (Salas et al., 2003) or rearrange to another colourless form through formation of an A-​type bond (Remy-​Tanneau et al., 2003). Anthocyanin oligomers based on the A-​type structures have been detected in grapes (Vidal et al., 2004) and wine (Salas et  al., 2005)  and recently formally identified (Oliveira et  al., 2013). The flavylium cation can also react with compounds having a polarizable double bond to form other pigments called pyranoanthocyanins. These include phenylpyranoanthocyanins, carboxypyranoanthocyanins (vitisins A), pyranoanthocyanins (vitisins B), formed by addition of vinylphenol (Fulcrand et  al., 1996) or hydroxycinnamic acids (Schwarz et  al., 2003), of pyruvic acid (Fulcrand et  al., 1998), and of acetaldehyde (Cheynier et al., 1997), respectively, onto the anthocyanin. The list of pyranoanthocyanins is constantly expanding. Moreover, pyranoanthocyanin dimers have been recently identified (Oliveira et  al., 2010). Finally, nucleophilic attack of water or hydrogen peroxide onto the flavylium induces opening of the C-​ring, which can subsequently cleave (Vallverdu-​ Queralt et  al., 2016a) or rearrange (Es-​Safi et al., 2008). All these reactions occur simultaneously, yielding a large diversity of products as intermediate products are involved in further

reactions. Thus, over 160 compounds have been detected after a few hours of reaction of a very simple solution consisting of an anthocyanin, a flavanol monomer, and acetaldehyde, confirming the occurrence of such reaction cascades (Vallverdu-​ Queralt et al., 2017).

Colour Changes Induced by Anthocyanin Reactions Anthocyanin reaction products show a wide range of colour properties. Thus, tannin–​anthocyanin adducts are red and susceptible to water and sulfite addition, like their anthocyanin precursors (Salas et al., 2004), while anthocyanin–​tannin adducts are colourless. In contrast, pyranoanthocyanins and condensation products with aldehydes are orange and purple pigments, respectively, while further reactions of pyranoanthocyanins yield blue portisins and even turquoise pyranoanthocyanin dimers (Figure  2.2). Moreover, these pigments show increased resistance to bleaching induced by water or sulfite addition. Water addition on pyranoanthocyanins is negligible, so that their prevalent form in mildly acidic solution is the neutral quinonoidal base

16 (de Freitas and Mateus, 2011). Aggregation of the red quinonoid base of some pyranoanthocyanins yields an intense blue colour and eventually leads to precipitation (Vallverdu-​Queralt et  al., 2016b). Similarly, ethyl-​linked anthocyanin derivatives resulting from condensation with acetaldehyde show high resistance to hydration and sulfite bleaching, and are present as pigments (flavylium cation and quinonoid base) in mildly acidic beverages (Duenas et al., 2006). Anthocyanin reactions and hence the colour depend on the medium composition and on processing or preparation conditions. For example, heating increases conversion of the hemiketal to the chalcone (Brouillard, 1982), which is easily cleaved to smaller colourless molecules and promotes rearrangements of the anthocyanin molecules, generating yellow pigments (Es-​Safi et  al., 2008). Oxidation steps (catalysed by metal ions such as iron or copper) are involved in numerous reaction mechanisms, including condensation with aldehydes and formation of pyranoanthocyanins. Oxidation can also induce cleavage of the anthocyanidin skeleton, yielding colourless breakdown products, especially at higher pH values and higher temperature (Vallverdu-​ Queralt et al., 2016a).

Pigment Diffusion and Changes during Cooking Anthocyanins are water-​ soluble pigments and thus easily extracted with water. Most fruits contain anthocyanins only in their skins, but some species or varieties also have pigmented flesh. Anthocyanin extraction is easier from the vacuoles of flesh cells than from those of skin cells, due to differences in tissue structure, so that varieties with coloured flesh are more suitable for the preparation of red juices. Physical processes, e.g., heating and maceration, or the use of pectolytic enzymes that degrade plant cell walls, are required to release pigments from skins. Anthocyanin extraction, for instance in the preparation of hibiscus beverages (known as bissap in West Africa, karkade in Egypt, and agua de Jamaica in Mexico), which are highly popular in Africa and Latin America, increases with infusion time and water temperature. Thus, colour density (calculated as (Absorption at 420 nm − Absorption at 700 nm) + (Absorption at 520 nm − Absorption at 700 nm), i.e., the sum of yellow and red

Véronique Cheynier colours) increased two-​fold between 30 and 120 minutes of infusion at 20 °C, while identical values were obtained after 4 minutes at 90 °C and 1 hour at 20 °C (Ramirez-​Rodrigues et al., 2011). Colour quality is also affected by temperature, higher infusion temperatures resulting in a shift from red to orange reflected by an increase in hue tint (i.e., (Absorption at 420 nm − Absorption at 700 nm)/​(Absorption at 520 nm − Absorption at 700 nm), the ratio of yellow to red), reflecting degradation of the anthocyanin structure. The colour of red cabbage extracts containing highly acylated anthocyanins is much more resistant than that of hibiscus extracts (Fernandez-​Lopez et  al., 2013); this enhanced stability is due to protection of the anthocyanin against nucleophilic attack by water and hydrogen peroxide and subsequent ring opening by intramolecular copigmentation. Addition of copigments can also provide some protection (Trouillas et  al., 2016). For example, ellagitannin copigments increased the heat and colour stability of pomegranate anthocyanins (Fischer et  al., 2013). The best stability was observed in juices with high initial anthocyanin contents and, in model solution, for a copigment to pigment ratio of 2:1. Cooking of red cabbage pigments, especially with alkaline water, results in a shift toward purple hints (due to conversion to blue quinonoidal forms) and to discolouration (conversion to hemiketal and chalcone forms followed by cleavage of the heterocyclic C-​ring) upon heating (Mateus et  al., 2003). The former is reversible, while the latter is irreversible but can be prevented by adding acid such as lemon juice, as illustrated in Figure 2.3. Interaction of extracted anthocyanins with the solid plant material is another conspicuous phenomenon. Thus, red cabbage anthocyanins localized in the external layers of the leaves are released during cooking and absorbed on the whole surface, which becomes uniformly pigmented. This may be related to their particular structure, as it has been recently shown that acylated anthocyanins are particularly prone to interacting with protein (Gil et  al., 2017). However, it may also reflect more complex mechanisms, involving extraction of anthocyanins, their subsequent reactions with other plant components such as tannins, as detailed earlier, and adsorption of the resulting tannin-​like polymeric pigments onto the plant matrix proteins and cell walls.

FIGURE 2.3  Colour of red cabbage extract with (A) or without (B) lemon juice (pH effect on reversible acid–​base equilibrium) and changes induced by heating in a water bath for 30 minutes (C: with lemon juice, no change, D: without lemon juice, irreversible discolouration).

Anthocyanins in Food Finally, some observations such as the reddening of pears after cooking in acidic medium remain unexplained, although one might speculate that this involves the release of anthocyanidins after acid-​catalysed cleavage of tannins, which are abundant in the flesh of some pear varieties.

REFERENCES Andersen OM, Jordheim M. 2006. The anthocyanins. In Flavonoids: chemistry, biochemistry and applications, ed. Andersen O and Markham K, 471–​552. Boca Raton, CRC Press, Taylor & Francis group. Awika J. 2008. Behavior of 3-​deoxyanthocyanidins in the presence of phenolic copigments. Food Research International, 41,  532–​8. Brouillard R. 1982. Chemical structure of anthocyanins. In Anthocyanins as food colours, ed. Markakis P, 1–​40. New York, Academic Press. Brouillard R, Mazza G, Saad Z, Albrecht-​Gary AM, Cheminat,A. 1989. The copigmentation reaction of anthocyanins: a microprobe for the structural study of aqueous solutions. Journal of the American Chemical Society, 111, 2604–​10. Cheynier V, Fulcrand H, Sarni P, Moutounet M. 1997. Application des techniques analytiques à l’étude des composés phénoliques et de leurs réactions au cours de la vinification. Analusis, 25, M14–​M21. De Freitas V, Mateus N. 2011. Formation of pyranoanthocyanins in red wines: a new and diverse class of anthocyanin derivatives. Analytical and Bioanalytical Chemistry, 401, 1463–​73. Duenas M, Salas E, Cheynier V, Dangles O, Fulcrand H. 2006. UV-​ Visible spectroscopic investigation of the 8-​8-​methylmethine catechin-​malvidin 3-​glucoside pigments in aqueous solution: structural transformations and molecular complexation with chlorogenic acid. Journal of Agricultural and Food Chemistry, 54, 189–​96. Es-​Safi N, Meudec E, Bouchut C, Fulcrand H, Ducrot P, Herbette G, Cheynier V. 2008. New compounds obtained by evolution and oxidation of malvidin 3-​O-​glucoside in ethanolic medium. Journal of Agricultural and Food Chemistry, 56, 4584–​91. Fernandez-​Lopez J, Angosto J, Gimenez P, Leon G. 2013. Thermal stability of selected natural red extracts used as food colourants. Plant Foods for Human Nutrition, 68, 11–​17. Fischer UA, Carle R, Kammerer D. 2013. Thermal stability of anthocyanins and colourless phenolics in pomegranate (Punica granatum L.) juices and model solutions. Food Chemistry, 138, 1800–​9. Fossen P, Andersen OM. 2003. Anthocyanins from red onion, Allium cepa, with novel aglycone. Phytochemistry, 62, 1217–​20. Fulcrand H, Cameira dos Santos P, Sarni-​Manchado P, Cheynier V, Favre-​Bonvin J. 1996. Structure of new anthocyanin-​derived wine pigments. Journal of the Chemical Society, Perkin Transactions, I, 735–​9. Fulcrand H, Benabdeljalil C, Rigaud J, Cheynier V, Moutounet M. 1998. A new class of wine pigments yielded by reactions between pyruvic acid and grape anthocyanins. Phytochemistry, 47, 1401–​7. Gil M, Avila-​ Salas F, Santos LS, Iturmendi N, Moine V, Cheynier V, Saucier C. 2017. Rosé wine fining using polyvinylpolypyrrolidone: colourimetry, targeted poly­ phenomics and molecular dynamics simulations. Journal of Agricultural and Food Chemistry, 65, 10591–​7. Mateus N, Silva AMS, Rivas-​Gonzalo JC, Santos-​Buelga C, De Freitas V. 2003. A new class of blue anthocyanin-​ derived pigments isolated from red wines. Journal of Agricultural and Food Chemistry, 51, 1919–​1923.

17 Oliveira C, Azevedo J, Silva A, Teixeira N, Cruz L, Mateus N, De Freitas V. 2010. Pyranoanthocyanin dimers: a new family of turquoise blue anthocyanin-​derived pigments found in port wine. Journal of Agricultural and Food Chemistry, 58, 5154–​9. Oliveira J, Alhinho da Silva M, Parola A, Mateus N, Brás N, Ramos M, de Freitas V. 2013. Structural characterization of a A-​type linked trimeric anthocyanin derived pigment occurring in a young Port wine. Food Chemistry, 141, 1987–​96. Remy-​Tanneau S, Le Guerneve C, Meudec E, Cheynier V. 2003. Characterization of a colourless anthocyanin-​ flavan-​ 3-​ ol dimer containing both carbon-​carbon and ether interflavanoid linkages by NMR and mass spectrometries. Journal of Agricultural and Food Chemistry, 51, 3592–​7. Salas E, Fulcrand H, Meudec E, Cheynier V. 2003. Reactions of anthocyanins and tannins in model solutions. Journal of Agricultural and Food Chemistry, 51, 7951–​61. Salas E, Le Guernev C., Fulcrand H, Poncet-​Legrand C, Cheynier V. 2004. Structure determination and colour properties of a newly synthesized direct-​linked flavanol-​anthocyanin dimer. Tetrahedron Letters, 45, 8725–​9. Salas E, Dueñas M, Schwarz M, Winterhalter P, Cheynier V, Fulcrand H. 2005. Characterization of pigments from different high speed countercurrent chromatography wine fractions Journal of Agricultural and Food Chemistry, 53, 4536–​46. Schwarz M, Wabnitz TC, Winterhalter P. 2003. Pathway leading to the formation of anthocyanin-​vinylphenol adducts and related pigments in red wines. Journal of Agricultural and Food Chemistry, 51, 3682–​7. Trouillas P, Sancho-​García J, de Freitas V, Gierschner J, Otyepka M, Dangles O. 2016. Stabilizing and modulating colour by copigmentation: insights from theory and experiment. Chemical Reviews, 116, 4937–​82. Vallverdú-​ Queralt A, Meudec E, Sommerer N, Dangles O, Cheynier V, Le Guernevé C. 2016a. A comprehensive investigation of guaiacyl-​ pyranoanthocyanin synthesis by one-​ /​ two-​dimensional NMR and UPLC-​DAD-​ESI-​MSn. Food Chemistry, 199, 902–​10. Vallverdú-​Queralt A, Biler M, Meudec E, Le Guernevé C, Vernhet A, Mazauric J, Legras J, Loonis M, Trouillas P, Cheynier V, Dangles O. 2016b. p-​Hydroxyphenyl-​pyranoanthocyanins: an experimental and theoretical investigation of their acid –​base properties and molecular interactions. International Journal of Molecular Sciences, 17, 1842. Vallverdú-​Queralt A, Meudec M, Eder M, Lamuela-​Raventos R, Sommerer N, Cheynier V. 2017. The hidden face of wine polyphenol polymerization highlighted by high-​resolution mass spectrometry. Chemistry Open, 6, 336–​9. Vidal S, Meudec E, Cheynier V, Skouroumounis G, Hayasaka Y. 2004. Mass spectrometric evidence for the existence of oligomeric anthocyanins in grape skins. Journal of Agricultural and Food Chemistry, 52, 7144–​51. Wu LC, Prior R. 2005a. Systematic identification and characterization of anthocyanins by HPLC-​ESI-​MS/​MS in common foods in the United States: fruits and berries. Journal of Agricultural and Food Chemistry, 53, 2589–​99. Wu LC, Prior R. 2005b. Identification and characterization of anthocyanins by high-performance liquid chromatographyelectrospray ionization-tandem mass spectrometry in common foods in the United States: vegetables, nuts, and grains. Journal of Agricultural and Food Chemistry, 53, 3101–​13. Yoshida K, Kondo T, Goto T. 1991. Unusually stable monoacylated anthocyanin from purple yam Dioscorea alata. Tetrahedron Letters, 32, 5579–​80. Zhu F. 2018. Anthocyanins in cereals: composition and health effects. Food Research International, 109, 232–​49.

Alcoholic Beverages: Production, Trends, Innovations Konstantin Bellut1, Kieran M. Lynch1 and Elke K. Arendt1,2 1 School of Food and Nutritional Sciences, University College Cork, Cork, T12 YN60, Ireland 2 APC Microbiome Ireland, University College Cork, Cork, T12 YN60, Ireland

Introduction Alcoholic beverages have a very long history of production and consumption. Their production by humans can be traced back to about 7,000 BC, but it is very likely that prehistoric humans had unknowingly consumed alcohol in overripe berries and fruits for many thousands of years before that (Philipps, 2014). For the production of alcohol, two things are required: a reasonable amount of sugar, and yeast. Ethanol is one of the end products of “alcoholic” fermentation by yeast. Yeast are small single-​cell microorganisms, which are classified as members of the fungus kingdom. Saccharomyces cerevisiae is the most well-​ known and commonly used yeast species for alcoholic fermentations. It has been harnessed as a trustworthy workhorse for alcoholic fermentations since ancient times, and has since been domesticated for its use in brewing, distilling, winemaking and baking. If a beverage contains alcohol, it is the product of alcoholic fermentation by yeast, which turn sugars into ethanol and carbon dioxide as part of their metabolism. The three most common classifications for alcoholic beverages are beer, wine and spirits. In beer brewing, the substrate is a sugary extract from malted grains, mostly barley, which is boiled with hops and then inoculated with yeast. In winemaking, grapes must be used as the substrate, and the fermentation usually involves several yeast strains. Spirits are an indirect product of alcoholic fermentation; exceeding the biological upper alcohol limit achievable through fermentation, spirits are the distilled product of alcoholic fermentation of various substrates.

Beer Production Method and Styles Only four ingredients are required to produce beer: water, barley malt, hops and yeast. Beer brewing is a multi-​step process (Figure  3.1). First, barley malt is crushed and mixed with water in the mash tun. During mashing, the so-​called “mash” is led through different temperature rests to release the sugars and nutrients from the malt via the action of its endogenous enzymes. After mashing, a filtration step is needed to separate the liquid sugary extract from the remaining grain particles. To achieve separation, different techniques can be applied; in a lauter tun, the

grain particles build their own filter bed by sedimentation on a false bottom, which acts as a sieve, and the clear extract, now called “wort”, can be separated from the leftover grain particles. This leftover grain is the so-​called “brewers’ spent grain” (BSG). Alternatively, a mash filter separates the wort from the BSG in a pressurized, membrane-​based process. During boiling to sterilize the wort, hops are added to give beer its distinct aroma and bitterness and to introduce hop-​derived anti-​microbial compounds into the wort. After boiling, hop leftovers and denatured proteins (“hot trub”) are removed physically in a “whirlpool” and the wort is cooled and transferred into a fermentation tank. During fermentation, after the addition of yeast, the yeast metabolizes the wort sugars into ethanol and CO2, which can take between one and three weeks. After fermentation and subsequent maturation, the beer is usually filtered again and is then ready for filling and sale. Through the almost infinite combinations of ingredients, there is no limit to the brewer’s fantasy and creativity in creating different beers. Brewers can choose from over 270 hop varieties, over 80 malt varieties and over 200 different brewing yeast strains (Deutscher Brauer-​Bund e.V., 2016; Stempfl, 2016; IHGC, 2018). In addition, in compliance with their countries’ legislation, brewers can add adjuncts such as sugars, unmalted grains, fruits and spices to further individualize their beers. However, the brewers’ creativity is not only limited to ingredients but also includes variations in brewing practices. All process steps, from mashing to fermentation and even maturation, can be altered to influence the beers’ characteristics. Generally, beers are classified into two main styles: lagers and ales. These come with many different sub-​styles, based on the use of ingredients and yeast, and brewing practices. Lager beer, the most popular beer worldwide, is brewed using Saccharomyces pastorianus, a hybrid sister of S. cerevisiae. It is generally fermented and matured at lower temperatures compared with ales, which are brewed using S. cerevisiae. Commonly used parameters to define beer styles, besides lager and ale, are original and final gravity, bitterness, alcohol content and colour.

Home Brewing Home brewing, the brewing of beer on a small scale for personal, non-​commercial purposes, has its roots in the very beginnings of beer brewing. Historians believe that in the early days of brewing 19

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Konstantin Bellut et al.

FIGURE 3.1  Illustration of the beer brewing process.

in the Mesopotamian and Egyptian cultures, beers were first brewed at home. Only when towns developed did brewing grow from a household enterprise to larger scales (Papazian, 2003). Today, home brewing is practised all over the world by beer enthusiasts who want to live out their experimental and creative spirit or simply save money on their beer consumption. While the early home brewers had to build their equipment themselves, today home brewing kits exist in all varieties and sizes. A standard home brewing kit is shown in Figure 3.2. Alongside the standard home brewing kits, brewing equipment manufacturers have developed high-​quality home brewing equipment, designed for high functionality and convenience. The degree of automation of such equipment can reach from temperature control and programming to a fully automated system. Ingredients and equipment can easily be sourced from local or online home brewing shops. In terms of the choice of ingredients and the level of skill, home brewers can choose between extract brewing and all-​grain brewing. While the beginner would start with liquid or dry malt extract, the more sophisticated home brewer will opt for all-​grain brewing. For all-​grain brewing, mashing is performed in a heated vessel or by mixing the crushed grain with hot water. Temperature control of some sort during mashing is necessary to lead the mash through the desired temperature rests. Subsequently, a filtering step (lautering) is required to separate the spent grain from the sugary extract. With the “brew-​in-​a-​bag” method, mashing is performed in what could be considered as a large tea bag. The bag

FIGURE 3.2  Typical home brewing starter kit.

can then easily be removed and drained upon completion of the mashing process, leaving behind only the sugary extract, which can subsequently be boiled in the same vessel. Extract brewing makes the whole process of mashing and lautering redundant, since the extract itself, in its dry or liquid form, is already the concentrated product of mashing. However, whether all-​grain or extract, a subsequent boiling step is essential to sanitize the wort. Hops can be added in hop bags to facilitate the easy removal of

21

Alcoholic Beverages: Production, Trends, Innovations hop residue after the boil. Pre-​hopped malt extract eliminates the necessity of adding hops during the boil. To separate the clear wort from hop residue and “hot trub”, home brewers can create the whirlpool effect applied in commercial brewing by vigorously stirring the wort. Subsequently, the clear wort needs to cool before it can be inoculated with yeast. The sophisticated and impatient home brewer can use special cooling equipment such as an “immersion cooler”, a hollow stainless-​steel spiral which can be connected to a water supply, to dissipate the heat. An option that does not require additional equipment is to fill the hot wort into the fermenter. By tumbling slightly, the hot wort can even be used to sanitize the fermenter surfaces before it is left alone to cool. Upon reaching the desired fermentation temperature, yeast is added in the form of dried or liquid yeast. Plastic buckets or glass or plastic carboys are well suited for use as fermentation vessels. The vessels are closed with a fermentation lock, which allows the escape of carbon dioxide produced during fermentation but prevents oxygen or contaminants from entering the fermentation vessel. At the end of the fermentation process, the beer can be filled into sanitized bottles. Small, calculated amounts of sugar are added, and the bottles are capped. The remaining live yeast in the beer will consume this sugar and give the homebrew its fizz in the closed bottles. As with commercial brewing, a high standard of hygiene is essential to get a safe, good-​tasting product. As well as equipment and ingredients, sanitizing agents can be sourced in any home brewing shop. Home brewers are engaging in local home brewing clubs or online home brewing communities, which serve as platforms to share recipes and experiences, equipment and, importantly, home brewed beer!

Craft Beer According to the Brewers Association, a craft brewery is defined as small (with an annual production 80%), and heat treatment will lead to starch gelatinization. The properties of these protein/​ starch gels will, therefore, be considerably modified by heating during cooking, pasteurization, and/​or sterilization, but how? Figure  9.2 presents the storage modulus G´ of whey protein isolate (WPI) gels formed by calcium-​induced cold gelation as a

FIGURE 9.2  Storage modulus (G′) of gels as a function of PS content (pure WPI gel, composite gels WPI 10% + PS 1%, 3%, 5%, 7%, or 9%), at 20 °C before heat treatment, at 90 °C, and at 20 °C after heat treatment up to 90 °C. Values were extracted from temperature sweep measurements. (From Lavoisier and Aguilera (2019b), reprinted with permission of the publisher.)

function of their potato starch (PS) content at 20 °C before heat treatment, at 90 °C, and at 20 °C after heat treatment. As can be observed on this graph, starch did not significantly modify G´ before heat treatment. Starch granules in their native state behaved as inactive filler particles, and the architecture of the protein network alone is responsible for the viscoelastic properties of the composite gels. However, during heating from 20 to 90 °C, starch gelatinization occurred, and G´ increased. This reinforcement effect seems to be connected to three different phenomena related to gelatinization. First, starch granules encased in the WPI gel absorbed water, swelled, and exerted pressure on the protein network. Then, due to this increase in volume, starch granules were able to touch and interact with each other. Finally, starch swelling removed water from the gelled protein phase, which induced changes in the microstructure and the properties of the WPI matrix (Lavoisier and Aguilera, 2019b). Interestingly, after cooling, different rheological properties were obtained depending on the PS content of the composite gels.

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FIGURE 9.3  Scheme representing the changes in the microstructure of the composite gels after heat treatment. (From Lavoisier and Aguilera (2019b), reprinted with permission of the publisher.)

For small amounts of starch, a weakening of the WPI gel was observed: gelatinized granules probably ruptured the protein network, creating flaws in the microstructure (i.e., antagonist effect). On the contrary, higher amounts of starch led to a strengthening of the protein gel. In this case, an interpenetrating network formed between PS and WPI (i.e., synergistic effect). These conclusions are summarized in Figure 9.3. This example illustrates how starch gelatinization can influence the properties of a food matrix. This process involves multiple factors, and it must be taken into consideration when designing new foods, either to understand changes that may occur during thermal processing, or to improve the product by tailoring its texture. Note: this chapter is based on the following doctoral thesis: Lavoisier, A. (2019). Effect of a whey protein network formed by cold gelation on starch gelatinization and digestibility. Pontificia Universidad Católica de Chile, Santiago, Chile (available at https://​repositorio.uc.cl/​handle/​11534/​23344).

REFERENCES Ai Y, Jane J. 2018. Understanding starch structure and functionality. In Sjöö M & Nilsson L (Eds.), Starch in food: Structure, function and applications, second edition. Woodhead Publishing, Cambridge, UK, 151–​178.

Bertoft E. 2017. Understanding starch structure: Recent progress. Agronomy, 7(56), 129. Carvalho A. 2008. Starch: Major sources, properties and applications as thermoplastic materials. In Belgacem M & Gandini A (Eds.), Monomers, polymers and composites from renewable resources. Elsevier Science, Amsterdam, The Netherlands, 321–​342. Cuq B, Abecassis J, Morel H. 2014. Chapitre 1, la physique et la chimie au service de l’élaboration des pâtes alimentaires. In Lavelle C (Ed.), Science culinaire: matière, procédés, dégustation. Belin, Paris, 28–​51. Jane J, Kasemsuwan T, Leas S, Ames IA, Zobel H, Il D, Robyt JF. 1994. Anthology of starch granule morphology by scanning electron microscopy. Starch Stärke, 46(4), 121–​129. Lavoisier A, Aguilera JM. 2019a. Effect of a whey protein network formed by cold gelation on starch digestibility. Food Biophysics, 14(2), 214–​224. Lavoisier A, Aguilera JM. 2019b. Starch gelatinization inside a whey protein gel formed by cold gelation. Journal of Food Engineering, 256, 18–​27. Mahmood K, Kamilah H, Shang PL, Sulaiman S, Ariffin F, Alias AK. 2017. A review: Interaction of starch/​non-​starch hydrocolloid blending and the recent food applications. Food Bioscience, 19, 110–​120. Matignon A, Tecante A. 2017. Starch retrogradation: From starch components to cereal products. Food Hydrocolloids, 68, 43–​52.

56 Schirmer M, Jekle M, Becker T. 2015. Starch gelatinization and its complexity for analysis. Starch Stärke, 67(1–​2),  30–​41. Semeijn C, Buwalda PL. 2018. Potato starch. In Sjöö M & Nilsson L (Eds.), Starch in food: Structure, function and applications, second edition. Woodhead Publishing, Cambridge, UK, 353–​372.

Anaïs Lavoisier Wang S, Copeland L. 2013. Molecular disassembly of starch granules during gelatinization and its effect on starch digestibility: A review. Food & Function, 4(11), 1564–​1580.

Baking: Sourdough Bread Mark Traynor1 and Imran Ahmad2 Department of Nutrition, Dietetics, and Hospitality Management, College of Human Sciences, Auburn University, Auburn, Alabama, United States 2 Chaplin School of Hospitality and Tourism Management, Florida International University, North Miami, Florida, United States

1

A Mixture with Many Names If a mixture of flour and water is left at ambient temperature for a prolonged period it transforms into a remarkably sour, aromatic, and gaseous mass of dough. This transformation, caused by endogenous and exogenous microflora, is a biochemical phenomenon known as fermentation. A  variety of labels are given to this mixture: pre-​ferment, levain, poolish, biga, barm, pâte fermentée, mother, chef, and sponge, but sourdough starter is one of the most common terms used. The specific label used is usually dependent upon the geographic location of production, the choice of production method, and the particular formulation used. The breadth of terms used can often confuse and bewilder many bakers. Nonetheless, while the label for the mixture may differ, the fundamental scientific principles behind its production and application do not.

Protecting the Heritage of Sourdough The primary role of a sourdough starter is to aerate (leaven) a bread dough (Decock and Capelle 2005). Sourdough starter typically accounts for no more than 50% of the bread dough (Hansen and Schieberle 2005). The resulting breads are collectively known as sourdough bread. Rye or wheat flours are the most commonly used flours for sourdough production, although a variety of flours can be used, such as barley and sorghum (Mariotti et al. 2014; Sluková et al. 2016). Sourdough fermentation is considered to be one of the oldest food biotechnological processes for fermenting cereal foods (Gobbetti et  al. 2008). Today, sourdough breads are common in a wide range of cuisines around the globe, in particular in Europe, the Middle East, North America, and the Mediterranean (Spicher 1999). In Italy alone, almost all of the approximately 200 traditional Italian breads today use sourdough starters in their production (Minervini et al. 2012). Traditional sourdough production is quite a labour-​intensive and time-​consuming process, considerably more so than the conventional ‘straight dough method’ that uses a commercial yeast to leaven bread. Baker’s Yeast (Saccharomyces cerevisiae) is the

primary leavening agent used in the ‘straight dough method’ (Jayaram et  al. 2013). This involves a relatively short bulk fermentation, usually lasting a couple of hours at 27  °C (McGee 2004). Due to the ubiquity of these yeast breads, the ‘culture’ and craft surrounding sourdough baking has been, for the most part, lost by many bakers. A select few specialists and enthusiast bakers are preserving the heritage of sourdough bread production (Catzeddu 2011).

Artisan over Convenience In recent times, there has been a global upsurge in demand and consumer appreciation for the flavour and taste of authentic, artisan-​ style sourdough bread (Minervini et  al. 2012). The nutritional, technological, and shelf life properties of sourdough bread have been found to be superior to those of yeast bread (Gobbetti et al. 2014). Moreover, their sensory properties set these breads apart. The deep sour taste, pronounced complex aromas and flavours, dense crumb, and thicker crust are just some of the distinctive sensory properties that a sourdough starter can bestow on a bread (Pétel et al. 2017). Research has shown that sourdough bread contains a broader range of volatile compounds, which contribute to bread flavour (especially in the bread crumb), in comparison to yeast bread (Decock and Cappelle 2005). The baking process is an important baking step for the development of bread crust flavour, whereas complex microbial interactions during fermentation are responsible for the formation of the characteristic flavour of the bread crumb (Hansen and Schieberle 2005). A  multitude of environmental and ecological factors govern the fermentation process: temperature, pH, redox potential, ionic strength, dough composition, dough yield, and microbial enzymatic reactions (Font de Valdez et  al. 2010; Spicher 1999). Thus, a deeper understanding of the sourdough fermentation process can assist the baker in controlling the production of consistent-​quality bread through a more straightforward process.

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Sourdough Fermentation –​It Is All About the Microbes Traditional sourdough fermentation is a complex process. The microflora involved in the fermentation process are composed of stable associations of cultures of symbiotic Lactic Acid Bacteria (LAB) and wild yeasts (Gänzle and Ripari 2016). These are derived from natural contaminants present in the flour or as spores throughout the environment, and convert simple sugars, found in the flour, into carbon dioxide gas (CO2), organic acids, and alcohols under anaerobic conditions (Gobbetti 1998).

Lactic Acid Bacteria Lactic Acid Bacteria (LAB) are bacteria characterised by the production of lactic acid as the primary by-​product of fermentation (Monedero et  al. 2017). In addition to lactic acid, many LAB strains can produce carbon dioxide gas (CO2) as a by-​ product, which contributes to leavening. There is immense diversity among the strains of LAB in a sourdough; approximately 50 different species of sourdough LAB have been isolated from traditional sourdough starters (De Vuyst and Neysens 2005). The majority of strains belong to the genus Lactobacillus, with L. sanfranciscensis, L. brevis, and L. plantarum among the key species of Lactobacillus (Gänzle et al. 2007). Growth temperature for Lactobacillus is in the range 2–​53  °C; however, the optimum range is 30–​40 °C (Hammes and Vogel 1995). In mature sourdough starters, the ratio of LAB to yeast ranges from 10:1 to 100:1 (Minervini et  al. 2012). The dominance of LAB is a result of their adaption to the unique environment. LAB possess several stress response mechanisms that are activated to enable the bacteria to overcome the hostile environment: acidity, oxidation, periods of starvation and dehydration, and extremes of temperature (De Angelis et al. 2001). Broadly speaking, LAB fall into three general categories based on specialised carbohydrate fermentation pathways: (i) facultatively homofermentative, (ii) facultatively heterofermentative,

and (iii) obligately heterofermentative. Facultatively homofermentative LAB almost exclusively degrade simple sugars into lactic acid (>85%) as a sole by-​product of fermentation. On the other hand, heterofermentative types produce ethanol, acetic acid, and CO2 in addition to lactic acid ( quantity in channels; or quantity in channels >> quantity in parenchyma cells. For each case, there are two possibilities: release from parenchyma cells faster than release from channels, or release from channels faster than from parenchyma cells. Another case could be quantity in parenchyma cells ~ quantity in channels, with the two transfer rates being equal or different (Figure 12.10). Fitting curves of the dry matter against time measured during experiments indicates that approximately the same quantity of saccharides is extracted from parenchyma cells and from channels, with a large difference in transfer rate (0.048 for the former, 0.876 for the latter; arbitrary units). At this point, we do not know which compartment releases bioactive compounds faster, but, if pure molecular diffusion occurs from channels, as was observed when plant tissues were soaked in methylene blue, it is likely that this compartment is responsible for faster exchanges than the other. Of course, these studies are far from finished, in particular because one can see from Figure  12.10 that methylene blue migrating toward the inside of plant tissues diffuses not only in the channels but also in the surrounding tissues. The question of measuring the fluxes and the kinetic parameters associated

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Bioactivity and Measurement 0.7

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FIGURE 12.10  One particular behaviour that can be obtained for the model of Figure 12.8 restricted to two compartments (“cells” and “channels”). Curve 1 corresponds to the compartment richest in metabolites; Curve 2 to the compartment with the lowest concentration in metabolites; and Curve 3 to the sum of the two phenomena.

with them remains an important goal for understanding the mechanisms of a simple culinary process such as a stock.

REFERENCES Alzamora SM, Hough G, Chirife J. 1985. Mathematical prediction of leaching losses of water soluble vitamins during blanching of peas, J. Food Sci. Technol. 20, 251. Atkins PW. 1990. Physical Chemistry, p. 687. W. H. Freeman & Co, New York. Boyer J, Liu RH. 2004. Apple phytochemicals and their health benefits, Nutr. J. 3, 5. Campbell N. 1995. Biologie. De Boeck-​Wesmael, Québec. Cazor A, Deborde C, Moing A, Rolin D, This H. 2006. Sucrose, glucose, and fructose extraction in aqueous carrot root extracts prepared at different temperatures by means of direct NMR measurements, J. Agric. Food Chem. 54, 4681. Clegg JS. 1984. Properties and metabolism of the aqueous cytoplasm and its boundaries, Physiol. Regul. Integr. Comp. Physiol. 246, R133. Collins CD, Craggs M, Garcia-​Alcega S, Kademoglou K, Lowe S. 2015. Towards a unified approach for the determination of the bioaccessibility of organic pollutants, Environ. Int. 78C,  24–​31. Davis F, Terry LA, Chope GA, Faul CFJ. 2007. Effect of extraction procedure on measured sugar concentrations in onion (Allium cepa L.) bulbs, J. Agric. Food Chem. 55 (11), 4299–​4306. Dickinson E. 2006. Colloid science of mixed ingredients, Soft Matter. 2, 642. Engel E, Nicklaus S, Septier C, Salles C, Le Quere JL. 2001. Evolution of the taste of a bitter Camembert cheese during ripening: characterization of a matrix effect, J. Agric. Food Chem. 49, 2939.

FDA. 2002. Guidance for industry. Bioavailability and bioequivalence studies for orally administered drug product. General consideration, www.fda.gov/​cder/​guidance/​index.htm, last access 4 December 2019. Fan TWM. 1996. Metabolite profiling by one-​and two-​dimensional NMR analysis of complex mixtures, Prog. Nucl. Mag. Res. Spec. 28, 161. Faulks RM, Southon B. 2005. Challenges to understanding and measuring carotenoid bioavailability, Biochim. Biophys. Acta. 1740, 95–​100. Garrote RL, Bertone RA, Silva ER. 1984. Effect of thermal treatment on steam peeled potatoes, Can. Int. Food Sci. Technol. J. 17(2), 111. Gregory JF, Quinlivan EP, Davis SR. 2005. Integrating the issues of folate bioavailability, intake and metabolism in the era of fortification, Trends Food Sci. Tech. 16, 229–​40. Hedren E, Mulokozi G, Svanberg U. 2002. In vitro accessibility of carotenes from green leafy vegetables cooked with sunflower oil or red palm oil, Int. J. Food Sci. Nutr. 53, 445–​53. IUPAC. 2004. Glossary of terms used in toxicokinetics (IUPAC Recommendations 2003), PAC. 76, 1033–​1040. IUPAC. 2007. Glossary of terms used in toxicology, 2nd edition, IUPAC recommendations, Chem. Int. 31(4), 29. https://​ envirotoxinfo.nlm.nih.gov/​toxicology-​glossary-​b.html, last access 14 November 2019. Kahane R, Vialle-​Guerin E, Boukema I, Tzanoudakis D, Bellamy C, Chamaux C, Kik C. 2001. Changes in non-​structural carbohydrate composition during bulbing in sweet and high-​solid onions in field experiments, Environ. Exp. Bot. 45, 73. Kördel W, Peijnenburg W, Klein CL, Kuhnt G, Bussian BM, Gawlik BM. 2009. The reference-​matrix concept applied to chemical testing of soils, Trends Anal. Chem. 28(1), 51. Le Gall G, Colquhoun IJ, Davies AL, Collins GJ, Verhoeyen ME. 2003. Metabolite profiling of tomato (Lycopersicon esculentum) using 1H NMR spectroscopy as a tool to detect potential unintended effects following a genetic modification, J. Agric. Food Chem. 51, 2447. Ly-​Nguyen B, Van Loey AM, Fachin D, Verlent I, Indrawati M, Hendickx ME. 2002. Partial purification, characterization, and thermal and high-​ pressure inactivation of pectin methylesterase from carrots (Daucus carota L.), J. Agric. Food Chem. 50, 5437. Nonnecke IL. 1989. Vegetable Production, p.  75. Van Nostrand Reinhold, New York, NY. Oliveira FAR. 1988. Mass transfer analysis for the leaching of water soluble components from food, PhD thesis. University of Leeds, Leeds. O’Donoghue EM, Omerfield SD, Bendall M, Hedderly D, Eason J, Sims I. 2004. Evaluation of carbohydrates in Pukekohe Longkeeper and Grano cultivars of Allium cepa, J. Agric. Food Chem. 52, 5383. Oomen AG, Rompelberg CJM, Bruil MA, Dobbe CJG, Pereboom DPKH, Sips AJAM. 2003. Development of an in vitro digestion model for estimating the bioaccessibility of soil contaminants, Arch. Environ. Contam. Toxicol. 44, 281–​7. Shargel L, Yu AB. 1999. Applied Biopharmaceutics & Pharmacokinetics (4th ed.). McGraw-​Hill, New York, NY. Shi J, Le Maguer M. 2000. Lycopene in tomatoes: chemical and physical properties affected by food processing, Crit. Rev. Biotechnol. 20, 293–​334. Stapley AGF, Sousa Gonçalves JA, Hollewand MP, Gladden LF, Fryer PJ. 1995. An NMR pulsed field gradient study of the electrical and conventional heating of carrot, Int. J. Food Sci. Technol. 30, 639.

80 Tardieu A, Guerez A, Phana S, De Man W, This H. 2009. Quantitative Nuclear Magnetic Resonance (qNMR) analysis of mono-​and disaccharides in aqueous solutions obtained by soaking raw or fried dice of onion bulbs (Allium cepa L.), J. Food Sci. 74(4), C319–​325. Tardieu A, France MB, This H. 2011. NMR Determination of a Model of Solute Release from Plant Tissues in an Aqueous Environment, Fruit&Veg Processing, Avignon, France (to be published). This H. 2005. Modelling dishes and exploring culinary “precisions”: the two issues of molecular gastronomy. Br. J.  Nutr. 93 (4), S139–​S146. This H, Weberskirch L, Plassais M, Luna A, His A, Skoglund S. 2010. La RMN du liquide voit le coeur des légumes et des viandes... puisque ce sont des gels, L’Actualité chimique. 337 (1), 10.

Hervé This vo Kientza This H. 2016. Statgels and dynagels, Notes Académiques de l’Académie d’agriculture de France /​Academic Notes from the French Academy of Agriculture. 12, 1–​12. This H. 2012. Solutions are solutions, and gels are almost solutions, Pure Appl. Chem. 85 (1), 257–​276. Turner NC. 1981. Techniques and experimental approaches for the measurement of plant water status, Plant and Soil. 58, 339–​366. Varoquaux P, Varoquaux F, Tichit L. 1986. Loss of nitrate from carrots during blanching, J. Food Technol. 21, 401–​407. Viola R, Davies HV. 1992. A microplate reader assay for rapid enzymatic quantification of sugars in potato tubers, Potato Res. 35,  55–​58. Weberskirch L, Luna A, Skoglund S, This H. 2011. Comparison of two liquid-​state NMR methods for the determination of saccharides in carrot (Daucus carota L.) roots, Anal. Bioanal. Chem. 399 (1), 483–​487.

Browning: The Glycation and Maillard Reactions –​Major Non-​Enzymatic Browning Reactions in Food Frédéric J. Tessier Université de Lille, Inserm U1167, F-​59000 Lille, France

Glycation is the most general internationally accepted term for adduction of a sugar to another biomolecule. In particular, glycation of proteins by reducing sugars is considered the first step in the “Maillard reaction”, a central group of chemical reactions that occurs when food turns brown in cooking and is involved in formation of flavour compounds and new textures. For some 60 years, food chemists and other scientists have been working to elucidate the mechanisms involved when glycation reaction products form, and further, to interpret the impact of the glycation reaction on the quality of the food and to assess the physiological consequences related to the ingestion of such neoformed compounds. When specific glycation reaction products such as acrylamide are revealed as potentially harmful to human health, mitigation strategies should be developed, both at home and in commercial production, to reduce the risk of exposure. On the other hand, the chemical pathways of beneficial glycation products, such as melanoidins, must be promoted to ensure their optimum content in food. The most difficult task of all in food chemistry is to strike a balance between the formation of desirable and undesirable glycation reaction products. Today, most of the foods that we eat are thermally processed either during cooking at home, in restaurants or in the course of more or less complex processes in the food industry. The heat treatments applied to fresh foods such as meat and eggs, and to mixtures of raw ingredients such as flour, butter, milk and sugar, induce thousands of chemical reactions between molecules. These are responsible for the formation of a diverse range of sensory-​active compounds, which usually enhance the sensory qualities of food (Cerny, 2008). The new odorant and taste compounds formed during cooking are usually derived from glycation reactions, which were first discovered by Emil Fischer and particularly thoroughly investigated by Louis-​ Camille Maillard (1912). They play a crucial role not only in the synthesis of odorant and taste compounds but also in the formation of colours, from yellow to brown, in cooked food. Another complex combination of chemical reactions that leads to the browning of food is caramelisation (Kroh, 1994). These two reactions, the Maillard reaction and caramelisation, are sometimes considered to be one and the same, which is probably because: (1) they can result in the formation of the same compounds, such as

hydroxymethylfurfural (HMF), (2) they often take place in parallel during cooking and (3)  they lead to the formation of the same brown colour. However, the main difference between caramelisation and the Maillard reaction is that the former needs only simple saccharides such as sucrose, glucose or fructose, whereas the latter requires both reducing sugars and amino groups from proteins, which react together to form the so-​called Maillard reaction products (MRPs). Compared with caramelisation, which needs high temperatures, the Maillard reaction can start at low temperatures (around 30 °C) (Tessier, 2010). Initially, research on the Maillard reaction focused on the discovery of its complex chemical pathways (Hodge, 1953), with the major goals of controlling the formation of key odorants in foods and producing a full range of synthetic flavourings for the food industry (Belitz and Grosch, 1999). However, since the discovery of the potential relationship between the intake of MRPs other than the aroma compounds and potential health effects (Tessier and Niquet, 2007; Delgado-​ Andrade and Fogliano, 2018) questions related to the toxicity of some MRPs and other neoformed molecules have been gaining increasing attention recently. The discovery of the formation of acrylamide by glycation reactions in certain foods such as coffee and French fries (Mottram et al., 2002; Tareke et al., 2002) has also attracted the attention of health scientists and public health authorities. In this chapter, a brief presentation of the three main chemical steps of the Maillard reaction will be followed by a focus on the browning of butter during cooking and on the formation of acrylamide in a selection of foods, particularly starchy foods like bread and potatoes, and roasted plant ingredients like coffee and cocoa beans. Finally, possible beneficial or harmful health effects associated with the consumption of food high in MRPs will also be discussed.

The Basic Chemical Pathways of the Maillard Reaction The reaction between simple sugars and amino groups from amino acids was first observed in 1884 by Emil Fischer, before Maillard explored the reaction between reducing sugars and

81

82 amino acids (Maillard, 1912), but it was not chemically described for another 40 years. John E. Hodge (1953) was the first to propose a general scheme of the Maillard reaction, and his scheme remains the most accurate reference for all scientists in this field.

Initial Phase In food matrices, the initial step in the reaction corresponds to the formation of imine intermediates (also known as Schiff bases or N-​substituted-​glycosylamines) between reducing sugars such as glucose or fructose and free amino acids or protein-​bound amines. The unstable imines can then regenerate the initial substrates (i.e., sugar and amine), generate dicarbonyl compounds through oxidative reaction (e.g., glucosone), or rearrange themselves into stable Amadori and Heyns products (Rufian-​Henares and Pastoriza, 2016). The two types of early Maillard products are formed from aldoses (e.g., glucose) and ketoses (e.g., fructose), respectively. They represent the first stable MRPs that can be detected and thus quantified in foods. Neither the Amadori nor the Heyns compounds absorb wavelengths of visible light. Therefore, in the early step of the Maillard reaction, no browning can be observed on the food. The main early product found in food is fructoselysine (FL), which is an amino-​deoxy-​ketose formed from the nucleophilic attack of the ε-amino group of lysine (an essential amino acid) on a molecule of glucose. The FL is either found on dietary proteins or as a free Maillard product when the reaction occurs on free lysine. Different studies indicate that 1  L of UHT milk contains between 130 and 600 mg FL (Henle, 2003; Erbersdobler and Faist, 2001). Due to the difference in the intensity of the heat treatment, pasteurised milk always contains less FL than sterilised milk (also known as UHT milk). The concentration of FL is usually proportional to the heat load as long as the heat treatment of food is moderate. When the thermal processing is more intense, such as in baking or roasting, the degradation of FL may become more intense than its formation, and therefore an apparent decrease of FL can be observed over the cooking time. Overall, the daily intake of FL has been estimated at around 100 mg per day when a Western diet is consumed (Tessier and Birlouez, 2012).

Propagation Phase The propagation phase of the Maillard reaction, also called the intermediate stage, is a cascade of parallel chemical reactions that lead to the formation of thousands of compounds, some of which are more or less stable (Figure  13.1). Depending on the physicochemical conditions of the food matrices (pH, water activity, temperature, etc.), different intermediate molecules will be formed after the rearrangement of the Amadori or Heyns products (Rufian-​ Henares and Pastoriza, 2016). For instance, fission reactive products such as glyoxal, methylglyoxal and other carbonyls will react with amino groups to generate more stable compounds, including odorants (e.g., pyridines, pyrazines and imidazoles) and browning products. Some of the stable products formed during the propagation phase are UV active, exhibit the

Frédéric J. Tessier typical brown colour of the Maillard reaction, have fluorescent properties and can be classified as flavour compounds. Among the stable products formed during the propagation phase, some of them, such as Nε-​carboxymethyllysine (CML), glucosepane, pentosidine and pyrraline, were discovered almost at the same time in human tissues and in foods. It must be noted that the Maillard reaction also occurs at 37 °C in the human body (Tessier, 2010). In this case, the reaction is called “glycation” and the products formed are named the AGEs, or advanced glycation end products (Henning and Glomb, 2016). Thus, AGEs and MRPs may define the same molecules, and it is simply the case that biologists and physicians use the term AGEs whereas food chemists use the term MRPs. In physiological conditions, CML, pentosidine and other AGEs are the most advanced products that can be found in living organisms, and it can be said that they “finish” the reaction. In food, the situation is quite different, especially when the duration of the treatment is long and the temperature level is high (e.g., roasting, baking, frying or grilling). In such cases, the final compounds are called “melanoidins”.

Termination Phase The termination phase in food occurs during prolonged heating and at temperatures usually above 100  °C. In such conditions, the water content and activity are usually low, at least at the surface of the food matrices (e.g., the high dehydration of the bread crust, the roasting of coffee), and the intermediate MRPs undergo interactions between themselves and other compounds to promote the formation of brown polymers named melanoidins (Wang et al., 2011) (Figure 13.1). Although the term “melanoidins” was very likely created more than a century ago by Schmiedeberg (1897) and used by Maillard to define the end products of his chemical reaction (1912), no precise chemical structure for them has been fully characterised so far. The complexity of isolating and solubilising them makes their identification and quantification difficult, if not impossible (Helou et  al., 2016). What is known so far is that their structures vary greatly depending on the composition of the raw ingredients and the processing of the foodstuff (Martins and van Boekel, 2003). For instance, the chemical structures of melanoidins found in coffee are considerably different from those in cereal-​based products. Even in a single food type, such as bread, a complex variety of melanoidins can be observed. At least four hypotheses have been proposed concerning the formation of melanoidins: (1) a polymerisation of furan and pyrrole units (Tressl et al., 1998); (2) a polymerisation of sugar degradation products formed during the Maillard reaction and linked by amino compounds (Cämmerer el al., 2002); (3) a protein skeleton cross-​linked by carbohydrate-​derived structures (Hofmann, 1998) or MRPs (Hofmann et al., 1999); and (4) ketal and acetal linkages between phenolic compounds, hydroxyl acids and sugars (Moreira et al., 2017). Overall, melanoidins can be defined as high-​molecular-​weight nitrogen-​containing brown-​coloured pseudo-​polymers (Wang et al., 2011).

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Browning: Glycation and Maillard Reactions

FIGURE 13.1  Schematic representation of the three phases of the Maillard reaction.

Yield of Different Maillard Reaction Products

What Is behind the Browning of Butter?

The yield of formation of the different MRPs at each stage of the reaction has been estimated through different tests with model systems for the Maillard reaction (e.g., the reaction between sugars and amino acids in test tubes) and more importantly, through the analysis of a variety of food products over recent decades (Cerny, 2008). Depending on the food products, 1% to 10% of the initial reactants, both amino acids and sugars, can be transformed into Amadori or Heyns products, 0.1% to 1% are modified into stable AGEs such as CML, deoxyglucosone and other reactive intermediate compounds, 0.01% to 0.1% lead to the formation of fragmentation products such as glyoxal, and, lastly, odorant compounds represent less than 0.1% of the initial reactants. The yield of formation for the most chemically reactive, most thermodynamically unstable or most volatile MRPs may have been underestimated, since they do not accumulate over time in food matrices but either react with other compounds or are partly eliminated in the atmosphere.

Traditional butter contains fat (81% w/​ w), proteins (0.85%), sugars (0.06%) and water (16%). We are all familiar with the fact that the colour of butter changes from yellow to brown when it is heated. One might believe that the formation of a brown colour is due to lipid peroxidation, but our data indicate that the oxidation of fat from butter is always very low and cannot explain the browning after heating (Niquet-​Léridon et  al., 2015). The absence of browning in clarified butter (100% fat) indicates that the fatty acid residues present in the triglycerides of butter are stable even at temperatures above 150 °C. The high proportion of saturated fatty acid residues and low proportion of unsaturated fatty acid residues in butter account for this stability. The lack of browning in heated clarified butter also suggests that proteins and saccharides, alone or together, are the key players of the browning reaction. In order to confirm the involvement of the Maillard reaction in the process of browning, CML and HMF, two markers of this reaction, have been quantified in raw and cooked butter. No detectable HMF and only traces of CML were

84 found in raw butter. But after an exaggerated heat treatment of butter (25  min at 150  °C), CML and HMF were found in significant amounts (2 to 3  µg/​g and 51 to 58  µg/​g, respectively). CML and HMF were not detected in either raw or heated clarified butter. While CML is indeed a unique marker of the Maillard reaction, HMF is not fully specific for this reaction and can also be a marker of caramelisation. This implies that carmelisation, in addition to the Maillard reaction, is a potential agent in the formation of browning in cooked butter. CML and HMF are currently under investigation for their potential deleterious effects on health (Tessier et  al., 2016; Murkovic and Pichler, 2006). However, a reasonable daily intake of cooked butter (20 g) would correspond to approximately 1% and 6% of the total exposure to CML and HMF, respectively, when foods commonly consumed are taken into account. A recent study also showed that the concentration of volatile α-​dicarbonyl compounds found in cooked butter was much lower than that in heated beef fat, margarine and safflower oil (Jiang et al., 2013). Based on current scientific knowledge, we can conclude that there is no evidence that heated butter is potentially toxic and, therefore, that it is, in fact, perfectly suitable for cooking.

Acrylamide, the Dark Side of Glycation Acrylamide is a colourless and odourless amide (CH2CHCONH2) of low molecular weight, 71.08 g/​mol. In 1994, it was classified

Frédéric J. Tessier by the International Agency for Research on Cancer (IARC) as probably carcinogenic to humans (Group 2A), and its status has remained unchanged. The IARC originally stated that acrylamide was not known to occur as a natural product and that it was chemically produced for various industrial practices such as water treatment (IARC, 1994). Exposure to it was believed for that reason to be essentially occupational (and related to smoking) until the discovery by Tareke and her colleagues in 2002 that acrylamide is present in heated foods (Tareke et al., 2002). This was rapidly confirmed by other groups who found acrylamide in grilled, baked, fried and toasted foods but not in raw or boiled foods (Ahn et al., 2002). Several mechanisms may be involved in the formation of acrylamide during the cooking process, but the predominant reaction is the Maillard reaction (Mottram et al., 2002). It is, in fact, the glycation between asparagine in its free form (not protein-​bound) and reducing sugars or dicarbonyl compounds that leads to acrylamide forming at temperatures usually higher than 120  °C and where the water content is low (Figure  13.2). Free asparagine is a natural constituent of many plant-​derived foods, but it is hardly found in animal-​derived foods. Consequently, acrylamide is found, in significant amounts, mainly in plant-​based foods such as cereals, potatoes, chicory and coffee beans, where the raw materials contain its two precursors, asparagine and reducing sugars, and when the foods are processed at high temperatures. After more than a decade of investigation by public and private scientific organisations, the European Food Safety Authority

FIGURE 13.2  Schematic major pathway for the formation of acrylamide in food.

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Browning: Glycation and Maillard Reactions (EFSA) has now taken a stance on acrylamide in foodstuffs (European Food Safety Authority, 2015). Based on animal studies, EFSA has confirmed the previous evaluations, which stated that acrylamide in food potentially increases the risk for consumers in all age groups of developing cancer. In 2017, the European Commission adopted a regulation asking European food companies to apply mitigation measures with a view to achieving levels of acrylamide as low as reasonably achievable below the benchmark levels defined by the EFSA for each category of “at risk” food (Commission Regulation 2017/​2158). While the most concerned food processors have already put mitigation measures in place by changing their raw ingredients and recipes (e.g., selecting potato varieties with low quantities of reducing sugars for the production of French fries or chips) or by adjusting their processes (e.g., using new baking techniques), the same cannot be said for home cooking. Indeed, the relative contribution to exposure to acrylamide between foods processed by the food industry and those processed at home is difficult, if not impossible, to estimate. Bender’s (1978) view from 40  years ago remains true; he commented on the widespread belief that only commercially processed foods posed a danger to health and emphasised that even fresh food prepared at home could be equally damaging when cooked at high temperatures, such as by the common methods of frying, roasting and grilling. The same can be applied to acrylamide, for which some non-​ governmental organisations have demanded a total ban in food. While it is possible to limit the formation of acrylamide in food, and this is what the European regulation is demanding of food producers, it is not possible to “ban” acrylamide, since it is formed naturally in various foodstuffs that require cooking. Rather than trying to ban acrylamide in industrially processed food, private and public organisations should contribute to a general awareness and understanding by the population of the risks associated with a high intake of starchy foods such as potatoes and bread cooked and toasted at home at high temperatures for long periods. Some national organizations like the UK Food Standards Agency have sent out a message, “Go for Gold”, to encourage people preparing food at home to make small changes in their cooking habits (UK Food Standards Agency, 2018). Moderate cooking temperatures and slow cooking are already among the alternative techniques used by chefs in restaurants, and those methods should be extended to domestic practice. The risk associated with the dietary exposure to acrylamide must not, however, be used as an argument to encourage “raw food diets”, which will eventually lead to an unbalanced diet and most likely reduce food diversity.

Concluding Remarks and Perspectives The glycation reactions are key reactions that influence not only the sensory quality of cooked food but also its nutritional quality. They can, though, be harmful to health when high temperatures are used in cooking. People who cook, whether chefs, those in the food industry or people at home, are fortunately, however, in control of the Maillard process to the extent that they have evidence

of the process when the food turns brown. As long as they are aware of the potential dangers of cooking at high temperatures, they are fully able to strike a balance between making the food taste good and putting health at risk. However, the recent discovery of acrylamide in food, together with the potential physiopathological role of other MRPs, has raised new questions about the safety of foods cooked at high temperatures and brought new concerns for consumers already aware of the problems. There are conflicting opinions about the ultimate effects of MRPs on human health. Some studies show that MRPs contribute to the development of non-​ communicable and age-​ related pathologies (Cai et  al., 2008; Cai et  al., 2014; Grossin et  al., 2015). In this state, the MRPs are referred to as “glycotoxins”. Another study indicates that CML, a major Maillard product, accumulates in the body over time (Tessier et  al., 2015). However, there are voices challenging the conclusions drawn from these and similar studies because of the methods underlying the evidence on which these conclusions are based (Delgado-​Andrade and Fogliano, 2018). The fact that experiments used in some of those studies are animal-​rather than human-​based is one important argument. There are other concerns in terms of the methodologies of the experiments, chiefly about the specificity and the accuracy of the dietary exposure to glycotoxins and other MRPs. As well as MRPs being detrimental to human health, certain MRPs, such as melanoidins, are potentially beneficial for human health. Taking all this into account, it is the overall composition of the diet, in addition to isolated molecules or ingredients, that must be focused on in future studies. Until the toxic effects of some MRPs have been conclusively refuted or confirmed, the best recommendation is to use a variety of cooking methods (e.g., boiling, blanching, steaming, poaching, simmering, searing, braising, roasting and grilling) as well as eating a variety of foods. Our message is not “Cook Less to Age Better” but “Cook Better to Age Better”.

REFERENCES Ahn JS, Castle L, Clarke DB, Lloyd AS, Philo MR, Speck DR. 2002. Verification of the findings of acrylamide in heated foods. Food Additives & Contaminants, 19,1116–​1124. Belitz HD, Grosch W. 1999. Aroma substances. In Food Chemistry, Springer-​Verlag, Berlin Heidelberg New York. Bender AE. 1978. Beneficial effects of food processing. In Bechtel PJ (ed.), Food Processing and Nutrition, Academic Press, London,  19–​23. Cai W, He JC, Zhu L, Chen X, Zheng F, Striker GE, Vlassara H. 2008. Oral glycotoxins determine the effects of calorie restriction on oxidant stress, age-​ related diseases, and lifespan. American Journal of Pathology,173, 327–​336. Cai W, Uribarri J, Zhu L, Chen X, Swamy S, Zhao Z, Grosjean F, Simonaro C, Kuchel GA, Schnaider-​Beeri M, Woodward M, Striker GE, Vlassara H. 2014. Oral glycotoxins are a modifiable cause of dementia and the metabolic syndrome in mice and humans. Proceedings of the National Academy of Sciences, 111, 4940–​4945. Cämmerer B, Jalyschko W, Kroh LW. 2002. Intact carbohydrate structures as part of the melanoidin skeleton. Journal of Agricultural and Food Chemistry, 50, 2083–​2087.

86 Cerny C. 2008. The aroma side of the Maillard reaction. Annals of the New-​York Academy of Sciences, 1126, 66–​71. Commission Regulation (EU) 2017/​ 2158 of 20 November 2017 establishing mitigation measures and benchmark levels for the reduction of the presence of acrylamide in food. Official Journal of the European Union. L 304/​24–​44. Delgado-​ Andrade C, Fogliano V. 2018. Dietary advanced glycosylation end-​products (dAGEs) and melanoidins formed through Maillard reaction: physiological consequences of their intake. Annual Review of Food Science and Technology, doi. org/​10.1146/​annurev-​food-​030117-​012441. European Food Safety Authority. 2015. Scientific opinion on acrylamide in food. EFSA Journal, 13(6), 4104. Erbersdobler HF, Faist V. 2001. Metabolic transit of Amadori products. Nahrung, 45, 177–​181. Grossin N, Auger F, Niquet-​ Léridon C, Jacolot P, Durieu N, Montaigne D, Schmidt AM, Susen S, Jacolot P, Beuscart JB, Tessier FJ, Boulanger E. 2015. Dietary CML-​enriched protein induces functional arterial aging in a RAGE-​dependent manner in mice. Molecular Nutrition & Food Research, 59, 927–​938. Helou C, Jacolot P, Niquet-​Léridon C, Gadonna-​Widehem P, Tessier FJ. 2016. Maillard reaction products in bread: a novel semi-​ quantitative method for evaluating melanoidins in bread. Food Chemistry, 190, 904–​911. Henle T. 2003. AGEs in foods: do they play a role in uremia? Kidney International, 84, S145–​147. Henning C, Glomb MA. 2016. Pathways of the Maillard reaction under physiological conditions. Glycoconjugate Journal, 33, 499–​512. Hodges JE. 1953. Dehydrated foods, chemistry of browning reactions in model systems. Journal of Agricultural Food Chemistry, 1, 928–​943. Hofmann T. 1998. Studies on the relationship between molecular weight and the colour potency of fractions obtained by thermal treatment of glucose/​amino acid and glucose/​protein solutions by using ultracentrifugation and colour dilution techniques. Journal of Agricultural Food Chemistry, 46, 3891–​3895. Hofmann T, Bors W, Stettmaier K. 1999. Radical-​assisted melanoidin formation during thermal processing of foods as well as under physiological conditions. Journal of Agricultural Food Chemistry, 47, 391–​396. International Agency for Research on Cancer (IARC). 1994. Acrylamide. In IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, ed. World Health Organization, 60, 389–​433. Jiang Y, Hengel M, Pan C, Seiber JN, Shibamoto T. 2013. Determination of toxic a-​ dicarbonyl compounds, glyoxal, methylglyoxal, and diacetyl, released to the headspace of lipid commodities upon heat treatment. Journal of Agricultural Food Chemistry, 61, 1067–​1071. Kroh LW. 1994. Caramelisation in food and beverages. Food Chemistry, 51, 373–​379. Maillard LC. 1912. Action des acides aminés sur les sucres : formation des mélanoïdines par voie méthodique. Comptes Rendus de l’Académie des Sciences, 54, 66–​68.

Frédéric J. Tessier Martins SIFS, van Boekel MAJS. 2003. Melanoidins extinction coefficient in the glucose/​ glycine Maillard reaction. Food Chemistry, 83, 135–​142. Moreira ASP, Nunes FM, Simões C, Maciel E, Domingues P, Domingues MRM, Coimbra MA. 2017. Transglycosylation reactions, a main mechanism of phenolics incorporation in coffee melanoidins: inhibition by Maillard reaction. Food Chemistry, 227, 422–​431. Mottram DS, Wedzicha BL, Dodson AT. 2002. Acrylamide is formed in the Maillard reaction. Nature, 419, 448–​449. Murkovic M, Pichler N. 2006. Analysis of 5-​hydroxymethylfurfural in coffee, dried fruits and urine. Molecular Nutrition and Food Research, 50, 842–​846. Niquet-​Léridon C, Jacolot P, Niamba CN, Grossin N, Boulanger E, Tessier F.J. 2015. The rehabilitation of raw and brown butters by the measurement of two of the major Maillard products, Nε-​carboxymethyl-​lysine and 5-​hydroxymethylfurfural, with validated chromatographic methods. Food Chemistry, 177, 361–​368. Rufian-​ Henares JA, Pastoriza S. 2016. Maillard reaction. In Caballero B, Finglas PM, Toldra F (eds) Encyclopedia of Food and Health, Elsevier, Oxford, 593–​600. Schmiedebergs O. 1897. Über die Elementarformeln einiger Eiweißkörper und über die Zusammensetzung und die Natur der Melanine. Naunyn-​Schmiedebergs Archives, 31, 1. Tareke E, Rydberg P, Karlsson P, Eriksson S, Törnqvist M. 2002. Analysis of acrylamide, a carcinogen formed in heated foodstuffs. Journal of Agricultural and Food Chemistry, 50, 4998–​5006. Tessier FJ. 2010. The Maillard reaction in the human body. The main discoveries and factors that affect glycation. Pathologie Biologie, 58, 214–​219. Tessier FJ, Birlouez-​ Aragon I. 2012. Health effects of dietary Maillard reaction products: the results of ICARE and other studies. Amino Acids, 42, 1119–​1131. Tessier FJ, Niquet C. 2007. État des connaissances sur la biodisponibilité et la toxicité des produits de Maillard issus de l’alimentation. Journal de la Société de Biologie, 201, 199–​207. Tessier FJ, Niquet-​Léridon C, Jacolot P, Jouquand C, Genin M, Schmidt AM, Grossin N, Boulanger E. 2016. Quantitative assessment of organ distribution of dietary protein-​ bound 13C-​labeled Nɛ-​ carboxymethyllysine after a chronic oral exposure in mice. Molecular Nutrition & Food Research, 60, 2446–​2456. Tress lR, Wondrak GT, Garbe L, Kruger RP, Rewicki D. 1998. Pentoses and hexoses as sources of new melanoidin-​ like Maillard polymers. Journal of Agricultural and Food Chemistry, 3, 1765–​1776. UK Food Standards Agency. 2018. Go for Gold. www.food.gov.uk/​ science/​acrylamide-​0, last access 4 December 2019. Wang H-​Y, Qian H, Yao W-​R. 2011. Melanoidins produced by the Maillard reaction: Structure and biological activity. Food Chem., 128, 573–​584.

Canning: Appert and Food Canning Jean-​Christophe Augustin Ecole Nationale Vétérinaire d’Alfort, 7 avenue du Général de Gaulle, 94704 Maisons-​Alfort, France

Appertization: The Art of Preserving Animal and Vegetal Substances At the end of the 18th century, Nicolas Appert (1749–​1841), a French confectioner, developed an original method of preserving large amounts of foods for long periods. He found that the application of heat to food in sealed glass bottles preserved the food from deterioration. The “appertization” industry was thus born in 1795 at Ivry-​sur-​Seine, near Paris, where Appert began to elaborate heat-​preserved food products (Barbier, 1994). In 1802, he relocated his activity to a bigger factory in Massy and produced preserved meat, vegetables and fruits. At that time, foods were preserved with the addition of salt, alcohol, vinegar, sugar or fat or by drying and smoking. Unfortunately, all these processes deeply modified the intrinsic properties of food, and in particular their flavour. The preservation method proposed by Appert had the advantage of not altering the food. Gourmets and marine officers were the first to praise his invention. The former emphasized the “freshness” of preserved foods, allowing them to “enclose seasons in glass bottles”. The latter underlined the remarkable capacity of the process to preserve a variety of foods and the lack of spoilage of bottled perishable food products after durability studies performed aboard ships. The Navy was moreover particularly interested in this process, since it could prevent scurvy among ship crew members, which was attributed to the consumption of salted meats. In 1809, a committee of three members (Louis-​ Bernard Guyton-​ Morveau, Antoine Parmentier and Bernard-​ Felix Bouriat) of the Society supporting the National Industry published a report after testing ten animal and vegetable food products preserved for more than eight months. The committee reported that all foods were perfectly edible and expressed a positive opinion of Appert’s process. The recognition of his work came in 1810, when Nicolas Appert chose to publish and disseminate his findings and received a 12,000 Franc grant from the Ministry of the Interior for his work. He published a book entitled Le livre de tous les ménages ou l’art de conserver pendant plusieurs années toutes les substances animales et végétales (The book for all households or the art of preserving animal and vegetable substances for many years) (Appert, 1810). He described the following successive steps in the process:

• enclose, in bottles or jars, the food to be preserved; • cork the bottle carefully; • submit the bottles to the action of boiling water for various lengths of time, depending on the food; • remove the bottles after the prescribed time. Food canning was empirically invented. The process was soon improved in England, where a method of sealing food into unbreakable cylindrical tin or wrought-​iron canisters (“cans”) was developed. The efficiency of the thermal process was also improved, and the first pressure retort was built during the mid-​ 19th century, allowing steam temperatures higher than 100 °C to be used. This provided the benefit of reduced thermal process times, which in turn improved the flavour, texture and nutritional value of the food. Despite not understanding why the heat process prevented the food from going bad, Appert was convinced that his method depended on the action of heat inhibiting food spoilage. The French chemist Louis-​ Joseph Gay-​ Lussac (1778–​ 1850) thought that oxygen present in bottles was modified by heating, allowing the preservation of foods. It was not until 50  years later that Louis Pasteur (1822–​1895) provided the explanation for the effectiveness of canning when he demonstrated the role of microorganisms in food spoilage. In 1862, he performed experiments that proved that heat-​treating beverages (milk, beer and wine), a process that became known as pasteurization, could stop them from spoiling.

The Botulinum Cook Although the heating times prescribed by Appert were short, between one and two hours in boiling water (Appert, 1810), food spoilage and foodborne illnesses involving canned foods were not formally reported before the end of the 19th century. After 1895, the William Underwood Company in the USA, which experienced tin cans swelling, decided to launch studies to fix this problem. Many cases of canned food spoilage were studied by William Underwood and Samuel Prescott, who demonstrated that heat-​resistant bacterial spores were the causative agents of spoilage and that the proper time–​temperature combinations could prevent it (Wanucha, 2009). They were

87

88 the first to perform time–​ temperature studies and published time–​temperature requirements of canned foods to control their spoilage. More dramatically, at the same period, canned foods were also implicated in botulism outbreaks. Foodborne botulism is a very serious form of food poisoning, a flaccid paralysis with a high case-​ fatality rate caused by ingesting preformed Clostridium botulinum neurotoxin. The name “botulism” is derived from the Latin word “botulus” meaning sausage and came to be used in Europe in the 18th century to describe a disease associated with muscle paralysis, breathing difficulties without loss of cognition and a sensory system remaining intact, which was frequently linked to the consumption of blood sausage. At the end of the 18th century, cases of fatal poisoning were observed in Württemberg in southern Germany following the consumption of smoked blood-​ sausage (Erbguth and Naumann, 1999). In 1820 and 1822, the District Medical Officer Justinus Kerner published monographs detailing more than 200 cases of “sausage poisoning”, describing accurately the clinical presentation of foodborne illness and speculating on the mechanism of “the fat poison”. The causative agent, named Bacillus botulinus, was isolated in 1896 by Emile van Ermengem from inadequately cured ham in Belgium (Van Ermengem, 1979). He established that foodborne botulism was an intoxication, not an infection, and that the toxin was produced by a spore-​forming anaerobic bacterium. Spores of C.  botulinum are ubiquitous in the environment. Spore germination and bacterial growth allowing the elaboration of toxin occur in anaerobic, low-​salt ( 4.6) environments at temperatures above 10 °C for classical proteolytic strains and 3 °C for nonproteolytic strains (Peck et al., 2011). The canning of foods is conducive to creating anaerobic conditions; hence, low-​acid foods even slightly contaminated with C.  botulinum spores receiving inadequate heat treatment, allowed to stand for a time and eaten undercooked (the toxin is heat-​labile) may cause botulism. In November 1913, an outbreak involved 12 young people at Stanford University who had consumed a string bean salad. Dickson (1918) was the first to observe that spores of C.  botulinum could survive for two hours in boiling water and recommended that an educational campaign be instituted so that all who practised the home-​canning of fruits and vegetables might be informed of the danger. The first outbreak of foodborne botulism recorded in the UK happened in August 1922 at Loch Maree, in Scotland. The consumption of sandwiches containing wild-​duck paste preserved in a glass jar caused the death of eight persons (Leighton, 1923). There was little suspicion of commercial canning at that time. In the USA, from 1910 to 1919, there were 48 outbreaks attributed to home-​processed food and 14 to commercially processed food. However, between August and November 1919, commercially canned ripe olives caused a total of 28 intoxications with 17 deaths. Hence, in December 1919, the Canners League of California, the California Olive Association and the National Canners Association proposed and financed a detailed investigation by the University of California and the Stanford University. This project involved assessing the dangers of botulism in commercially prepared food, how it originates, and how it could best

Jean-Christophe Augustin be eliminated. Karl Friedrich Meyer and his co-​workers were entrusted with the investigation, and studies were carried out between 1919 and 1926 (Meyer, 1973). Sterilization standards were scientifically established, derived from experiments exposing the spores of 109 different strains of C.  botulinum to different heating temperatures and demonstrating that thermal destruction of the most resistant pathogens required a four-​minute exposure at 120  °C and 330 minutes at 100  °C (Esty and Meyer, 1922). Sanitary standards of packing plants and inspection services regulating the processing of low-​ acid foods improved the safety of canned foods (Meyer, 1973). From this period, botulism did not disappear, but home-​canned rather than commercial-​canned foods became the leading cause of foodborne botulism (Hall, 1943; Sobel et  al., 2004). In the USA, commercial-​canned foods accounted for 32% of the 125 outbreaks of known source from 1910 to 1929 but for only 3.3% of the 305 outbreaks of known source reported during the period 1930–​1959 (Sabin, 1980).

Thermobacteriology: Modelling the Thermal Processing of Foods Early time–​temperature studies conducted at the beginning of the 20th century were carried out, giving rise to what was named “thermobacteriology” (Stumbo, 1949). Thermobacteriology is dedicated, on the one hand, to studying undesirable heat-​ resistant microorganisms potentially present in canned foods, and, on the other hand, to studying heat transfers in canned foods during thermal processing. Mathematical models were developed to describe the destruction of microorganisms during heat treatment. These models are very simple and allow the heat resistance of microorganisms to be characterized with only two parameters: D, the decimal reduction time (required heating time for a survival ratio of 10% of the microbial population) (Katzin et  al., 1943), and z, the thermal death-​time curve parameter (interval in temperature yielding a ten-​fold change in D) (Bigelow, 1921). Measurement of temperature during the heating and cooling of foods in hermetically sealed containers was performed and allowed models to be developed that described the transfer of heat to the food’s thermal centre for conduction or convection heating (Ball, 1923; Ball and Olson, 1957). Parameters characterizing bacterial resistance to heat were integrated with parameters describing heat transfer and heat intensity to estimate the amount of heating required to produce safe and “commercially sterile” food products (Stumbo et  al., 1975). The lethality requirement depends on the acceptable statistical probability of observing surviving microorganisms. This includes both pathogenic and spoilage organisms that are capable of growth under the intended storage conditions. The classical acceptable survival probability of spores of C. botulinum is no greater than 1 viable spore in 1012 containers. Since the approximate maximum heat resistance of C. botulinum spores may be represented by a D-​value of 0.21 min at 121.1 °C, the lethality requirement at 121.1 °C was rounded up to three minutes, known as the standard F03 sterilization process (Stumbo et al., 1975).

Canning: Appert and Food Canning An F03 process is safe with respect to C. botulinum spores, but products may contain a small number of surviving spores of more heat-​resistant spoilage microorganisms, jeopardizing the “commercial sterility” of canned foods. For spoilage, spore-​forming mesophilic bacteria more heat resistant than C.  botulinum, the acceptable probability of survival to ensure reasonably minor economic losses should be no greater than 1 viable spore per about 104 containers (Stumbo et al., 1975). Quality degradation calculations must therefore be performed in order to select the thermal process that results in the highest product quality retention. Indeed, while destruction of spoilage agents is achieved during the thermal process, quality degradation (nutrients, vitamins, colour, texture and flavour loss) also occurs. Therefore, from all the time–​temperature combinations that comply with the minimum conditions necessary to free foods of microorganisms that might spoil the foods or endanger the health of consumers, those less deleterious to organoleptic and nutritive properties must be chosen. This optimization can be achieved by knowing the kinetics of quality degradation as a function of the time–​ temperature profiles (Holdsworth, 1985). Risk–​benefit models can therefore be developed according to the time–​temperature profile of the canning process and the intrinsic parameters of foods (pH, oxygen content) to manage the microbial spoilage risk versus the nutritional benefit of preserved foods (Rigaux et al., 2012). These studies allow a compromise on process parameters to be identified, optimizing nutritional and organoleptic quality while keeping microbial risk at an acceptable level. These kinds of studies emphasize the never-​ending need for greater precision in calculating and assessing heat processes for food preservation (Mafart et al., 2010).

REFERENCES Appert N. 1810. L’art de conserver pendant plusieurs années toutes les substances animales et végétales. Patris et Cie, Paris. Bibliothèque Nationale de France; http://​gallica.bnf.fr/​, last access 23 November 2019. Ball CO (1923) Thermal process time for canned food. Bulletin of the National Research Council, 7, 10. Ball CO, Olson FCW. 1957. Sterilization in Food Technology. McGraw-​Hill, New York, p. 654. Barbier JP. 1994. Nicolas Appert: Inventeur et humaniste. Royer, Paris, France.

89 Bigelow WD. 1921. The logarithmic nature of thermal death time curves. The Journal of Infectious Diseases 29, 528–​536. Dickson EC. 1918. Botulism –​A  clinical and experimental study –​Monograph No. 8, Rockfeller Institute for Medical Research. Erbguth FJ, Naumann M. 1999. Historical aspects of botulinum toxin: Justinus Kerner (1786–​1862) and the “sausage poison”. Neurology 53, 1850–​1853. Esty JR, Meyer KF. 1922. The heat resistance of the spores of B. botulinus and allied anaerobes. XI. The Journal of Infectious Diseases 31, 650–​663. Hall IC. 1943. The danger of botulism. American Journal of Public Health 33, 818–​820. Holdsworth SD. 1985. Optimisation of thermal processing –​a review. Journal of Food Engineering 4, 89–​116. Katzin L, Sandholzer L, Strong E. 1943. Application of the decimal reduction time principle to a study of the resistance of coliform bacteria to pasteurization. Journal of Bacteriology 45, 265–​272. Leighton GR. 1923. Botulism in Scotland. Nature 111, 415. Mafart P, Leguerinel I, Couvert O, Coroller L. 2010. Quantification of spore resistance for assessment and optimization of heating processes: a never-​ ending story. Food Microbiology 27, 568–​572. Meyer KF. 1973. The rise and fall of botulism. California Medicine, The Western Journal of Medicine 118, 63–​64. Peck MW, Stringer SC, Carter AT. 2011. Clostridium botulinum in the post-​genomic era. Food Microbiology 28, 183–​191. Rigaux C, Renard CMGC, Nguyen-​The C, Albert I, Carlin F. 2012. Modeling risk-​ benefit in a food chain: nutritional benefit versus microbial spoilage risk in canned green beans. AgroStat 2012, Paris, France. Sabin AD. 1980. Karl Friedrich Meyer 1884–​1974. Biographical memoirs. National Academy of Sciences 42, 269–​332. Sobel J, Tucker N, Sulka A, McLaughlin J, Maslanka S. 2004. Foodborne botulism in the United States, 1990–​ 2000. Emerging Infectious Diseases 10, 1606–​1611. Stumbo CR. 1949. Thermobacteriology as applied to food processing. Advances in Food Research 2, 47–​115. Stumbo CR, Purohit KS, Ramakrishnan TV. 1975. Thermal process lethality guide for low acid foods in metal containers. Journal of Food Science 40, 1316–​1323. Van Ermengem E. 1979. A new anaerobic bacillus and its relation to botulism. Reviews of Infectious Diseases 1, 701–​719. Wanucha G. 2009. Two happy clams: The friendship that forged food science. Food Technology 63, 11.

Capillarity in Action Hervé This vo Kientza 1 INRAE, AgroParisTech, UMR 0782 SayFood, 75005, Paris, France 2 Group of Molecular Gastronomy, INRAE-​AgroParisTech International Centre for Molecular Gastronomy, F-​75005, Paris, France

The phenomenon of capillarity can occur during culinary processes because the thermal treatment of animal or plant tissues weakens the material (whether collagenic tissue or plant cell walls) holding together the cells making up the tissues. This weakening is mainly the result of the hydrolysis of the polymers (collagen and pectin, respectively) that are responsible for coherence (Sila et  al., 2006). When pores or cracks are created as a result of cell separation, a surrounding liquid (e.g., a sauce in which the plant or animal tissues are thermally treated) can enter the tissues by capillarity (De Gennes et al., 2004). In this chapter, we discuss this mechanism of capillarity in the context of culinary processes, but we mainly observe that, whereas this mechanism certainly plays a role in exchanges of matter between food ingredients and their liquid environment, the question is not so much to admit the possible involvement of the mechanism as to assess its quantitative importance. We shall finish by proposing an easy way of introducing a flavour into a piece of plant or animal tissue.

Diffusion versus Capillarity Before analysing particular phenomena that can be observed in the kitchen and investigated from a physical and chemical (i.e., molecular gastronomy) point of view, it is interesting to observe that, as we show here, capillarity was sometimes underestimated; Aguilera et al. (2004) discussed the fact that it occurs in chocolate, for which the mechanism is often said, incorrectly, to be “diffusion”. Such a confusion can be observed both in culinary and in scientific circles, as discussed in the chapter in this book about osmosis (“Osmosis in the Kitchen”). For all the phenomena for which capillarity, diffusion, osmosis or “imbibition” is considered as the interpretation, the core question is the exchange of matter between food and its (often liquid) environment (This, 2019). For example, when roots of Daucus carota L. are heated in water with a view to producing a “carrot stock”, the aqueous environment is progressively enriched with many compounds, the most abundant being saccharides (D-​glucose, D-​fructose and sucrose) and amino acids (Cazor

et al., 2006). The same holds for tomato sauces containing pieces of the bulb of Allium cepa L.  (Tardieu et  al., 2009), or when producing the beverage called “coffee” from ground, thermally processed seeds of Coffea (Febvay et al., 2019). Water loss from the interior of foods during thermal treatments has sometimes been attributed solely to “diffusion”, and it is possible that some molecular diffusion occurs when the open channels of xylem and phloem of a cut plant tissue are in direct contact with an aqueous environment (Bauchard and This, 2015). However, other mechanisms can occur, such as the escape of water vapour when high temperature causes water to evaporate, either in the “crust” or inside the tissue if microwaves are used for heating (see chapter in this book on evaporation). Fu et  al. (2003) studied moisture movement from the interior of a porous and moist food matrix (for example, bread dough) during microwave heating; the loss of water was due not only to moisture diffusion but also to bulk vapour flow (convection) caused by the positive pressure built up inside the product. Osmosis can also come into play when plant and animal tissues are in an aqueous environment, and the contraction of denaturated collagenic tissue can lead to “juice” release when animal tissues are thermally processed (Kopp et al., 1977). It is not the purpose of this chapter to repeat why the word “diffusion” is sometimes overused in food science and technology, as this was very well explained by Aguilera et al. (2004), but it is useful to reiterate that the different velocities for material transfer obey different laws, even if diffusion and capillarity cannot always be readily distinguished after the solely kinetic determination of some compounds. For molecular diffusion, the German physiologist Adolf Eugen Fick (1829–​1901) stated that, in the presence of a concentration gradient, the net migration of molecules due to their random motion occurs from a region of high concentration to one of lower concentration; as seen in the chapter about osmosis, this can be expressed using the chemical potential. The second Fick’s law states that the rate at which this process proceeds at a point M(x,y,z) in space, for a diluted binary system, is proportional to the variation of the slope of the concentration gradient (Fick, 1855). The so-​called diffusion equation reads:

91

92

Hervé This vo Kientza ∂c ( x,y,z,t ) ∂t

= D∆c ( x,y,z,t )

where c(x,y,z,t) is concentration at the point M, at time t, D is the diffusion coefficient, or diffusivity, and ∆ is the Laplacian operator. Aguilera et al. (2004) showed that this model is widely used (and abused) by food engineers as a general model for mass transfer. The approach has the advantage that, by plotting experimental data in the form of log (unaccomplished ratio of mass transferred) against time, an apparent or effective diffusion coefficient, Deff , can be determined from the straight portion of the curve (Schwartzberg, 1987). The parameter Deff is to be correctly redefined as a mass transfer coefficient, because it encompasses all possible forms of mass transfer involved in the process, not only diffusional ones. A simple analysis used to adopt the Fickean diffusion model is to check whether the ratio of the mass m(t) transported as measured experimentally by the mass m(+∞) when the equilibrium is reached varies proportionally to

t (Geurtz and

Oortwijn, 1975). For capillarity, on the other hand, the most common expression for capillary rise is the so-​called Lucas–​Washburn equation, which assumes that the capillary pressure in a cylindrical capillary in contact with an infinite liquid reservoir is compensated by viscous drag and gravity (Krotov and Rusanov, 1999):

2 8 dh γ cos (θ ) = 2 µh + ρgh r dt r

where h is the distance the fluid is drawn into the capillary, γ the surface tension of the fluid, θ the contact angle between the fluid and the capillary wall, r the radius of the capillary, ρ and μ the density and viscosity of the liquid, respectively, and g the acceleration of gravity. The asymptotic solutions to this for short and long times were discussed by Quéré (1997) and by Zhmud et al. (2000). The short time limit (t→0) predicts that the liquid movement in a horizontal capillary should be proportional to the square root of time according to the expression:

h=

r γ cos (θ ) 2µ

t

A plot of h against t is a straight line. Overall, curves for diffusion as well as for capillary flow show a square-​root-​of-​time dependence for mass transport at short times.

Sometimes, There Is No Entrance; Sometimes, There Is One As stated earlier, the issue is generally not so much to know whether some capillarity can take place as how much of the matter transfer is due to capillarity at various times after the

FIGURE 15.1  In this picture of the inside of a muscle that was cooked for 2  h in a solution of fluorescein, it can be seen that the fluorescent marker diffused into the gelatinized collagenic tissue (green colour at the bottom and on top left) by about 1 cm. A lower amount of fluorescein also entered the meat by capillarity (top) between the muscular fibres. No fluorescein could be found in the inside of the meat.

initial contact between food and its liquid (aqueous solution or oil) environment. Regarding marination of red meat (Aktas et  al., 2003; Vlahova-​Vangelova and Dragoev, 2014), one can observe that some publications discuss how the composition of wines determines the tenderizing of cubes of meat marinated in wines, but the mechanism is seldom discussed. Figure  15.1 shows the inner part of a muscle (longissimus dorsi) of Bos taurus that was thermally treated for 2 h in a solution of “fluorescein” (sodium;3-​oxospiro[2-​benzofuran-​1,9′-​xanthene]-​3′,6′-​ diolate). Fluorescence spectroscopy was performed on ethanolic extracts of samples from this muscle, but the green fluoresence is enough to assess that some fluorescein diffusion occurred in the gelatinized thick collagenic tissue and that some fluorescein also penetrated the meat by capillarity when the collagenic tissue was destroyed between muscular fibres. It it noteworthy that this entry of the marker, through diffusion and capillarity, occurred in spite of the strong meat contraction (by about one-​third in mass). The study of grilled meat (see chapter on salt and meat by This et  al. in this handbook) corroborates this result; as shown by analysis of sodium chloride in meat being grilled, no salt was found inside the meat (processing times 100 °C) and in dry conditions in the presence of oxygen, carotenoids may undergo pyrolysis, leading to a number of oxygenated derivates with aromatic properties. This oxidative process can also take place through culinary fermentation processes such as wine making or green tea leaf drying (Rodriguez-​Amaya, 2001). Curcumin (Figure  22.4), or turmeric yellow, is obtained by solvent extraction of turmeric (ground rhizomes of Curcuma longa L.); the extract is purified by crystallisation. This process eliminates the pungent and aromatic essential oil in turmeric,

153

Colour: Natural Pigments in Foods leaving deodourised turmeric, which is used in dairy products and baked goods. Curcumin is relatively inexpensive and heat stable but has poor light stability.

Chlorophylls and their Derivatives Chlorophylls (Figure  22.5) and their derivatives are another group of lipid-​soluble pigments, industrially obtained by organic solvent extraction of grasses, alfalfa (Medicago sativa L.), nettles (Urticaceaes) and other plants (Spinacia oleracea L.) or algae materials. During the extraction of chlorophylls and the subsequent solvent removal, the naturally present coordinated magnesium may be wholly or partly removed from the chlorophyll molecules (pheophytinisation), causing a change of colour towards dark olive green pheophytins.

In higher plants, only chlorophylls a and b are present. The a/b ratio is normally between 1 and 3, depending on a multitude of factors, both genetic (species, variety, etc.) and environmental (luminosity, water stress, mineral nutrition, etc.) (Lichtenthaler, 1987). For instance, plants exposed to the sun tend to have higher chlorophyll a / c​ hlorophyll b ratios than plants in the shade. Chlorophylls generally contribute 0.6% to 1.2% of the dry weight of plants (Scheer, 1991). Some marine organisms such as seaweeds and bacteria have completely different chlorophylls but are still sensitive to colour degradation due to the pheophytinisation process. This dramatic change of colour has been studied for centuries, as many chefs and scientists tried to understand why this was happening and how to avoid it. It is quite common to find recommendations in cookbooks on how to retain the green colour of vegetables. It must be said that most of them have no scientific basis and are of limited effect (Valverde, 2008; Figure 22.6).

FIGURE 22.5  Chlorophylls (the numbering is the IUPAC one).

FIGURE 22.6  Green beans after cooking for 15 min at 100 °C in buffer solutions at pH = 5.0 (left) or at pH = 8.0 ± 0.5 (right).

154 Therefore, it is not unusual to find scientific literature dedicated to this phenomenon and technological proposals to avoid it. There are four main technological pathways that the food industry has explored and used commercially to obtain higher-​quality products: 1. neutralisation of acids released during thermal processing; 2. reduction of the processing time by increasing the processing temperature; 3. enzymatic conversion of chlorophylls into more stable products; 4. transformation of chlorophylls into more stable metallo-​complexes.

Juan Valverde botulinum spores (Canjura et al., 1999). Temperature dependence can be expressed in terms of Z-​value. The Z-​value is the change in °C required to generate a 10-​fold change in the destruction rate of bacteria. The Z-​values for the formation of pheophytins a and b in heated spinach purée have been determined to be 51 and 98 °C, respectively (Fennema, 1996). The high values for both as compared with that for the inactivation of C. botulinum spores (10 °C) result in greater colour retention when HTST processing is used (Schwartz et  al., 1981). In this way, by increasing the temperature of the treatment, the processing time can be reduced. Other studies of plant tissue combined HTST processing with pH adjustment. Samples treated in this manner were initially greener and contained more chlorophylls than control samples (typical processing and pH). However, the improvement in colour, as previously mentioned, was generally lost during storage (Buckle et al., 1969).

Neutralisation of Acids The addition of alkalising agents to boiling water can result in improved retention of chlorophylls during thermal processing. Techniques have involved the addition of CaO and NaH2PO4 into blanching water to maintain the pH of the product or even to raise it to 7.0. When vegetables are cooked, their biological structures break down, liberating acids retained in the vacuoles and thus reducing the pH of the solution. Chlorophylls and other pigments (anthocyanins) are sensitive to these pH changes. Compounds such as MgCO3 or Na2CO3 in combination with Na3PO4 were proposed in the past (Fennema, 1996). However, this process has a trade-​off, resulting also in tissue softening and a soapy flavour. In 1940, Blair recognised the toughening effect of Ca2+ and Mg2+ when added to vegetables. This observation led to the use of Ca(OH)2 or Mg(OH)2 for the purpose of raising pH and keeping the consistency. This combination of treatments became known as the “Blair process” (Blair, 1937). The commercial application of these processes was not successful because of the inability of the alkalising agents to effectively neutralise interior tissue acids over a long period of time, resulting in substantial colour loss after less than two months of storage (Fennema, 1996). However, this process is generally useful in culinary and catering preparations, as they are able to maintain bright green colour generally for a number of days. In industrially canned peas, an elevated pH (8.0 or above) can cause the formation of struvite, i.e., glass-​like crystals consisting of a magnesium and ammonium phosphate complex. Struvite is believed to result from the reaction of magnesium with ammonium generated from the protein in peas during heating (Fennema, 1996).

Enzymatic Conversion of Chlorophylls to Chlorophyllides Blanching at lower temperatures than those conventionally used to inactivate enzymes has been suggested as a means of achieving better retention of colour in green vegetables, in the belief that chlorophylls can degrade to chlorophyllides, which are precursors/​derivates of chlorophylls but without the phytol chain. Chlorophyllides are known to have greater thermal stability than chlorophylls. Early studies showed that, when spinach was blanched for canning at 71  °C for 20  min, better colour retention resulted. This occurred as long as the blanching temperature was kept between 54 °C and 76 °C. It was concluded that the better colour of processed spinach blanched under low-​temperature conditions (65 °C for up to 45 min) was caused by heat-​induced conversion of chlorophylls to chlorophyllides by chlorophyllases. However, the improvement in colour retention achieved by this approach was insufficient to warrant commercialisation of the process (Clydesdale et al., 1970). Activation of chlorophyllases takes place in spinach when it is blanched at 71  °C; however, when it is blanched above 88  °C, the enzymatic process is stopped due to the inactivation of the enzyme. This process is not commercially viable, as the temperatures used in this process are not high enough to inactivate micro-​organisms that may spoil the food over the product’s shelf-​life. However, it can be useful in culinary preparations.

High-​Temperature Short-​Time Processing

Transformation of Chlorophylls into More Stable Metallo-​Complexes

Commercially sterilised foods processed at a higher temperature for short times (HTST) are processed at a higher temperature than 100 °C for several minutes (Fennema, 1996). These products often exhibit better retention of vitamins, flavour and colour than conventionally processed foods. The greater retention of these constituents in HTST foods is because their destruction is more temperature dependent than the inactivation of Clostridium

In order to keep the bright green colour of chlorophylls, copper and/​or zinc complexes of chlorophyll can be synthesised. These metallo-​complexes of chlorophylls can be obtained by addition of an organic salt of copper and/​or zinc. However, due to the toxicity of copper salts, their use is limited, and maximum levels recommended in Codex Alimentarius of the GSFA (General Standard for Food Additives) are rarely above 500 mg/​kg.

155

Colour: Natural Pigments in Foods Current efforts to improve the colour of green processed vegetables and to prepare chlorophylls that might be used as food colourants have involved the use of either zinc or copper complexes of chlorophyll derivatives. Copper complexes of pheophytins are available commercially under the name Cu-​ chlorophylls. Their use in canned foods, soups, candy and dairy products is permitted in most European countries under regulatory control of the European Economic Community. The Food and Agriculture Organization (FAO) of the United Nations certified their use as safe in foods provided that no more than 200 ppm of free ionisable Cu is present. Chlorophylls are extracted from dried grass or alfalfa with acetone or chlorinated hydrocarbons. Sufficient water is added, depending on the moisture content of the plant material, to aid penetration of the solvent while avoiding activation of chlorophyllases. Some pheophytins form spontaneously during extraction. Copper acetate is added to form oil-​ soluble Cu-​chlorophyll. Alternatively, pheophytins can be acid hydrolysed before Cu2+ ions are added, resulting in the formation of water-​soluble Cu-​chlorophyllide (Fennema, 1996). The Cu complexes have greater stability than comparable Mg complexes; for example, after 25 h at 25 °C, 97% of chlorophyll degrades, while only 44% of the Cu-​chlorophylls degrades. The complex industrial process can also be carried out in a simple kitchen. The process involves cooking the vegetables to ensure complete pheophytinisation of chlorophylls in the presence of copper/​zinc salts or copper/​zinc-​rich foods (i.e., seafood, offal or dried yeast). In the latter case, the process is less efficient because copper and zinc ions are generally bound to proteins and are therefore less readily available for the reaction. It has been observed that, when vegetable purées are commercially sterilised, small bright-​green areas occasionally appear (Von Elbe et al., 1986). It was determined that pigments in the bright-​green areas contained Zn2+ and Cu2+, and the formation of bright-​green areas in vegetable purées was termed “regreening”. Regreening of commercially processed vegetables takes place when Zn2+ and/​or Cu2+ ions are present in process solutions. Okra’s retention of bright green colour when processed in a brine solution containing ZnCl2 is attributed to the formation of zinc complexes of chlorophyll derivatives. This process (known as the Veri-​Green process) involves blanching vegetables in water containing sufficient amounts of Zn2+ or Cu2+ salts to raise the tissue concentration of the metal ions to between 50 and 500 ppm (depending on the vegetable). Green vegetables processed in modified blanching water are greener than conventionally processed vegetables (Canjura et al., 1999).

Conclusions Pigments provide organoleptic properties often related to product quality. Thermal processing and other culinary processes have an impact on colour due to alterations in the chemical structure of the pigments. Knowing the mechanism of these chemical structure changes can lead to technological proposals to reduce the impact of these culinary processes. When these pigments are used in pure form in culinary formulations (such as Note by Note cooking), one must pay attention to these.

REFERENCES Azeredo HMC. 2009. Betalains: properties, sources, applications, and stability –​a review, International Journal of Food Science and Technology, 44(12), 2365–​2376. Belitz HD, Grosch W, Schieberle P (eds). 2004. Food Chemistry, Springer-​Verlag, Berlin-​Heidelberg. Blair JS. 1940. Patent US2189774A, March 1937, 1940. Buckle KA, Edwards RA. 1969. Chlorophyll degradation products from processed pea puree, Phytochemistry, 8, 1901–​1906. Canjura FL, Watkins RH, Schwartz SJ. 1999. Colour improvement and metallo-​chlorophyll complexes in continuous flow aseptically processed peas, Journal of Food Science, 64(6), 987. Cianci M, Rizkallah PJ, Olczak A, Raftery J, Chayen N, Zagalsky PF. 2002. The molecular basis of the colouration mechanism in lobster shell: β-​crustacyanin at 3.2-​Ả resolution, Proceedings of the National Academy of Sciences USA, 99(15), 9795–​9800. Clydesdale FM, Fleischman DL, Francis FJ. 1970. Maintenance of colour in processed green vegetable, Food Product Development, 4(5), 127–​130. Dangles O, Brouillard R. 1992. Polyphenol interactions. The copigmentation case: thermodynamic data from temperature variation and relaxation kinetics. Medium effect, Canadian Journal of Chemistry, 70, 2174–​2189. Dangles O., Fenger JA. 2018. The chemical reactivity of anthocyanins and its consequences in food science and nutrition, Molecules, 23(8), 1970. Ducauze CJ. 2006. Fraudes Alimentarios, Acribia, Zaragoza, Spain. Fennema OR. 1996. Food Chemistry. 3rd ed., Marcel Dekker Inc., New York. Heaton JW, Marangoni AG. 1996. Chlorophyll degradation in processed foods and senescent plant tissues, Trends in Food Science & Technology, 7, 8–​15. Kahn MI, Giridhar P. 2015. Plant betalains. Chemistry and biochemistry, Phytochemistry, 117, 267–​295. Lichtenthaler HK. 1987. Chlorophylls and carotenoids: pigments of photosynthetic biomembranes, Methods in Enzymology, 148, 350–​382. Rodriguez-​ Amaya DB. 2001. A Guide to Carotenoid Analysis in Food, International Life Sciences Institute (ILSI), Washington, DC. Scheer H. 1991. Chlorophylls and Chlorophyll Derivatives, CRC Press, Boca Raton, Florida. Schieber A, Stintzing FC, Carle R. 2001, By-​products of plant food processing as a source of functional compounds  –​recent developments, Trends in Food Science & Technology, 12(11), 401–​413. Stintzing FC, Carle R. 2007. Review betalains –​emerging prospects for food scientists, Trends in Food Science & Technology, 18(10), 514–​525. Schwartz SJ, Woo SL, Von Elbe JH. 1981. High-​performance liquid chromatography of chlorophylls and their derivatives in fresh and processed spinach, Journal of Agricultural and Food Chemistry, 29, 533–​537. Tanaka Y, Sasaki N, Ohmiya A. 2008. Biosynthesis of plant pigments: anthocyanins, betalains and carotenoids, The Plant Journal, 54(4), 733–​749. Valavanidis A, Vlachogianni T. 2013 Plant polyphenols: recent advances in epidemiological research and other studies on cancer prevention in studies in natural products chemistry. In Atta-​ur-​Rahman (ed.) Studies of Natural Products Chemistry. Elsevier, Amsterdam, The Netherlands, 269–​295.

156 Valverde J. 2008. Study of the modifications induced by various culinary and industrial treatments of pigment systems from immature pods of green beans (Phaseolus vulgaris L.): introduction of new analytical methods for the study of these systems, PhD dissertation of the University Paris 6, www. theses.fr/​2008PA066677, last access 5 December 2020.

Juan Valverde Von Elbe JH, Huang AS, Attoe EL, Nank WK. 1986. Pigment composition and colour of conventional and Veri-​Green canned beans, Journal of Agricultural and Food Chemistry, 34, 52–​54. Wrolstad RE. 2005. Handbook of Food Analytical Chemistry (1st ed.), Vol. 2, John Wiley & Sons, Inc., Hoboken, New Jersey.

Cooking Hervé This vo Kientza1,2 Université Paris-​Saclay, INRAE, AgroParisTech, UMR 0782 SayFood, 75005, Paris, France 2 Group of Molecular Gastronomy, INRAE-​ AgroParisTech International Centre for Molecular Gastronomy, F-​75005, Paris, France

1

Cooking? If it is “the activity or skill of preparing food” or “the process of preparing food by heating it” (Lexico, 2020; Stevenson, 2010), this whole handbook deals with it, and no particular description is needed. However, here, I  shall envision a general description of some thermal processes based on thermal transfer, before introducing an innovative method of cooking. Then I shall discuss the chemical basis behind changes triggered by the various thermal processes.

A Table for Innovation Let us first observe that the definition of cooking that includes heating is debatable, as we would certainly not consider as “cooked” a frozen chicken that was taken out from the deep freeze and put at room temperature: it would be heated, certainly, but the temperature for physical and chemical changes characteristic of what we consider “cooked” would not occur. Also, macerating some animal tissues in an acidic medium (lime juice, for example) gives a result that looks as if these tissues have been heated, but here, there would be no temperature increase (fish à la Tahitienne, ceviche, etc., for which the word “coction” was proposed) (This, 2001). Before considering the chemical changes occurring during thermal processing of plant or animal tissues, let us begin by observing that the various traditional methods for the processing of food include either thermal processing or another way of changing the chemical state of the tissues (This, 1997). Indeed, we can distinguish between giving energy to the inside of the food ingredients by conduction, and giving energy by radiation. In the former, the hot source can be a solid, a liquid (aqueous solution or oil) or a gas (air, steam). In the latter, it is traditionally infrared, with a limited depth being reached because of radiation absorption by the tissues, but of course, microwave radiation is now also almost traditional (and other sources producing energy in the form of visible light, such as lasers, could be used). For non-​ thermal processing, one can trigger some coagulation of proteins using acids and bases or ethanol, and also by the application of a high pressure, in “pascalization” or high-​pressure processing (Chauhan, 2019).

Of course, a classification such as the one given in the previous paragraph is very crude, because “hot” only means something in relationship with the various phenomena that can occur, as shown in the chapter dealing with eggs at 6X°C of this book, “Let Us Have an Egg” (This, 2009). But it is already useful, as it allows analysis of the various traditional cooking methods (This, 2014). For example: • grilling means heat transfer by conduction from a hot solid; • braising means heat transfer from a mildly hot aqueous solution; • boiling means heat transfer from a hot aqueous solution at 100 °C; • frying means heat transfer from a hot oil (with differences between deep frying and flat frying, the latter creating a unidirectional gradient); • baking means heat transfer from a hot gas (air); • steaming means heat transfer from a hot gas (steam); • roasting means heat transfer from infrared radiation; • microwaving means heat transfer from microwave radiation; • and, as said, transformations of tissues can occur after contact with acids (fruit juices and vinegar, for example), bases (lime in longevity eggs, for example), brandies (as in the “beaumés”; see the chapter on egg coagulation), sugar and salt, all creating “coctions”. All these processes could be discussed in more detail, but it is proposed now to observe that some of these “elementary”, or “unit”, processes can be applied in a row. For example, in braising, there is (traditionally) first the contact with hot air, and then a processing in a closed vessel, for which the heat transfer occurs from hot humid air at a temperature below 100 °C. Or for some frying processes, potatoes can be boiled before being fried, i.e., receiving heat from hot oil. What about envisioning a systematic table with the elementary processes in rows and in columns? This creates more than a hundred possibilities with very different results (Table 23.1). Let us observe that this general idea of coding processes was already put into action when considering the cooking of eggs (see 157

158

Hervé This vo Kientza

TABLE 23.1 Double Cooking: 144 Different Results Are Obtained When a Process Given in a Column Is Applied Before a Second Process from a Row Heat transferred from

Hot solid

Simmering aqueous solution

Boiling aqueous solution Hot oil

Hot air

Steam

Infrared

Microwaves

High pressure

Acids

Bases

Ethanol

Hot solid Simmering aqueous solution Boiling aqueous solution Hot oil Hot air Steam Infrared Microwaves High pressure Acids Bases Ethanol

TABLE 23.2 The Tree of Doughs Is Horizontal, Growing from Left to Right Flour and water

With fat

Not kneaded, but egg added

With yeast Without yeast

Kneaded, no egg

With yeast Without yeast

Without fat

Not kneaded, but egg added

With yeast Without yeast

Kneaded, no egg

With yeast Without yeast

this chapter) in a different manner. More abstractly, this means always having a number for a particular process, creating “formulas” that correspond to particular processes. Alternatively, the ingredients can also be described in this way, as in the “dough tree” that we are going to examine now (Table 23.2). The idea is to observe that doughs are generally made of flour and water. Some include fat (oil, butter), and some don’t. Some are leavened, and some are not. Some contain eggs, and some don’t. Some are steamed, and some are boiled. This can be arranged systematically as a “tree of doughs” (here from left to right) showing many possibilities that are not practised traditionally. Some names are given in the last column.

Boiled Steamed Boiled Steamed Boiled Steamed Boiled Steamed Boiled Steamed Boiled Steamed Boiled Steamed Boiled Steamed

Dampnudel

Echaudes

Knepfle Dry noodles Cornuau

In More Depth: Some Chemistry of Cooking The diversity of plant and animal tissues, and the diversity of processes applied to them, can appear infinite, but the processes are used because they trigger some particular changes, based on the molecular and physical characteristics of the food ingredients. Animal tissues, on the one hand, and plant tissues, on the other, belong to categories sharing physical and chemical characteristics. Most animal tissues are muscular tissues, made of grouped bundles of muscular fibres, chemically based primarily on water, proteins and fat (Girard, 1990). For plant tissues, the plant cell wall (with polysaccharides such as cellulose, hemicelluloses and

159

Cooking

pectins, the more hydrolysed they are during thermal treatment (Sajjaanantakul, 1989). More generally, β-​elimination is the primary process responsible for pectin degradation during thermal treatment at pH 6.1. However pectins can also be degraded by acidic hydrolysis (Krall and McFeeters, 1998). Methyl groups are needed for β-​elimination but not for acid hydrolysis. Polypectates (degree of methylation (DM) < 5%) can be degraded in acidic medium. For pH higher than 3.5, β-​elimination would be the primary process. Experimentally, the importance of β-​elimination for the texture of plant tissues has been studied, but with contradictory results. During thermal treatment, the softening of carrot (Daucus carota L.) tissues was described as a first-​ order kinetic mechanism (Paulus and Saguy, 1980) or due to two different mechanisms (Huang and Bourne, 1983), the first being the cause of pectin transformation in the middle lamella (responsible for 95–​97% of firmness loss). The two-​step loss of firmness was later observed again (Huang and Bourne, 1983), with a loss of firmness observed during the first 5–​8 min (60%) (Greve et al., 1994). β-​elimination is increased by higher temperatures: 1% solutions of lemon

pectins) surrounds the inside of cells, which are mainly composed of water and sometimes starch and fat (Bowes, 1988). For plant tissues, first, low temperature can cause hardening of the tissues through the release of calcium ions that will bridge pectins (Anthon and Barrett, 2006). But at higher temperatures: • starch will gelatinize (Véchambre et al., 2010); • amylose and amylopectin will be hydrolysed (Utrilla-​ Coello et al., 2014); • pectins will be hydrolysed (Figure 23.1); • some small saccharides can undergo intramolecular dehydration (Wunderlin et al., 1998; Lewkowski, 2001). In particular, pectin degradation is a very important phenomenon because of its contribution to the softening of plant tissues. Many studies have shown that this elimination process (E1 mechanism) occurs in two steps involving a carbocation intermediate (Neukom and Deuel, 1958). The process seems to be accelerated by the presence of a methyl group on the C5 of galacturonic acid residues (Albersheim et  al., 1960). The more methylated the

O

HO

OMe H

H H

O

O

OH

R

OH

OH

O

H

O H

R

β

O

H O

H

O

OH–

H

α

H H HO

H

O

H H

OMe

OH

H

HO

O

H

–

α O R

β

+H2O

Ö

O

O O

OMe H

H

O

O R

H

OH

O

H

O H

R OH

H

4

H H HO

OH

OH

H H OH 5

H

O

H O

HO

H

OH

O

H

OMe

FIGURE 23.1  Cooking plant tissues (and softening them) is associated, for example, with degradation of pectin (mostly polymers of galacturonic acid, or (2S,3R,4S,5R)-​2,3,4,5-​tetrahydroxy-​6-​oxohexanoic acid) through β-​elimination, a hydrolysis process due to breaking chemical bonds between galacturonic residues. (Keijbets and Pilnik, 1974)

160 (Citrus citrus) pectins have a viscosity that decreases from 14 to 13 or from 14 to 1 for thermal treatments at 35°C and 95°C, respectively. Moreover, the length of galacturonic acid chains drops from 350 units to 16 or 63 units after 1 h at 95 and 80 °C, respectively. Of course, pectin is not the only polysaccharide that can be hydrolysed during thermal processing. Whereas cellulose is highly heat resistant, starch (amylose and amylopectin) or proteins dissociate slowly with time, in particular when the environment is acidic, such as in meat or in most dishes (Belitz and Grosch, 1999a). These processes generate saccharides or amino acids, which can then react by processes such as dehydration of hexoses or Streker degradation (Belitz and Grosch, 1999b). For example, 5-​(hydroxymethyl)-​2-​furaldehyde (HMF) is formed by hexose dehydration (Wunderlin et al., 1998; Lewkowsi, 2001). For animal tissues: • water is not chemically transformed, but it can evaporate; • some proteins (actins, myosins) can “coagulate”, i.e., be denatured and link through intermolecular bonds such as disulfide bridges (see the chapter on “uncooking an egg”); • some other proteins are hydrolysed, such as collagen, which makes up the envelope of the muscular fibres and also creates bundles and superbundles of such fibres; this creates peptides and amino acids; • fat melts and can be oxidized (see the chapter on fat oxidation in this book). Of course, such a short description does not include all processes, and many culinary phenomena remain unexplained. For example, the formation of hydrogen sulfide during egg processing is readily observed (from the egg white, or from the yolk) using a piece of filter paper impregnated with a solution of lead acetate; it has been studied (Germs, 1973), but no publication explains why this process does not occur when eggs are thermally processed for more than 12 h at 65 °C. Also, “pyrolysis” mechanisms seem to be very important during culinary processes, as very high temperatures are obtained at the surface or even inside food products when water is evaporated. For example, the temperature under cubes of meat 5  cm wide was measured to be about 100 °C when the heating power was low, so that the flow of juice expelled because of collagenic contraction (Kopp et al., 1977) is enough to keep the lower surface moist; but the temperature under the meat can reach very high temperatures when the heating power overcomes water evaporation (temperatures as high as 290 °C were measured). Also, during the first step of shrimp bisque production, when shrimp shells are heated in oil, temperatures as high as 320 °C have been measured in a professional kitchen, which is much higher than the temperatures studied in model systems (Mar’in and Shlyapnikov, 1980). We invite the reader to consult the chapter on glycation reactions in this book. As a conclusion to this chapter, it should also be observed that the “most important” changes in terms of mass are not always the

Hervé This vo Kientza most important in terms of flavour, as exemplified by the evaporation of odorant compounds from a solution.

REFERENCES Albersheim P, Neukom H, Deuel H. 1960. Splitting of pectin chain molecules in neutral solutions, Archives of Biochemistry and Biophysics, 90, 46–​51. Anthon GE, Barrett DM. 2006. Characterization of the temperature activation of pectin methylesterase in green beans and tomatoes, Journal of Agricultural and Food Chemistry, 54, 204–​211. Belitz HD, Grosch W. 1999a. Food Chemistry, Springer Verlag, Heidelberg, Germany, 81. Belitz HD, Grosch W. 1999b. Food Chemistry, Springer Verlag, Heidelberg, Germany, 22. Bowes BG. 1988. Structure des plantes, INRA Editions, Paris. Chauhan OP (ed.). 2019. Non-​Thermal Processing of Food, CRC Press, Boca Raton, USA. Germs AC. 1973. Hydrogen sulfide production in eggs and egg-​ products as a result of heating, Journal of the Science of Food Agriculture, 24, 7–​16. Girard JP (ed.). 1990. Technologie de la viande et des produits carnés, Lavoisier Tec et Doc, Paris. Greve LC, McArdle RN, Gohlke JR, Labavitch JM. 1994. Impact of heating on carrot firmness: changes in cell wall components, Journal of Agricultural and Food Chemistry, 42, 2900–​2906. Huang J, Bourne MC. 1983. Kinetics of thermal softening of vegetable, Journal of Texture Studies, 14, 1–​9. Keijbets MJH, Pilnik W. 1974. Beta-​Elimination of pectin in the presence of anions and cations, Carbohydrate Research, 33, 359–​362. Kopp J, Sale P, Bonnet Y. 1977. Contractomètre pour l’étude des propriétés physiques des fibres conjonctives: tension iométrique, degré de réticulation, relaxation, Canadian Institute of Food Science and Technology Journal, 10(1), 69–​72. Krall SM, McFeeters FR. 1998. Pectin hydrolysis: effect of temperature, degree of methylation, pH, and calcium on hydrolysis rates, Journal of Agricultural and Food Chemistry, 46, 1311–​1315. Lewkowski J. 2001. Synthesis, chemistry and applications of 5-​ hydroxymethyl-​furfural and its derivates, Arkivoc, 1, 17–​54. Lexico. 2020. www.lexico.com/​ definition/​ cookery, last access 5 December 2020. Mar’in AP, Shlyapnikov YA. 1980 Thermal and oxidative thermal degradation of chitin, Vysokomol Soedin, Ser A, 22(3), 589–​594. Neukom H, Deuel H. 1958. Alkaline degradation of pectin, Chemistry and Industry, 683. Paulus K, Saguy I. 1980. Effect of heat treatment on the quality of cooked carrots, Journal of Food Science, 45, 239–​245. Sajjaanantakul T. 1989. Effect of methyl ester content on heat degradation of chelator-​soluble carrot pectin, Journal of Food Science, 54, 1272–​1277. Stevenson A. 2010. Oxford Dictionary of English, Oxford University Press, Oxford. This H. 1997. La cuisson, Pour la Science, 235, 14. This H. 2001. Lettre à la Secrétaire perpétuel de l’Académie française. This H. 2009. Molecular Gastronomy, a chemical look to cooking, Accounts of Chemical Research, 42(5), 575–​583. This H. 2014. Mon histoire de cuisine, Belin, Paris.

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Utrilla-​Coello RG, Hernández-​Jaimes C, Carrillo-​Navas H, González Wunderlin DA, Pesce SF, Ame MV, Faye PF. 1998. Decomposition of hydroxymethylfurfural in solution and protective effect of F, Rodríguez E, Bello-​Pérez LA, Vernon-​Carter EJ, Alvarez-​ fructose, Journal of Agricultural and Food Chemistry, 46, Ramirez J. 2014. Acid hydrolysis of native corn starch: 1855–​1863. morphology, crystallinity, rheological and thermal properties, Carbohydrate Polymers, 103(2014), 596–​602. Véchambre C, Chaunier L, Lourdin D. 2010. Novel shape-​memory materials based on potato starch, Macromolecular Materials and Engineering, 295, 115–​122.

Cooking: Culinary Precisions and Robustness of Recipes Hervé This vo Kientza1,2 Université Paris-​Saclay, INRAE, AgroParisTech, UMR 0782 SayFood, 75005, Paris, France 2 Group of Molecular Gastronomy, INRAE-​ AgroParisTech International Centre for Molecular Gastronomy, F-​75005, Paris, France

1

Culinary books include technical descriptions of dishes, artistic prescriptions and “social” advices. Within the technical part, one can distinguish a “definition” and added information, which were named “culinary precisions” (This, 2010). For French cuisine only, more than 25,000 of them were collected, and many were tested over the last decades. A proportion of them are conveying incorrect technical information, and it would be an improvement for culinary education to get rid of them. Because of their huge number, an international collaboration is needed. This recognition did not occur immediately after the creation of molecular and physical gastronomy, and it calls for an analysis of the history of both culinary practices as well as food science and technology. Indeed, in culinary books of the past (and still today), there are remnants of old magical ideas, such as when it is written that women having periods prevents the making of emulsions such as mayonnaise sauce (Thiebaut, 2017). And in food science and technology, one can remember that, some decades ago, it was suggested that mayonnaise (again!) could be made better in a copper vessel using an iron whisk, because there was a “battery effect” (This, 2010). The fact is that molecular and physical gastronomy was partly based on the investigation of such ideas: culinary circles have been distributing incorrect explanations of phenomena and using out-​ of-​ date scientific information. This view goes back several centuries: since the beginning of the modern natural sciences, their application to phenonema occurring during food preparation (“cooking”) has been considered. For example, in 1560, Ambroise Paré introduced the word “emulsion” (from the Latine word emulgere, “to draw milk”) to describe “thick white products”, as he worked on what we would call today drug formulations (TLFi, 2018). In 1783, the French chemist Antoine-​Laurent de Lavoisier published the result of his studies of meat stock (Lavoisier, 1783), quoting the French pharmacist Claude Joseph Geoffroy, also called Geoffroy le Cadet, who scientifically studied such preparations as early as 1730 (Cadet de Vaux, 1818; Geoffroy, 1733; This et al., 2006). Studies of fat by Michel-​Eugène Chevreul led him to the discovery of the chemical constitution of triglycerides (Chevreul, 1823). Discussing whether all this was science or technology is not beyond the scope of this chapter (This and Kurti, 1994), because it is related to what is called “culinary precisions”.

Unquestionably, many chemical and physical phenomena occurring during culinary transformations were studied by food science or food technology before 1988, when the concept of molecular and physical gastronomy was introduced (sometimes shortened to “molecular gastronomy”) as a discussion during the preparation of the first International Workshop on Molecular and Physical Gastronomy. However, it is a fact that, in the 1980s, research in food science and technology generally neglected culinary processes. In France, the microbiologist Edouard Pojersky de Pomiane interpreted (without scientific experiments) culinary practices in the second half of the 20th century, but he also promoted incorrect ideas, such as that you could avoid crying when you peel onions if you bite on a wooden spoon. Japan was also an exception, with articles published (in Japanese) in the Japanese Journal of Home Economics and in the Japanese Journal of Cookery Science, but this work was mostly ignored by the English-​speaking scientific community. In the 1980s, “food chemistry” textbooks, such as the classic Food Chemistry (Belitz and Grosch, 1999), contained almost nothing about culinary transformations; even as late as 1999, most of the chapter on meat described either meat composition and structure, or industrial products (e.g., sausages, meat extracts), but nothing was said about braisés, sauté, roasts and all other preparations including meat. Also, less than 0.5% of the text considered “culinary phenomena” (such as meat shrinkage during heating because of collagen denaturation). In addition, nothing was said about the effect of thermal processing on wine, despite the wide use of this beverage in culinary activities; 48% of French classical sauces contain wine (This, 2015). Probably because culinary transformations are complex, and also because the food industry did not support studies outside its field, food science and technology had drifted slowly toward the science of ingredients and toward technological questions, neglecting phenomena that occur when making home or restaurant dishes, such as cassoulet, goulash, hollandaise sauce, etc. It was even considered a conspicuous eccentricity when an article on bearnaise was published in a scientific journal in the 1970s (Perram et al., 1977). This is why the late Nicholas Kurti (1908–​1998), former professor of physics in Oxford (This, 1999), and I decided in March 163

164 1988 that a “new scientific discipline” was to be created under the name “molecular and physical gastronomy”. The choice of “gastronomy” in this title was obvious, as it means “knowledge” (Brillat-​Savarin, 1825). Personally, I had the feeling that the “and physical” part of the name was useless, but Kurti, being a physicist, insisted on keeping it, because he thought that there would be too much emphasis on chemistry with only “molecular gastronomy”, and perhaps also because he wanted to escape from a former proposal (which I did not know about) by the American cook Elizabeth Thomas to name all culinary activities based on scientific explorations “molecular gastronomy” (Lersch, 2018). The objective of molecular and physical gastronomy was and remains scientifically clear; if one wants to discover new phenomena, which is the goal of the natural sciences, the exploration of a new range of phenomena is probably a better strategy than looking to already well-​considered objects. Indeed, at the beginning of molecular gastronomy, Kurti was upset by the technically old-​fashioned way of cooking, and he insisted on transferring ideas from physics into the kitchen, which was technology; he felt, however, that knowledge of the mechanisms was needed. Personally, from March 1980, I thought that it was necessary to collect and to test rigorously “old wives’ tales” and analogous pieces of information. More generally, we both thought that culinary traditions should be scientifically analysed, and because our ideas on science and technology were not clear enough, we introduced the following faulty scientific programme (This, 1995; This, 2002): (1) collect and test old wives’ tales; (2) model recipes; (3) introduce new tools, products and methods; (4) invent new dishes using the three previous steps; (5) use the appeal of food in order to promote science. It is easy to see, today, that this “scientific programme” was not clear, and it is surprising that nobody noticed: if aims (1) and (2)  are really scientific, items (3)  and (4)  are technological applications of the first two, and (5) is the educational application of the first four. As it had been clear for Kurti and me since the beginning of our discussions that molecular and physical gastronomy was a scientific discipline, and not a technology, a new programme had to be designed. Considering recipes, it appeared in 2003 that the technical information in any traditional recipe includes two parts. The first one is a “definition”: for example, a soufflé is a foamy product that expands during cooking and goes down as it is opened (otherwise it is a cake); a mayonnaise sauce is an emulsion obtained with only egg yolk, salt, pepper, vinegar and oil, etc. (This, 2005a). Generally, culinary definitions are given as protocols, and they are mixed with “culinary precisions”, i.e., added technical information that is not absolutely necessary for making the dish but conveys ideas of technical improvement. Such pieces of information were often called old wives’ tales, lore, ways of doing, tricks, sayings, adages or maxims. It was recognized that all these terms were too many, and that one term could encompass all of them: “culinary precisions”. Finally, any recipe can be considered to have three main parts: (1) one “definition”, (2) “culinary precisions” and (3) commentaries about the art or social components of the culinary practice (This, 2003). Let us examine, for example, a recipe from a culinary book published in France at the beginning of the 20th century (Anonymous, 1905):

Hervé This vo Kientza Take a dozen pears of middle size, remove the skin and put them immediately in cold water. Then melt 125  g of sugar with some water in a pan at low heat: as soon as the sugar is melted, add the pears, add some lemon juice if you want to keep the pears white; if you prefer them red, do not add lemon juice and cook them in a pan lined with tin. In this recipe, the words in bold are enough to give the definition of the dish (this definition here is less than 10% of the recipe). The words in italics add “precisions”. Here, there is no “third part” (art, social). However depending on the recipe and the author, the precision ratio of recipes can vary considerably; for example, in some recipes from the French chef Jules Gouffé (Gouffé, 1867), the percentage of culinary precisions is nil. Making this difference between culinary definition and culinary precisions was the basis for an improved scientific strategy for molecular and physical gastronomy: (1) modelling definitions and (2) exploring precisions. However, this new programme was rapidly discovered to be insufficient, because the main point in culinary practice is to produce “good” dishes; this is art, and not technique, because “good” means “beautiful to eat”. Moreover, even technically successful and artistically well-​designed dishes are not appreciated if the guests are neglected or treated poorly, so that the “social component” of culinary practice also needs to be considered. Of course, the natural sciences cannot have the last word on such topics, but evolutionary biology of psychology, for example, can explain a lot about human behaviour and, accordingly, about culinary practice. Today, the scientific programme of molecular and physical gastronomy could be more appropriately stated as: (1) explore scientifically the technical part of cooking (definitions and precisions); (2)  explore scientifically the art component of cooking; (3) explore scientifically the “social link” component of cooking. Now that this scientific programme is clearer, what is the most rational way of exploring the field of culinary phenomena? As culinary transformations are dynamic processes involving systems with structure (Dickinson, 2006), it is natural to make complementary descriptions of the physical state, on one hand, and of the chemical state, on the other. The bioactivity (organoleptic, nutritious or toxic) of such systems is considered later as the result of the two states (This, 2012).

Testing Culinary Precisions We now see why culinary precisions (in short, “precisions”) are important for molecular and physical gastronomy. Since the 1980s, a lot of precisions have been tested, and the number of precisions now collected from French culinary books alone is more than 25,000. Many have been given online (This, 2019a) or discussed within the framework of a yearly course on molecular and physical gastronomy (This, 2019b); some have been analysed in a book (This, 2010). The open questions are many, and we give some examples in the following. Is it true that pears (Pyrus communis L.) stay white when lemon (Citrus citrus L.) juice is added, in a pear jam? The answer is yes, and it is well understood that ascorbic acid

Cooking: Culinary Precisions ((R)- ​ 5 - ​ ( (S)- ​ 1 ,2- ​ d ihydroxyethyl)- ​ 3 ,4- ​ d ihydroxyfuran-​ 2 (5H)-​ one) inhibits o-​diphenol O2 reductase enzymes (EC 1. 10. 301, PPO) (Zawistowski et  al., 1991). When pears and other fruits are cut, these enzymes transform the released phenolics, such as chlorogenic acid and (−)-​epicatechin, into reactive quinones, which can in turn polymerize into dark pigments (Grotte et al., 2000; Goupy et al., 1995). Is it true that pears turn red when cooked in tin-​covered copper pans? During public lectures and laboratory experiments with Valérie Michaud, we performed numerous tests with common pears (Passe Crassane, Williams, Red Williams, Comice, Conference …) and never observed the expected red colour. Model tests using tin ions did not produce the red colour, but such a colour was obtained for many varieties when the pH was lower than 2 (Figure 24.1). This is attributed to anthocyanidins from the fruits, which turn red in an acidic medium (UV-​visible spectroscopy: λmax = 522 nm) (Belitz and Grosch, 1999). Colouration is particularly deep near the pear skin, where anthocyanidins are more concentrated. Is it true that mayonnaise sauce fails when made by menstruating women? This precision has been tested experimentally and was proved wrong (of course!). Is it true that mayonnaise sauce fails when made during a full moon? Students at Tours University (MST Le goût et son environnement, promotion 2000–​ 2001) tested this old wives’ tale, and the first mayonnaise that they made failed; i.e., a phase separation occurred after the addition of oil. However, the same students were told to repeat the process and got well-​formed emulsions. Of course, this means only that the students did not perform well the first time, probably adding oil too rapidly, but anyway, as only one counter-​example is enough to refute a general law, it can be said that the culinary precision about the influence of the moon is wrong. Is it true that for making mayonnaise sauce, eggs and oil have to be at the same temperature? Experiments were performed with eggs at 4 °C and oil (sunflower) at 24 °C, and with eggs at 24 °C and oil at 4 °C, and emulsions were obtained. Indeed, there is no

FIGURE 24.1  Pears of the Passe Crassane variety were cooked with water (same weight as fruits) and sucrose (same weight as fruits). For some, Sn2+ ions were added, and the colour did not change much (centre). On the other hand, a red colour appeared when the pH was low (left and right).

165 reason why the sauce could fail in all the circumstances described by old wives’ tales, as it is only an emulsion, i.e., a dispersion of oil droplets in the water of the yolk and vinegar, with proteins and phospholipids from the egg stabilizing (it would be more appropriate to write “metastabilizing”, as emulsions are not thermodynamically stable) the droplets. Finally, tests of culinary precisions performed since 1980 show that all possibilities arise: (1) some precisions seem wrong, and they are wrong; (2) some seem wrong, and they are true; (3) some seem true, and they are wrong; and (4) some seem true, and they are true. We shall now give a brief example of each, adding a fifth class, of uncertain precisions (5). (1) As we demonstrated, it does not seem true, and indeed it is not true, that women’s menstruation prevents them from preparing mayonnaise, as has been proposed in France. Indeed, it is strange that this culinary precision is widespread in France yet not in England (on the other hand, in England, menstruating women should not rub meat with salt) (Kurti, 1995) or in other countries. This demonstrates how much cooking is rooted in culture, and why comparative molecular gastronomy across countries and cultures could be an interesting project (This, 2006). (2) In 1994, the question of whether the skin of suckling pigs crackles more (i.e., becomes more crispy) when the head of the pig is cut off immediately after being roasted was examined (This, 1994). This advice seemed intuitively wrong, but it was proven to be true. The culinary precision was found in L’Almanach des gourmands from the French gastronome Alexandre-​ Balthazar Grimod La Reynière: “suckling pigs should have the head cut immediately when the pigs are taken out from the oven, otherwise their skin softens”. The same advice is found in many other culinary books. For example, the French chef Marie Antoine Carême indicates that a cut should be made around the neck (“When you are ready to serve, you separate immediately, with the tip of the knife, the skin of the neck, so that the skin says crisp, which makes most of the interest of roasted sucking pigs”). These remarks are strange, as in roasted pigs, no fluid seems to exchange between the head and the skin; it was highly unlikely that the advice was true, but the experiment was performed (public experiment at Saint-​ Rémy-​l’Honoré, Yvelines, France, July 7, 1993) with four suckling pigs from the same parents, reared together on the same farm, weight 7.1–​7.3 kilograms, cooked on a large outside fire from 4.00 PM to 9.00 PM, with one head being cut off for each pair of pigs. Blind tasting for 143 people showed that the skin of pigs with head cut was crispier. The mechanism behind this was easily discovered, as it was observed during cooking that a stream of steam was escaping one pig from a hole made during the preparation. This means that heat is causing water to evaporate from the surface of the meat during cooking, making the crust, and that vapour formed inside the meat is not enough to compensate for the loss of surface water. When the pigs are no longer being heated, the crust softens if vapour goes through; cutting the head prevents vapour perfusion, as it escapes through the opening. (3) It is said that a pan in which green beans are cooked should not be covered, as this would entrap volatile acids, which would promote pheophytinization of chlorophylls, but tests show that

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there is no colour difference. The idea seemed true, but it is wrong (Valverde, 2008). (4) It is sometimes said that the soufflés should be made from very firm whipped egg whites, added to a viscous preparation. It was demonstrated that this precision holds, as vapour bubbles formed in the bottom part of soufflés during cooking escape less easily through a firmer foam (This, 2002; see also the chapter on Expansion in this book). The advice seemed true, and it is true. (5) Let us now discuss a fifth class, with culinary precisions having a non-​clear-​cut status. For example, it was said by the French chef Pierre Gagnaire (This, 2005b) that wine sauces are more “brilliant” (in French, this would mean shiny or luminous) when shaken than when whipped. Such a declaration, even by a famous chef, should be considered with caution, because in many circumstances experiments have shown that cooks were influenced by tradition rather than experiments and facts. In such cases, one has to carefully define the question, because it would be useless to make tests in conditions different from the ones in which an observation is made; in particular, the production of wine sauces depends both on the authors and on the period in the history of cooking. The “wine sauces” discussed by Gagnaire are made from a veal “fond”, wine and butter. The “fond” is a solution obtained by grilling veal bones until they have a brown (not black) colour. Then water, carrot (Daucus carota L.) roots, onions (Allium cepa L.) bulbs and possibly other plant tissues are added. A thermal treatment at a temperature lower than 100 °C

for some hours (between 2 and 20, depending on the author, but also on each particular sauce) is achieved. Then, the fond is filtered, and its volume is reduced by boiling to about one-​tenth of its initial volume, after which red wine is added. The sauce is reduced again, and red wine is again added, before butter is added while the sauce is heated at a temperature lower than 100 °C, so that there is no boiling (when details are not given here, it has to be understood that the cooks themselves can make changes depending on any particular sauce; e.g., the exact quality of the “red wine” is not considered, and any red wine can be used). In our studies, the sauce was first modelled by a system containing distilled water (instead of stock and wine), gelatine (because gelatine is extracted in the first steps of fond production) and butter (approximate quantities of 100 mL water, 6 g gelatine and 60  g butter, based on preliminary analyses). The initial mixture of water and gelatine was divided into two parts, and the same quantity of butter was added to each. Then, the model sauce was heated and either shaken (the pan was moved forward and back over a distance of 5 cm, 23 times in 10 s, for 65 s) or whipped using a whisk (four whipping movements per second). Initial (triangular) visual tests with 52 people did not detect any difference in visual appearance. However the observation of the model sauces using optical microscopy (microscope Meiji Techno ML Série 2000, Model ML2300) showed clear differences (Figure 24.2) that were characterized.

2.0 20

1.5 ×106

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FIGURE 24.2  Wine sauces with the butter emulsified by whipping with a whisk (left) or by shaking the pan (right). Under each picture, a histogram of the diameters of fat droplets is given.

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Cooking: Culinary Precisions These differences can be explained by the fact that the energy given to the sauce is very different when the sauce is shaken or whipped. This energy is used to increase the surface energy of the emulsion and, hence, the size of the melted butter droplets dispersed in the water phase. At that point, it should be concluded that, if there is no difference in “brilliancy” (or shining), there is, however, a difference in flavour, as it was demonstrated for these model emulsions that the composition of odorant molecules above an emulsion differs depending on the microstructure of the emulsion, where composition is constant (Relkin et al., 2004).

As long as the parameters vary within certain limits (xi, min < xi < xi, max, yj, min < yj < yj, max), the recipe is successful: a product is the result of a successful recipe if it is associated with a point inside a limited hypervolume in the multidimensional space of the parameters; when the representing point is outside the recipe hypervolume, the recipe is considered as failed. For each parameter of the recipe, the size of the interval [xi, min, xi, max] can be a measure of the robustness, but, in order to get a non-​dimensional value that can be compared with others, we need to divide xi, max − xi, min by a number having the same units. We proposed to normalize by the uncertainty i(xi) on the considered variable xi: ρi = Δ xi /​  i(xi)

Reasons for Culinary Precisions: The Robustness Assumption Why did precisions arise in the past? Considering the empirical way culinary technique developed, we assume that failures and successes generated assumptions concerning the experimental protocol used. For example, in the old recipe for emulsion given now (Bernardi et  al., 1853), the inverse order of ingredients should have frequently led to failure: Green Rémolade. Take a handful of chervil, tarragon, you will blanch these herbs that are called Ravigote; press and grind, add salt, pepper, mustard: grind all together, then add half a glass of oil that you amalgamate with the ravigote and mustard; finally you add two or three egg yolks. Such a process is strange; the authors are describing an emulsion but they add the surfactants (proteins and phospholipids from the yolk) at the end (happily, there are some phospholipids and proteins, as well as water, in the ground herbs!). The frequent failure of such a method could have led them to investigate the causes of the irregularity of the process, and culinary precisions should have arisen naturally. This observation leads to a prediction: if it is true that precisions arise from failures, then an inverse quantitative relationship should exist between the “robustness” of a recipe and the number of precisions written in culinary books. In order to test experimentally this theoretical prediction, “robustness” has first to be made quantitative. Let us consider that a recipe R is a function of many variables: various times (t1, t2 …), temperatures (T1, T2 …), quantities of ingredients (m1, m2 …) and very general details of process (p1, p2 …). For example, in a mayonnaise recipe, the process can be described by the amount of egg yolk (a parameter including water content, protein content and phospholipid content), the amount of vinegar (i.e., primarily water), the rate of oil addition and the energy of whipping. A product P obtained through the recipe using particular conditions is given by the equation:

Of course, orders of magnitudes have to be calculated instead of exact values, as the uncertainty is only known by estimation. For example, mayonnaise can be defined by the mass of yolk, m(y), the mass of vinegar, m(v), the mass of oil, m(o), the mass of salt, m(s), the mass of pepper, m(p), the mass of oil in each successive addition, m(d), the whipping power, Pw, and the efficiency of dispersion, Ed. As the critical parameter is clearly the oil addition, let us focus on robustness related to oil addition; at the beginning of mayonnaise preparation, oil should not be added too fast, or a water-​in-​ oil (W/O) emulsion is obtained instead of an oil-​in-​water (O/W) emulsion. As the quantity of water from one yolk and one teaspoon of vinegar is about (15 g + 5 g = 20 g), and considering the uncertainty on the oil quantity added each time (estimation based on experiments 7.5 g), robustness related to oil addition is equal to 20/​7.5 = 2.7. In more “robust” recipes, such as beef meat roasted in the oven, the calculated robustness is bigger; for a piece of meat of mass 1 kg, cooked at 180 °C for a time between 20 and 60 min, assuming that the precision on time measurement is 5 min, robustness is equal to (60 − 20)/​5 = 8. If the cooking temperature is lower (e.g., 70 °C), then the cooking time interval would be still bigger, and robustness higher; the time interval could be estimated to be between 60 min and 1 day, so that the robustness is equal to 1440/​5 = 276. For some recipes, parameters are not independent, and success is obtained only if more than one condition is simultaneously verified. Particular robustnesses have to be aggregated. In order to consider this, let us assume that robustness is inversely related to the number of precisions: ρi = 1/​ni. If the n is the total number of precisions: n = n1+n2+ … +nm. Then

P = R (t1, t2, …, T1, T2, …, p1, p2, …).

ρ = 1/​(n1 + n2+ …) = 1/​(1/​ ρ1 + 1/​ρ2 + …).

Or, more generally, P = R(xi, yj), xi being parameters describing the ingredients, yj the parameters describing the process, and i and j being integers from 1 to n and m, respectively.

Does the inverse relation hold? In the corpus of precisions that we have collected since 1980, there are 105 paragraphs about mayonnaise preparation, compared with 12 paragraphs for roasts.

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FIGURE  24.3  Variation of robustness as a function of the number of culinary precisions. (a) No regular variation appears, but if the last point is eliminated, then the relationship can be fitted to a 1/​n curve (b). The last point corresponds to meat stocks, recipes that are robust but have been very important in the history of human food, because they are a way to keep all the nutrients from animal tissues during cooking.

In Figure 24.3, we show how the robustness ρ depends on the number of paragraphs containing precisions for grated carrots, stock, soufflé, boiled eggs, gougères, mayonnaise and roast beef. In Figure 24.3a, stock is included, and the curve does not correspond to an inverse relation: stock generated many precisions only because of its culinary importance, even if there is almost no risk of failure. In Figure 24.3b, stock has been excluded, and the relationship is more as expected. More work now needs to be done to test our assumption, using the aggregation of partial robustness, for example, and also the hyperspace needs to be explored more thoroughly.

As we said earlier, molecular and physical gastronomy developed initially through international workshops, but, after 1999, these meetings were complemented with monthly seminars and also courses, numerous lectures, articles and books. Today, more and more associations of molecular and physical gastronomy are in existence in various countries, making an international network of scientists in universities and research centres. Each country can focus its study on its own particular definitions and precisions, so that we can look forward to a time when “comparative molecular and physical gastronomy” will be possible. Of course, some assumptions on the origin of such precisions can be made on the basis of “robustness of recipes”, but a more comprehensive collection of culinary precisions associated with time periods would help to investigate such cases. As we said, dozens of thousands of culinary precisions were collected for French culinary books alone, but it would be scientifically interesting for other countries than France to do the same work, as it would allow some comparative analysis. The disperse system formalism (DSF, see the chapter on this in this book) also could be useful in this regard; in the same way as it was used for studying the evolution of the number of physical categories of sauces, it could be applied to comparing sauces between countries and thereby understanding cultural influences and transfers. This would also have educational interest, perhaps helpful in view of the current pandemic of obesity. Even in Crete, where the famous Cretan diet originated, up to one-​third of children aged 12 are now overweight or obese (WHO, 2013). Other important factors include the increasing concern for the environment, the increasing proportion of the population living in cities, and energy issues. Finally, there is a growing gap between the world of science and laypeople, along with a disaffection for scientific studies, which could easily be bridged by considering food, the most complex scientific process most people engage in on a daily basis, as a scientific phenomenon. All these data lead toward the idea that children should receive more information about food and food preparation. In particular, health programmes promoting a healthy diet cannot be successful if people cannot rationally choose the food they eat. In order to adapt food to particular cases, citizens need clear information. However, tradition is no guarantee of healthy food or rational preparation of food. This is why educational activities such as “Ateliers expérimentaux du gout” (This, 2001) and other programmes were introduced in France, linking cooking and science at school. Some of these programmes include fieldwork by children that contributes to the collection of culinary precisions in order to create a national database of such precisions. No doubt, all countries could do the same.

REFERENCES Anonymous. 1905. Le livre de cuisine de Tante Colette, François Tedesco, Paris, France. Belitz HD, Grosch W. 1999. Food Chemistry, Springer, Heidelberg, Germany.

Cooking: Culinary Precisions Bernardi M, Viart M, Fouret M, Délan M. 1853. Le cuisinier national de la ville et de la campagne (ex Cuisinier royal), augmenté de 200 articles nouveaux, Gustave Barbu, Paris, France, 49. Brillat-​Savarin JA. 1825. La physiologie du goût, Brillat-​Savarin, Paris, France. Cadet de Vaux AA. 1818. De la gélatine et de son bouillon, L. Colas fils, Paris, France. Dickinson E. 2006. Colloid science of mixed ingredients, Soft Matter, 2, 642–​652. Geoffroy le Cadet M. 1733. Mémoires de l’Académie royale, histoire de l’Académie royale des sciences, année MDCCXXX, Pierre Mortier, Paris, France, 312. Gouffé J. 1867. Le livre de cuisine, Henri Veyrier (new edition 1988), Paris, France. Goupy P, Amiot MJ, Aubert S, Nicolas J. 1995. Apple phenolic compounds and enzymatic oxidation in model solutions by apple polyphénoloxydases, in Scalbert A., Ed., Polyphenols 94, Inra editions, 183–​184. Grotte M, Duprat F, Loonis D, Pietri E. 2000. Aspects des meurtrissures des pommes, Science des aliments, 20, 575–​590. La Varenne PF. 1651. Le cuisinier françois, Pierre David, Paris. Lavoisier AL de. 1783. Œuvres complètes (1862–​1893), Imprimerie Nationale, 3, 563–​578. Lersch M. 2018. https://​blog.khymos.org/​molecular-​gastronomy/​history/​, last access 27 November 2018. Perram CM, Nicolau C, Perram JW. 1977. Interparticle forces in multiphase colloid systems: the resurrection of coagulated sauce béarnaise, Nature, 270, 572. Relkin P, Fabre M, Guichard E. 2004. Effect of fat nature and aroma compound hydrophobicity on flavour release from complex food emulsions, Journal of Agricultural and Food Chemistry, 52(20), 6257–​6263. Thiebaut E. 2017. Ceci est mon sang, La Découverte, Paris, France. This H. 1994. La cuisson: usages, tradition et science, in La cuisson des aliments, 7 e rencontres scientifiques et technologiques des industries alimentaires, Agoral, 94, 13–​21. This H. 1995. La gastronomie moléculaire, L’Actualité chimique, 42–​46. This H. 1999. Froid, magnétisme et cuisine: Nicholas Kurti (1908–​ 1998, membre d’honneur de la SFP), Bulletin de la Société française de physique, 119, 24–​25. This H. 2001. Les Ateliers expérimentaux du gout, Presse de la Sorbonne, Paris, France.

169 This H. 2002. Molecular gastronomy, Angewandte Chemie, International Edition in English, 41(1), 83–​88. This H. 2003. La gastronomie moléculaire, Sciences des aliments, 23(2), 187–​198. This. 2005a. Modelling dishes and exploring culinary “precisions”: the two issues of Molecular Gastronomy, British Journal of Nutrition, 93(4), S139–​S146. This H. 2005b. Séminaire INRA de gastronomie moléculaire N°49: le vannage des sauces les rend-​elles plus brillantes?, 15 September 2005. This H. 2006. When shall we have comparative Molecular Gastronomy? Japanese Journal of Cookery Science, 4(2006). This H. 2010. Les précisions culinaires, Cours de gastronomie moléculaire N°2, Editions Quae/​Belin, Paris. This H. 2012. Solutions are solutions, and gels are almost solutions, Pure and Applied Chemistry,  1–​20. This H. 2015. Gastronomie moléculaire. Applications. Techniques de l’ingénieur, September 2015. www.techniques-​ingenieur.fr/​ base-​documentaire/​procedes-​chimie-​bio-​agro-​th2/​biochimie-​ alimentaire-​ a nalyses-​ e t-​ a limentation-​ h umaine-​ 4 2470210/​ gastronomie-​moleculaire-​f1016/​, last access 22 August 2018. This H. 2019a. http://​blogs.inra.fr/​herve_​this_​cuisine/​, last access 16 November 2019. This H. 2019b. www2.agroparistech.fr/​podcast/​-​Gastronomie-​ Moleculaire-​.html, last access 16 November 2019. This H, Kurti N. 1994. Physics and chemistry in the kitchen, Scientific American, 270(4), 44–​50. This H, Méric R, Cazor A. 2006. Lavoisier and meat stock. Comptes rendus de l’Académie des Sciences Chimie, 9, 1510–​1515. TLFI. 2018. Emulsion, http://​stella.atilf.fr/​Dendien/​scripts/​tlfiv5/​ search.exe?23;s=2841039660;cat=0;m=%82mulsion, last access 22 August 2018. Valverde J. 2008. Study of the modifications induced by various culinary and industrial treatments of pigment systems from immature pods of green beans (Phaseolus vulgaris L.): introduction of new analytical methods for the study of these systems, PhD dissertation of the University Paris 6, www. theses.fr/​2008PA066677, last access 5 December 2020. WHO. 2013. Data and statistics, www.euro.who.int/​en/​health-​topics/​ noncommunicable-​diseases/​obesity/​data-​and-​statistics, last access 6 December 2020. Zawistowski J, Bibarderis CG, Eshin NAM. 1991. Polyphenol oxidase. In Robinson DS and Eshin NAM (Eds.) Oxidative Enzymes in Food, Elsevier Applied Science, 217–​273.

Cryogenics in the Kitchen Peter Barham H H Wills Physics Laboratory, University of Bristol, Bristol, BS8 1TL, United Kingdom

Probably, most foodies’ introduction to the use of cryogens (substances that change state at low temperatures, typically well below ca. −60 °C) was to see an ice cream or sorbet being frozen at the tableside using liquid nitrogen. As the cold liquid is added to the ice cream mix, the bowl overflows with white clouds of fog, creating the appearance of an alchemist’s cauldron and setting up expectations of a wonderful dish to come. But although many restaurants do indeed use liquid nitrogen in this way, there are far more uses that occur behind the scenes that customers never see.

A Short History of How Cryogens Came into Restaurants The use of liquefied gases in food preparation has a long and interesting history, much of which involves the regular ‘rediscovery’ of particular uses over time. The first recorded description of the use of a cryogen came very soon after gases were first liquefied. However, it was only recently that Myhrvold and Young (2001) managed to unearth the full history of making ice cream using nitrogen. As recently as 1994, in their Scientific American article, Kurti and This noted that, despite many people who said they had heard of someone making ice cream in this way, the first time they could be certain it had happened was when I  demonstrated the process at one of the Erice Workshops on Molecular and Physical Gastronomy (Kurti and This-Benckhard, 1994). However, the story is much more interesting and is full of strange twists and turns as the wheel kept on being invented and re-​invented over a century. In her magazine The Table, A.B. Marshall wrote: “Persons scientifically inclined may perhaps like to amuse and instruct their friends as well as feed them when they invite them to the house. By the aid of liquid oxygen [sic], for example each guest at a dinner party may make his or her own ice cream at the table by simply stirring with a spoon the ingredients of ice cream to which a few drops of liquid air has been added by the servant” (Marshall, 1901). Air was first liquefied in 1877 (Dumas, 1877) by Louis Paul Cailletet in France and Raoul Pictet in Geneva (but in very small quantities), but it was only in 1884 that James Dewar

managed to prepare enough liquid air that he could show it to the public. By 1894, Dewar was ready to make a number of public demonstrations of cryogenic gases including whole air, nitrogen and oxygen. On 19 January 1894, he gave a particularly well-​ attended lecture, which is when it has been surmised, but not proven, that Marshall may have seen liquid air added to water to make it freeze quickly and got the idea for the article she later published in her magazine (Marshall et  al., 1988). One of the reasons why the achievements of Marshall are not as well known as they probably should be is that after her death in 1905, her estate was sold to Isabella Beeton’s publisher Ward Lock, who had little interest in keeping her work in print. In 1957, William Harrison in the USA patented a process to make nitrogen ice cream using pretty much the same method described by Marshall, but no records have been found as yet showing anyone actually using this patent, and it seems to have been forgotten for many years. Then, in 1974, William Chamberlain, working in the USA, took some liquid nitrogen home and played with it to try to make ice cream; after a number of failures, he settled on a technique, adding nitrogen to an ice cream mix that was being stirred in a food mixer on a stand. This proved successful (and is the technique used in most nitrogen ice cream parlours today), but although he gave a number of parties where he made ice cream for his friends and colleagues, again, it appears to have been forgotten. In 1976, after seeing a demonstration of using nitrogen to freeze bovine sperm, André Daguin, the chef of the restaurant Jardin des Saveurs, started using nitrogen to make ice cream at the tables of his guests; this spectacular dish gained a lot of publicity, which continued into the early 1980s, but no other chefs took it up, and once again it was largely forgotten (see the chapter by Daguin in this book). During the early 1980s, as a result of a drive to encourage school pupils to take up studying science, scientists started giving ‘inspirational’ talks in schools. I  had used a demonstration of making ice cream with liquid nitrogen to teach undergraduates about entropy and adapted this in 1983 to give a series of school talks. These soon expanded rapidly as more young physicists, and later chemists and biologists, were trained to give similar talks. Brian Coppola and colleagues published details of how to make nitrogen ice cream in 1994 (Coppola et al., 1994). Today, pretty much any science department in any university can provide an 171

172 inspirational school speaker to talk about their research and combine that with a demonstration of ice cream making. As the use of nitrogen to make ‘instant’ ice cream became widely known through these school talks, chefs were introduced to liquid nitrogen not just to provide spectacle and make instant ice cream but also, for example, as a means to prepare finely ground herbs and spices. I  well recall taking a dewar of nitrogen to the Fat Duck restaurant as I was driving close by on my way back from a school talk one afternoon in 2000 and then spending an afternoon in the garden with Heston Blumenthal and his chefs, simply seeing what could be done. Not long after that, Heston Blumenthal introduced the green tea amuse bouche (Nitro Poached Green Tea and Lime Mousse) with the accompanying spectacle of dragon’s breath coming from the nostrils of the diners, that captivated his customers at the start of their meals. This was quickly adopted by Ferran Adria, and similar dishes were served at El Bulli. As these two restaurants were at the time frequently voted the best in the world, it is not surprising that other chefs quickly started investigating how to use liquid nitrogen for themselves, and so, within less than a decade, it became almost compulsory for any Michelin restaurant to make some use of liquid nitrogen in the restaurant, usually at the table, to provide a combination of spectacle and deliciousness.

How Do Cryogens Work? When a substance undergoes a change of state (or first-​order phase transition), its internal energy changes, often by a large amount, so it either absorbs or releases this energy in the form of heat, which is often termed the latent heat. When a solid is heated (for example. an ice cube in a cocktail), it starts to melt when the temperature reaches its melting point (0 °C in this case). But, to be able to melt, it has to absorb a large amount of heat (334 kJ/​kg), so it can take some time for all the ice to melt as it absorbs this heat from its surroundings. As we know from experience, it takes typically 10 to 20 minutes for the ice in our drinks to melt. As the ice melts, it extracts the heat required for the change of state from a solid to a liquid from the surrounding liquid, and so cools that liquid. In the case of a typical ice cube in a gin and tonic, the temperature is reduced by about 5 °C as the ice cube melts. Similarly, if we heat a pan of water to its boiling point (100 °C), as it boils, the temperature remains at 100 °C, and the heat applied to the pan goes into the latent heat required to change its state from a liquid (water) to a gas (steam). The absorption of heat as a liquid boils is the basis for the operation of nearly all air conditioners, refrigerators and freezers we use at home and work. In these devices, the coolant is kept in a closed system and is made to boil by reducing the pressure in the place where the temperature needs to be reduced and then compressed back into a liquid outside, where the excess heat released as the gas turns into a liquid can be vented into the atmosphere. One disadvantage of using conventional freezers in cooking is that the cooling can take a long time, as the heat is extracted from the foodstuff by transfer through the air inside the freezer. It would be much more efficient if the food were placed in a liquid rather than a gaseous environment, as the heat transfer through

Peter Barham the denser liquid would be much greater. So, if we have a liquid that boils at a very low temperature, we can use that to cool things down to that temperature; as the liquid boils, it will extract heat from whatever we want to cool, and, provided there is enough of the liquid available, it can cool our object right down to its own boiling point. Cryogens change state at low temperatures (typically well below ca. −60 °C) and as a result can be used to cool systems down to these temperatures. Provided they are also inert and safe to eat, drink or breathe, then they can be used in direct contact with foods. Two commonly available cryogens that are completely non-​toxic are liquid nitrogen and solid carbon dioxide. Nitrogen boils at −196 °C, and carbon dioxide sublimes (turns directly from a solid to a gas) at −78.5 °C. Thus, if solid carbon dioxide (often referred to as dry ice) is dropped into some liquid that you want to freeze, or cool rapidly, it will very quickly sublime, causing the liquid to bubble up as if it were boiling as the carbon dioxide gas emerges; the sublimation process extracts a lot of heat, and, depending on how much carbon dioxide was used and the amount and type of liquid being cooled, it will cool to its own freezing point and then, as further heat is extracted, it will start to turn solid. Dry ice is often dropped into buckets of water in this way, not to cool the water but to generate white clouds of carbon dioxide vapour and small ice crystals to provide the special effects of rolling clouds of fog we see at many events. Indeed, I recall back in the 1960s buying dry ice from a local supplier as the stage manager for my school play, Macbeth, so we could have a bubbling cauldron on stage for the witches’ scenes. Similarly, solid foods can be dropped into liquid nitrogen, which, as it boils, will rapidly cool them down (again generating the fog-​like effects). The limit to how fast the food cools depends largely on the size of the pieces; larger pieces take longer to cool, as the heat has to get from the inside to the outside by thermal conduction, which can take some time. If there is sufficient liquid nitrogen that it does not all boil off, then the food will eventually cool down to its boiling point (−196 °C), which is of course far too cold to eat! If cryogens are used in the kitchen, great care must be taken to ensure safety. First, the proper equipment is needed to store the cryogens safely and to prevent them from simply evaporating before they can be used. Secondly, as any contact of cryogens with naked skin can lead to severe frostbite (or cryo-​burns), personal protective equipment such as gloves, aprons, and face masks or goggles are necessary. You will also need to take care that, if the gas evaporates quickly, there is sufficient oxygen in the room to avoid the risk of suffocation. For example, when liquid nitrogen boils, the resulting gas takes up about 700 times the volume of the liquid, so 1 litre of liquid nitrogen dropped on the floor will rapidly become 700 litres of nitrogen gas, which will displace 700 litres of air from the room. If you work in a small kitchen, a few litres of liquid nitrogen boiling could easily displace enough oxygen that you could die of suffocation. Some details of the necessary equipment and safety procedures and how to perform a proper risk assessment for the use of cryogens in the kitchen will be addressed later in this chapter. You might reasonably ask whether, with all these difficulties, it is worth the trouble of using cryogens in the kitchen. However,

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Cryogenics in the Kitchen there are plenty of things that can only be achieved with the use of cryogens. For example, some disadvantages of normal freezing are easily overcome by freezing at low cryogenic temperatures. The very low temperatures that can be quickly achieved greatly affect the formation of ice crystals; very many crystals nucleate in a short time and grow quickly, so the overall crystallite size in ice formed using cryogenic cooling is very much smaller than that of ice formed by freezing in a standard freezer. Thus, instead of large needle-​or tree-​like crystals that grow relatively slowly during relatively slow cooling and can puncture the cell walls inside many foodstuffs, giving coarse and grainy textures as well as leading to both structural damage and negative flavour changes, cryogenically frozen foodstuffs contain very small (sub-​micron) crystals that have smooth and creamy textures and do not normally damage structure or flavour. The food industry uses significant quantities of both dry ice and nitrogen in the preparation of many different foods, not just in frozen produce. As we shall see later, many of these techniques have in recent years found their way into professional (and even a few domestic) kitchens.

Why Are Cryogens Useful in Food Preparation? Many foods have soft textures that make them difficult to cut into very precise thin slices, or to grate or grind to make garnishes, etc., as they will tear, rather than cut, and will, if ground, simply turn into a paste rather than a fine powder. However, if they can be frozen completely solid, then they can readily be ground or sliced as required. The problem is then how to freeze them so that they are truly solid throughout and the texture is not damaged. When you freeze food, the liquid water turns to ice, but the water is not pure, as it has dissolved in it a number of molecules that provide taste and flavour to the food, e.g., salt, sugar and amino acids. We all know that salty water has a lower freezing point than fresh water, which is why putting salt on the roads helps prevent ice formation in winter. So it is when we freeze our food; as some of the water turns to ice, it increases the concentration of these solutes in the remaining water, which in turn reduces the freezing point of the water, and thus a very low temperature is required before all the water becomes frozen and the product is fully solid. This temperature is known in the sciences as the eutectic point, i.e., the temperature below which all the phases present will, at equilibrium, be solid. For a solution of common salt, the eutectic temperature is −21.1 °C, for sucrose it is −9.5 °C, and for most amino acids it is around −12 °C. Most oils, such as olive oil, only freeze at temperatures around −40 °C, and any alcohol in the food generally will not freeze until the temperature is below −110 °C. So, to be sure your food is truly a solid so that it will not form a paste when ground, for example, it needs to be at a very low temperature (lower than −30 °C and preferably much colder). Although some professional freezers can reach such low temperatures, it is much simpler (and faster) to use either dry ice or liquid nitrogen to freeze the food you want to grind or slice. Further, if the texture is to be retained (for example, if you want to make sub-​millimetre slices of a cream cake), then it is

most important that the ice crystals do not damage the texture when they form. There are two ways in which ice crystals damage the texture of foods: firstly, from the expansion of ice when it freezes, and secondly, by growing large needle-​shaped crystals that can puncture cell walls. We all know that ice floats on water, because, when ice forms at 0 °C, its density is lower than that of water at the same temperature; another way of looking at the phenomenon is to note that water expands when it freezes (anyone who has lived through a really cold winter will probably have had experiences of water mains that burst because the water inside froze and the expansion fractured the pipe). So, if the water inside a cell freezes at around 0 °C, it will expand and is likely to burst the cell wall, which can massively affect the texture of some frozen foods. However, if the ice forms at a much lower temperature (lower than −20 °C) then it is much denser and the effect is much smaller, resulting in little or no damage to cells. The size and shape of ice crystals that grow when food is frozen depend not only on the molecules that are present in the water but also on the temperature at which the crystals actually grow. While ice can form at any temperature below 0 °C (in pure water), in practice creating a crystal is very difficult, so crystals do not usually start to grow at all until the temperature is well below zero, typically from about −4 °C and lower. The reason is that, as a crystal forms, there is an interface between the growing crystal and the surrounding liquid. While converting a liquid to a solid (at a temperature below the freezing point) liberates a lot of energy in the form of heat (the latent heat noted earlier), creating the interface actually requires the use of significant amounts of energy, so there is an energy balance between the interfacial energy (which is proportional to the surface area) and the energy released due to crystallization, which is proportional to the volume of the crystal. Thus, small crystals, which have a relatively high surface energy, are unstable, while large crystals, where the surface to volume ratio is low, are stable. The upshot is that, if a liquid is cooled quickly, it is possible to start the crystallization process at temperatures well below the melting temperature, where small crystals are quite stable and can grow quickly, leading to an overall small crystal size. If foods are frozen at a low temperature in this way, the ice crystals can be small enough that they cause little or no damage to the surrounding tissues and hence leave the texture of the frozen food, once defrosted, just as it was before it was frozen.

Industrial Use of Cryogens in Food Processing Before looking in detail at the various uses of cryogens in the professional and domestic kitchen, it is worth quickly reviewing industrial uses, as many techniques that have found their way into modernist kitchens and much of the so-​called molecular cuisine lexicon have their origin in industrial food science. However, as we shall see, many of the real advantages that are offered by cryogenic freezing are much better suited to the small professional kitchen than to industrial-​scale processing, where long transport and storage times are involved.

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Peter Barham

Freezing Fresh Foods

Producing Powders and Ground Spices

When food is frozen by simply placing it in a freezer and waiting for it to cool down, there is a major risk of damage to the food caused by the formation of large ice crystals. These can not only break the cell membranes and distort the surrounding tissues but can also lead to denaturation of some proteins (Sanz et al., 1999; Gomez and Calvelo, 1982). Liquid nitrogen has been used in the industrial-​ scale freezing of foods since the early 1960s (Davidge, 1981). In particular, it was used in tunnel freezers, where the food to be frozen is passed on a conveyor belt through a tunnel where the temperature is steadily decreased until the food is completely frozen. In the entrance region, food is cooled using cold air (or gaseous nitrogen or carbon dioxide); then, as it passes along, liquid nitrogen is sprayed from nozzles in the top of the tunnel directly onto the food; this reduces the temperature below ca. −30 °C so that the food is ready for packaging, storage and distribution. In tunnel freezers, the rate of cooling can be quite high. High freezing rates lead to small crystal sizes, due to the high nucleation rate of ice at the lower temperatures achieved during rapid cooling. This can have a number of benefits; the food quality is perceived as being better in terms of texture and appearance as well as flavour (Awonorin, 1997). Furthermore, not only can using cryogenic freezing lead to reduced losses from dehydration, prolonging the overall shelf life (Awonorin, 1997; Ramakrishnan et  al., 2004), but it is also more cost effective (Miller and Roberts, 2001). It can be shown through a number of simulations, modelling and experimental studies that the faster the freezing, the smaller the ice crystal sizes and the better the quality of the products. For example, Martino and Zaritzky (1988) demonstrated that frozen meat benefits from the fastest possible freezing using the greatest possible thermal gradients. Although they note that direct immersion in liquid nitrogen gives the best results, they also speculated on how much better it could be if greater thermal gradients were to be achievable (e.g., using pressurized systems). Other authors (e.g., Sanz et al., 1999) have demonstrated the superior quality of a range of produce, including dates, peas, pineapples and even sausages, when frozen at the highest possible rates (Alhamdan et al., 2001; Biglia et al., 2016). Despite the multiple advantages imparted by the very small crystals ( 10–​5] indicates that the structures considered are larger than 10–​5 m, so that smaller granules are not taken into account. Kinetic parameters such as time or energy can be added to describe the evolution of the system. The equation O/​W + G → (G+O)/​W can be replaced by the following formula: (Gt=0…50 + O30(100 − t)/​100)/​W70(100 − t)/​100 where the time t is in seconds, the gas would be introduced at a regular rate and indices give volume instead of mass. Up to now, no food system has resisted description by this formalism. However, do all formulae correspond to possible systems? Many dispersed systems are metastable and not thermodynamically stable. Indeed, they evolve, depending on the size of their structures or on the nature or quantity of stabilizing elements

Use of DSF for Scientific Explorations and for Innovation DSF was initially introduced for the analysis of the differences between different kinds of “gels”, but it is also useful for innovation. The importance of algebraic notation has been recognized for many centuries, and it was a major success of René Descartes, Wilhelm Gottfried von Leibniz and Isaac Newton to use it in mathematics and physics. In a treatise on logic published in 1918, the French logician Edmond Goblot discussed how notation can lead to discovery (Goblot, 1918): For the algebra of logic, its inventors probably never thought that it was only a notation of concepts, relationships and elementary operations for logicians, and they had never had any doubt on the difference between discovery of a truth and the invention of a notation for expressing it when it is discovered. Notation can lead to discovery, as it occurred frequently in algebra. To general and abstract concepts, intractable without formula, cumbersome to use with words and common

Dispersed System Formalism language, the algebra of logic, as ordinary algebra, substitutes concrete and regular symbols which can be organized in a wealth of combinations and reduce heavy mind operations to very simple written processes.

211 example, considering the many possible gels of class 2, one can now ask whether they can be grouped into categories of flavour releasing systems. This is shown in the chapter about gels in this book.

This idea to simplify operations through an algebraic formalism REFERENCES was already developed by Antoine Laurent de Lavoisier (1743–​ Anton M. 1998. Structure and functional properties of hen egg yolk 1794), and it was the basis of the introduction of modern chemconstituents. Recent Research Development in Agricultural ical notation (Lavoisier, 1782): and Food Chemistry, 2, 839–​864. In order to better show the state of the issue, and to present synthetically the result of what is going on during metal dissolutions, I  build formulas, that could be confused with algebra, but do not derive from the same principles; we are very far from the time when the precision of mathematics can be introduced in chemistry, and I invite the reader to consider the formula that I shall give only as simple annotations, whose aim is to think easier. Indeed, in 1995, a new dish named “Chocolate Chantilly” (see the chapter on Chantillys in this book) was based on the equation O/​W + G → (G + O)/​W (This, 1996). First, a chocolate dispersion ((O+S)/​W) is made by heating chocolate in water with the same final fat/​water ratio as in ordinary cream: here O stands for the melted cocoa fat and S for the cocoa plant particles, the sugar crystals having dissolved in the aqueous phase. Then, this dispersion (mainly an emulsion) is whipped (+G) at room temperature while cooling. The very unstable hot (G+O)/​W system is slowly transformed into a more stable “chocolate mousse” G/​g(O, S, W), when part of the fat crystallizes and traps the dispersed oil and gas structures (here we use again a function g to indicate the unknown continuous structure made of liquid fat, solids –​fat and plant pieces –​and aqueous solution). This mousse contains no eggs, in contrast to a traditional chocolate mousse (Larousse Gastronomique, 1996), and its texture can be the same as that of whipped cream. Of course, chocolate can be replaced by other food products, such as as cheese, foie gras or even butter, leading to “cheese Chantilly”, “foie gras Chantilly” or “butter Chantilly”, respectively. The use of DSF can lead to a wealth of other new systems with both scientific and culinary interests. For example, using four phases and four connectors, the number of formulae is 114,688, and more than 106 with six phases; there is plenty of room for innovation. Indeed, DSF will be a valuable tool for the creation of “note by note dishes” (This, 2016a, 2016b), i.e., dishes made from pure compounds, as the consistency will have to be built, and it is likely that the gels of class 1 will not be enough to achieve the target systems.

Conclusions and Perspectives As we said, DSF can lead to innovation (technology), but it can also contribute to the scientific development of molecular and physical gastronomy, as for the study of flavour release. For

Belitz HD, Grosch W, Schieberle P. 2009. Food Chemistry, Springer, Heidelberg. Cuvier G. 1810. Rapports à l’Empereur sur les progrès des sciences, des lettres et des arts depuis 1789. T. II. Chimie et Sciences de la nature, Belin, Paris, re-​edition 1989. De Gennes PG. 1997. Soft Interfaces, the 1994 Dirac Memorial Lecture, Cambridge University Press, Cambridge. Dickinson E. 1994. Structure, properties and functions. In Nishinari K, Doi E. (eds.). Food Hydrocolloids, Plenum Press, New York. Everett DH. 1988. Basic Principles in Colloid Science, Royal Society of Chemistry, London. Goblot E. 1918. Traité de logique, Armand Colin, Paris, 18. Hiemnez PC. 1986. Principles of Colloid and Surface Chemistry. Marcel Dekker Inc, New York. Hornyak GL. 2009. Fundamentals of Nanotechnology. Taylor & Francis, Boca Raton, FL. Israelachvili J. 1992. Intermolecular & Surface Forces, 2nd Ed. Academic Press, London. IUPAC. 2001. Manual of Symbols and Terminology for Physicochemical Quantities and Units, http://​ old.iupac.org/​ reports/​2001/​colloid_​2001/​manual_​of_​s_​and_​t/​node33.html Jones RAL. 2002. Soft Condensed Matter. Oxford University Press, Oxford. Larousse Gastronomique. 1996. Larousse-​Bordas, Paris. Lavoisier AL. 1782. Considérations générales sur la dissolution des métaux dans les acides. Mémoires de l’Académie des sciences, 492. Lopez C, Ollivon M. 2009. Triglycerides obtained by dry fractionation of milk fat 2.  Thermal properties and polymorphic evolutions on heating. Chemistry and Physics of Lipids, 159,  1–​12. Lyklema J. 1991. Fundamentals of Interface and Colloid Science, Academic Press, London. Mandelbrot B. 1982. The Fractal Geometry of Nature. W. H. Freeman and Co., New York. Mavrovouniotis ML and Stephanopoulos G. 1988. Formal order-​ of-​ magnitude reasoning in process engineering, Computer Chemical Engineering, 12(2/​10), 867–​880. Paré A. 1560. Emulsion. Trésor de la langue française informatisé, CNRS-​ Université de Lorraine. Access: http://​ atilf.atilf.fr, research “emulsion”. Raiman O. 1991. Order of magnitude reasoning, Artificial Intelligence, 51, 11–​38. This H. 1996. Le chocolat Chantilly, Pour la Science, 230, 12. This H. 2003. La gastronomie moléculaire, Sciences des aliments, 23(2), 187–​198. This H. 2006. Let’s have an egg. In Hosking R (Ed.), Eggs in Cookery, Proceeding of the Oxford Food Symposium on Food and Cookery, Prospect Books, Oxford. This H. 2007. Formal descriptions for formulation, International Journal of Pharmacy, 344(1–​2),  4–​8. This H. 2009. Molecular gastronomy, a scientific look at cooking. Accounts of Chemical Research, 575–​583.

212 This H. 2013. Molecular gastronomy is a scientific discipline, and note by note cuisine is the next culinary trend, Flavour, 2(1). This H. 2016a. Statgels and dynagels. Notes Académiques de l’Académie d’agriculture de France, /​Academic Notes from the French Academy of Agriculture, 12, 1–​12.

Hervé This vo Kientza This H. 2016b. What can “artificial meat” be? Note by note cooking offers a variety of answers, Notes Académiques de l’Académie d’agriculture de France (N3AF), 6, 1–​10.

Distillation: The Behaviour of Volatile Compounds during Distillation of Hydro-​Alcoholic Solutions and during Hydro-​Distillation Martine Esteban-​Decloux Unité Mixte de Recherche Ingénierie Procédés Aliments, AgroParisTech, INRAE, Université Paris-​Saclay, F-​91300 Massy, France

When food ingredients are cooked, many volatile compounds are formed and escape. The goal of this chapter is to give some information on the behaviour of odorant compounds during the distillation of hydro-​alcoholic solutions, and to make a link with their elimination through steam distillation.

compound of the mixture. In equation (31.2), Pi is the partial pressure of the ith compound in the vapour phase, and P is the total pressure: n

∑P = P

(31.2)

i



Characterization of Volatility Distillation is a unit operation that makes it possible to separate compounds all together present in one phase by using volatility differences. It is generally performed at pressures close to atmospheric pressure (from 0.2 bar to some bars). For each pure species separately, it is possible to calculate the saturating vapour pressure Psat as a function of temperature by using equation (31.1). The coefficients are generally given in handbooks or databases (Perry et al., 1997; ProSim, 2019).



b   P sat = exp  a + + c.ln(T ) + d.T e    T

(31.1)

where T (kelvin) is the absolute temperature, Psat (Pascal) is the saturating vapour pressure, and a, b, c, d and e are particular constants for the various compounds. For example, Table  31.1 gives the values for water, ethanol and some categories of volatile compounds of the families of alcohols, esters, aldehydes and terpenes. Separately, at a particular temperature, water and ethanol do not have the same partial pressure; the vapour pressure of ethanol is always higher than that of water. For example, at 100 °C, the vapour pressure for water is 101.3  kPa, and it is 224.5  kPa for ethanol. Ethanol, having a vapour pressure much higher than that of water (by a factor of 2.2), is thus said to be more volatile than water.

Partial Pressures for the Liquid–​Vapour Equilibrium According to Dalton’s law, the total pressure for a vapour of n compounds is equal to the sum of the partial pressures for each

i =1



At the pressure traditionally used in distillation or hydro-​ distillation, the behaviour of the vapour phase can be described using the ideal gas law. As a consequence, the partial pressure Pi of the ith compound in the vapour phase can be expressed as a function of the molar fraction yi of this compound in the vapour phase and the total pressure P (31.3):

Pi = yi .P (31.3)

Interactions in the liquid phase can be described using activity coefficients. Hence, the partial pressure Pi of the ith compound can be expressed using the activity coefficient γi of the molar fraction xi of the ith compound of the liquid phase and the saturating vapour pressure at the considered temperature (31.4):

Pi = γ i (T , xi ) xi .Pisat (T ) (31.4)

Absolute and Relative Volatility In order to know the behaviour of the volatile compounds in a solution, one needs to look for the data for the liquid–​vapour equilibrium (how the compounds distribute between the boiling liquid and the saturated vapour phases). Each compound, i, is characterized by an absolute volatility coefficient, Ki, which is the ratio of molar concentrations yi of the ith compound in the saturated vapour and the molar concentration xi in the boiling liquid (31.5): 213

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TABLE 31.1 Coefficients for the Riedel Equation (31.1) According to ProSim (2019) for Water and Some Compounds of the Families of Alcohols, Esters, Aldehydes and Terpenes Compound

Tmin (°C)

Water Ethanol Methanol 1-​propanol 3-​methyl-​1-​butanol 2-​phenylethanol Ethyl acetate Hexyl acetate Ethyl caproate Ethyl lactate Acetaldehyde Linalol D-​limonene

Tmax (°C)

0 −114 −98 −126 −117 −27 −84 −81 5 −26 −124 10 −74

Ki =



374 241 239 263 304 411 250 345 189 334 193 198 380

yi xi

a

(31.5)

Generally, the volatile compounds are present in very low concentrations, and the interactions between them can be neglected. In contrast, the influence of the solvent is often very important. For mixtures, one can define relative volatilities αi/​j, for example for the ith compound relative to the jth one (31.6):

α

i

j

=

Ki y x = i i K j yj xj

b

73.64900 73.30400 82.11780 84.66416 117.0740 65.07600 66.82400 74.72700 64.83120 72.66870 52.91070 128.0810 75.57400

(31.6)

All these concepts being defined, we shall now examine (1) the principle of the distillation of a hydro-​ alcoholic distillation, (2)  the behaviour of volatile compounds in a hydro-​alcoholic solution during a simple distillation and (3)  the behaviour of volatile compounds in water during hydro-​distillation.

Distillation of a Hydro-​Alcoholic Solution The equilibrium data for the water–​ethanol binary mixture are well known. They can be represented in a diagram of composition (Figure 31.1) giving, for a particular pressure, the molar concentration of the boiling liquid in equilibrium with the saturated vapour. These data were calculated using the Simulis Thermodynamics software (ProSim, 2019) using the Non-​ Random Two-​ Liquid (NTRL) coefficients given by Puentes et  al. (2018a). The two phases in equilibrium are at the same temperature. Using the diagram, one can determine, for a pressure of 1.013 bar, the temperature at which the liquid reaches its boiling temperature and the composition of the vapour in equilibrium at this temperature. One can observe that the water–​ethanol system forms an azeotrope at a molar fraction equal to 0.8943: for this composition, the mixture

−7258.20000 −7122.30000 −6905.50000 −8307.24422 −10743.2000 −8985.10000 −6227.60000 −8352.60000 −7853.33462 −8249.65000 −4643.14000 −12163.6320 −8079.70000

c

d

−7.30370 −7.14240 −8.86220 −8.57673 −13.1654 −5.70340 −6.41000 −7.29240 −5.82651 −6.91582 −4.50683 −14.7440 −7.55960

4.1653 × 10−6 2.8856 × 10−6 7.4664 × 10−6 7.0509 × 10−18 1.1670 × 10−17 4.6087 × 10−18 1.7914 × 10−17 7.1794 × 10−18 0.000 7.5520 × 10−18 2.7028 × 10−17 0.000 8.3872 × 10−18

e 2 2 2 6 6 6 6 6 0 6 6 1 6

behaves like a pure species, i.e., the vapour has the same composition as the liquid. Hence it is impossible, even in a highly complex distillation system, to produce pure ethanol. In the European food industry, the official unit for the characterization of the ethanol concentration of a solution is the Alcoholic strength By Volume (ABV in % v/​v) (Official Journal of the European Regulation, 2019, article 4, 23rd point). This is the ratio in percentage between the volume of pure ethanol at a temperature of 20  °C, in a solution, and the total volume of this solution at the same temperature. Using relationships published by the International Organization of Legal Metrology (1975) between ABV and densities, a polynomial relation was established between ethanol molar fraction and ABV (31.7), where xEt is the ethanol molar fraction and ci the constants of Table 31.2. 5



ABV =

∑ c .( x i

i (31.7) Et )

i =1

One can trace the composition diagram using the ABV unit system, more convenient for culinary uses (Figure 31.2). Such plots of the composition of a hydro-​alcoholic solution at its boiling temperature are useful for a simple batch distillation. Let us consider the example of a hydro-​alcoholic solution with an ABV equal to 10% v/​v. At a pressure of 1.013 bar, it reaches its boiling temperature at 92.8 °C (at this temperature, the vapour pressure due to the liquid is equal to 1.013 bar). The vapour from this solution is at 54.5% v/​v. The production of this vapour, richer in ethanol than the liquid, generates a drop in the ethanol concentration of the liquid, and in this way, an increase of the boiling temperature and a decrease in the ethanol concentration of the vapour produced. This phenomenon goes on until nearly all the ethanol of the liquid is evaporated. It is important to understand that, in a solution with many compounds, the vapour is composed of most volatile compounds, but in different proportions. In the

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FIGURE 31.1  Composition diagram for a water–​ethanol system at a pressure of 1.013 bar in molar fractions.

TABLE 31.2 Coefficients for the Determination of ABV Using Equation (31.7) c1 = 320.68

c2 = −560.14

c3 = 654.94

c4 = −439.14

c5 = 123.76

FIGURE 31.2  Composition diagram for a water–​ethanol system at a pressure of 1.013 bar in ABV (% v/​v).

case of a water–​ethanol solution with 10% v/​v that is set to boil with condensation of the vapour being formed, the evolution of the ethanol content of the liquid and the vapour can be described as in Figure 31.3. In some cases, distillations (simple evaporation) are performed at a pressure below atmospheric pressure (so-​called “vacuum pressure”). The equilibrium data are modified, with a strong decrease in boiling temperatures when the pressure is low (as shown in Figure 31.4 for the water–​ethanol system) and a slight increase of the relative volatility of ethanol.

Distillation of Volatile Compounds in a Hydro-​Alcoholic Solution The goal of distilling hydro-​alcoholic solutions is to extract from the initial liquid the volatile compounds and to separate them when possible or when needed. For fermented must (grapes, apples, pears or grain), the main compound (except water) is, of course, ethanol, but the primary interest is to fractionate volatile compounds that influence the quality of the distillate. The key question is then their volatility in the boiling medium.

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Martine Esteban-Decloux

FIGURE 31.3  Evolution of the ABV for the produced vapour (_​ _​ _​) and the liquid remaining in the boiler ( _​_​_​ ) as a function of time.

FIGURE 31.4  Equilibrium data in ABV (% v/​v) for a water–​ethanol system as affected by pressure.

For the distillation of hydro-​alcoholic solutions, the volatility of most compounds depends on the ethanol content of the solution. Puentes et al. (2018a) reviewed the published equilibrium data and determined the coefficient of the NRTL model (Non-​Random Two Liquid) for using such data in simulation software. It is then possible to determine the absolute volatility of these compounds according to the ethanol concentration of the liquid. As an example, Figure  31.5 shows the shape of the absolute volatility (Ki in equation (31.5)) of some compounds, including ethanol. Obviously, some compounds are always more volatile than ethanol (ethyl acetate and acetaldehyde), whereas others have a volatility that varies with the ethanol content in the liquid (3-​methylbutan-​ol and methanol) or a volatility always lower than that of ethanol (ethyl lactate, acetic acid and 2-​phenylethanol).

Puentes et  al. (2018a) proposed a classification for 44 compounds based on their relative volatility (31.6) with respect to ethanol or water (αi/​Et or αi/​W): (i) light compounds, which are always more volatile than ethanol, whatever the ABV of the liquid (αi/​Et > 1); (ii) intermediate compounds, with a relative volatility higher than that of ethanol at low ABV but a lower volatility at high ABV; and (iii) heavy compounds, always less volatile than water (αi/​W < 1) (Table 31.3). Depending on the distillation process (simple or with a column), the compounds do not have the same distribution in the distillate and the residue. During simple distillation (evaporation), the compounds are more concentrated in the first vapour fractions when their volatility with respect to ethanol is higher; this is the case, for example, for ethyl acetate. Because of their strong extraction in the vapour phase, the concentration of such compounds in the

Distillation: Volatile Compounds

217

FIGURE 31.5  Variation of the absolute volatility (Ki) of some compounds as a function of the ethanol content of the liquid: (a) volatilities always higher than ethanol and (b) volatilities lower than ethanol.

liquid decreases rapidly. As a consequence, their concentration in the vapour also decreases. Such compounds belong to “heads” because they are extracted at the beginning of the distillation process. The compounds with a volatility comparable to that of ethanol, such as methanol or 3-​methylbutan-​1-​ol, have a distillation behaviour similar to that of ethanol and are particularly difficult to separate from ethanol. Among intermediate compounds, some, such as 2-​phenylethanol or ethyl lactate, have a volatility that is always lower than that of ethanol, but their volatility with respect to water increases when the ethanol content decreases. These compounds are evaporated at the end of distillation, when the ethanol content is greatly decreased. Finally, the compounds with a volatility lower than water remain in the liquid. As an example (Figure 31.6), the behaviour of some compounds during a distillation of cognac was shown by Douady et al. (2019). One can consider that, in a column distillation, when the initial solution is separated into a distillate with high ethanol content and a residue with low ethanol concentration, all light compounds are extracted by the distillate and heavy compounds by the residue. In contrast, the distribution of the intermediate compounds depends on the ethanol content of the distillate; the lower this content, the greater the extraction of the compounds in the distillate. This occurs, for example, during Armagnac production, for which the ABV of the liquid in the column is less than 10% v/​v (Puentes et al., 2018b). Finally, for the production of ethyl alcohol of agricultural origin with a minimum ABV of 96.0% v/​v and limited concentrations of other volatile compounds (Official Journal of

the European Regulation, 2019, article 5), more than seven distillation columns, each with a high number of plates, are coupled in order to eliminate all the volatile compounds except ethanol (Esteban-​Decloux et al., 2014). It is important to remember that the matrix and the method of distillation play an essential role in the behaviour of volatile flavouring compounds during distillation.

Behaviour of Volatile Compounds in Boiling Water Hydro-​distillation (or steam distillation) is a particular distillation for which the solvent is water or steam. As we have seen before, the compounds are more volatile when the ethanol content is low. This is due to their relative affinity for organic and aqueous phases, which can be characterized by the partition coefficient kow (octanol/​water). The partition coefficient is a measurement of a compound’s differential solubility in octanol and water, but, because of very large values, the logarithm of this partition coefficient is used. The higher the log kow, the more lipophilic the compounds are (and the less hydrophilic they are). When the concentration of a compound in a solution is higher than its solubility limit, its partial pressure in the vapour does not depend on its concentration but only on the temperature. It can then be extracted at a temperature lower than its own boiling temperature when pure. For example, limonene is the main compound of the essential oil of most citrus fruits and extracts (Deterre et al., 2011). Its solubility in water at 25 °C is

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Martine Esteban-Decloux

TABLE 31.3 Classification of Volatile Compounds According to Their Relative Volatilities with Respect to Ethanol (αi/​Et) and Water (αi/​W) over a Wide Ethanol Composition Range in the Liquid Phase αi/​Et min Light compound 3-​methylbutanal Propan-​2-​yl ethanoate 2-​methylpropanal Ethyl acetate Butanal Acetaldehyde 1,1-​diethoxyethane Propanal 2-​methylpropyl methanoate Prop-​2-​enal (acrolein) Intermediary compound Ethyl decanoate Ethyl 2-​methylpropanoate Ethyl octanoate 3-​methylbutyl ethanoate Ethyl 3-​methylbutanoate Pentanal Linalool Ethyl hexanoate Linalool oxide 2-​methyl-​2-​propanol 1-​hexanol 2-​propanol

αi/​W max

min

αi/​Et

max

1.3 1.7 1.1 1.8 1.7 5.1 3.3 1.8 1.7 2.4

19.8 14.2 12.8 10.8 10.5 6.3 6.8 6.5 6.0 4.5

1.2 1.7 1.0 1.6 1.5 5.3 5.6 1.7 1.6 2.4

239.6 172.0 154.6 131.2 125.9 61.6 40.0 51.3 72.8 30.1

0 1.0 0 0.1 0.5 0.9 0 0.6 0 0.7 0.1 0.9

82.9 33.0 28.0 18.5 18.1 10.1 8.2 6.7 5.2 2.7 2.9 1.8

0 0.9 0 0.1 0.4 0.8 0 0.5 0.1 0.7 0.1 0.8

1,005.3 399.3 339.4 224.0 219.5 122.9 99.5 81.6 63.4 33.2 35.3 22.0

Intermediary compound 3-​methyl-​1-​butanol 2-​methyl-​1-​propanol 2-​methyl-​1-​butanol 1-​butanol cis-​3-​hexen-​1-​ol 1-​propanol Methanol 2-​phenylethyl ethanoate Octanoic acid Allyl alcohol Hexanoic acid Furan-​2-​carbaldehyde Ethyl lactate 3-​methylbutanoic acid 2-​methylpropanoic acid Butanoic acid Diethyl butane-​1,4-​dioate 2-​methylbutanoic acid 2-​phenylethanol Propanoic acid Heavy compound Acetic acid Formic acid

αi/​W

min

max

min

max

0.1 0.4 0.1 0.2 0 0.6 0.6 0 0 0.5 0 0.1 0.1 0 0 0 0.1 0 0 0.1

2.7 2.5 2.5 1.9 1.9 1.4 1.5 1.5 1.4 0.9 0.7 0.4 0.3 0.3 0.2 0.2 0.2 0.2 0.1 0.1

0.1 0.3 0.1 0.2 0 0.5 1.4 0 0 0.5 0 0.1 0.1 0 0 0 0.1 0 0 0

32.1 30.0 30.2 23.1 23.4 16.9 6.8 18.8 17.2 10.8 8.1 5.3 3.9 4.0 2.1 2.0 1.9 2.0 1.3 1.0

0.1 0

0.1 0.1

0.1 0.1

0.8 0.5

Source: Puentes et al. (2018a)

13.8 mg.L-​1, its boiling temperature at 1.013 bar is 176 °C and its molar mass is 136.23  g.mol-​1 (The Good Scents Company, 2019). When a water–​limonene system (with a concentration of limonene higher than the solubility limit) is put to the boil, the partial pressure for each compound depends only on temperature (equation (31.1) and the coefficients in Table 31.1), and the total pressure is equal to the sum of partial pressures (equation (31.2)). Because the solution reaches its boiling temperature when the total vapour pressure is equal to the atmospheric pressure, the boiling temperature of water with an excess of limonene is 97.49  °C (Table  31.4). This value is lower than the individual boiling temperatures of limonene and water at the same pressure. The molar fraction of limonene in the vapour phase can be calculated from the pressures of each compound at the boiling temperature and equation (31.3), giving

ylim =

sat Plim P sat = sat lim sat P Plim + Pwater

(31.8)

Thus, the molar fraction in the vapour phase at 97.49  °C is 8.66 × 10−2, corresponding to a mass fraction of 0.4178  g.g−1. This value is much higher than the solubility limit, and so the condensed vapour separates into two phases: a light organic

phase (the “essential oil”) and a heavy aqueous phase, called the floral water.

Conclusion This chapter intended to show the behaviour of volatile compounds when solutions are put to the boil, during distillation or hydro-​ distillation. The importance of knowing the properties of these compounds, in particular their saturating pressure vapour with respect to temperature, and their absolute volatility with respect to the nature of the solvent, is stressed. During the simple distillation of hydro-​alcoholic solutions, the volatile compounds distribute between some that are concentrated in the first fractions of vapour, some that distribute as ethanol, and some that are more concentrated at the end of the process. With hydro-​distillation or steam distillation, one can extract poorly water-​soluble compounds at a temperature much lower than their own boiling temperature; indeed, their vapour pressure depends only on temperature. Their concentration in the vapour depends only on the ratio of the saturating vapour pressures, and through the condensation of the vapour, they can be separated by simple decantation as essential oil. Finally, by simple distillation or by steam distillation, one can lower the boiling temperature by reducing the pressure.

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Distillation: Volatile Compounds

FIGURE 31.6  Evolution of some odorant compounds during the distillation of cognac; the experimental values are compared with simulated values with BatchColumn software from ProSim®. (Douady et al., 2019)

TABLE 31.4 Boiling Temperatures of Limonene and Water at Various Total Pressures P T (°C)

Plim (Pa)

Pwater (Pa)

P (Pa)

97.47 97.48 97.49 97.50 97.51

8768 8771 8775 8778 8782

92,459 92,493 92,526 92,560 92,593

101,227 101,264 101,300 101,338 101,375

REFERENCES Deterre S, Rega B, Delarue J, Decloux M, Lebrun M, Giampaoli P. 2011 Composition of aroma volatiles of bitter orange essential oil (Citrus aurantium L.) and its distillate to evaluate the impact of the maceration-​distillation process, Flavour Fragr. J. 27, 77–​88

Douady A, Puentes C, Awad P, Esteban-​Decloux M. 2019. Batch distillation of spirits: experimental study and simulation of the behaviour of volatile aroma compounds. J. Inst. Brew. 125, 268–​283. Esteban-​Decloux M, Deterre S, Kadir S, Giampaoli P, Albet J, Joulia X, Baudouin O. 2014. Two industrial examples of coupling experiments and simulations for increasing quality and yield of distilled beverages. Food Bioprod. Process. 92, 343–​354. International Organization of Legal Metrology. 1975. International Alcoholometric Tables, OIML R 22. Official Journal of the European Regulation. 2019. Regulation (EU) 2019/​787 of the European parliament and of the council of 17 April 2019 on the definition, description, presentation and labelling of other foodstuffs, the protection of geographical indications of spirit drinks, the use of ethyl alcohol and distillates of agricultural origin in alcoholic beverages, and repealing Regulation (EC) No 110/​ 2008. https://​ eur-​ lex. europa.eu/​legal-​content/​EN/​TXT/​?uri=CELEX:32019R0787 Perry JH, Green DW, Maloney JO. 1997. Perry’s Chemical Engineers’ Handbook. New York, Mc Graw Hill.

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ProSim. 2019. Simulis thermodynamics. www.prosim.net/​ en/​ Puentes C, Joulia X, Vidal JP, Esteban-​Decloux M. 2018b. Simulation of spirits distillation for a better understanding of volatile software-​simulis-​thermodynamics-​mixture-​properties-​and-​ aroma compounds behavior: application to Armagnac producfluid-​phase-​equilibria-​calculations. tion. Food Bioprod. Process. 112, 31–​62. Puentes C, Joulia X, Athès V, Esteban-​Decloux M. 2018a. Review Limonene. www.thegood­ and thermodynamic modeling with NRTL model of vapour-​ The Good Scents Company. 2019. D-​ scentscompany.com/​ data/​ rw1013772.html. Last access 19 liquid equilibria (VLE) of aroma compounds highly diluted in November 2019. ethanol-​water mixtures at 101.3 kPa. Ind. Eng. Chem. Res. 57, 3443–​3470.

Eggs: Let Us Have an Egg Hervé This vo Kientza1,2 Université Paris-​Saclay, INRAE, AgroParisTech, UMR 0782 SayFood, 75005, Paris, France 2 Group of Molecular Gastronomy, INRAE-​ AgroParisTech International Centre for Molecular Gastronomy, F-​75005, Paris, France

1

Using codes for describing egg transformations can lead to a wealth of new culinary preparations. Sometimes, the physical or chemical mechanisms of the transformations are known, but often, more scientific research (molecular and physical gastronomy) is needed in order to explain the changes observed during simple processes. Since the beginning of molecular and physical gastronomy, scientific knowledge has been used for making technical innovations (This, 2006a). This “culinary technique” has been called “molecular cooking”, when new tools (in particular, equipment from chemistry or physics laboratories) or methods were used. For example, when considering innovative transformations of eggs, the following systematic system has been proposed (This, 2006b; This, 2007).

A Code for Innovation This system is given in Table 32.1, applied to eggs, but one can easily generalize it to any other food ingredient, and indeed, this has also been done for plant and animal tissues. But let us begin with eggs: the full, whole egg, comprising a shell, yolk and egg white, is given the code number 1, and this makes the first row of a table. Then, the various possibilities of dividing the whole egg are considered as cells of the second row. The full, intact egg is assigned the label 1.1, the shell alone 1.2, the non-​mixed yolk and the white out of the shell 1.3, the mixed yolk and white 1.4, the yolk alone 1.5, and the white alone 1.6 (Figure 32.1). In the third row of the table examples of transformations of these various constituent “parts” by various processes are found; options include the addition of nothing (1), gas (2), “water” (i.e., any aqueous solution) (3), “oil” (any fat liquid phase) (4), solids (5), ethanol (6), acid (7), alkali (8) or heat (9). Generally, the gas used in the kitchen is air, but many possibilities exist; for example, during an educational dinner organized in 2008 by the Institut des hautes etudes du goût, de la gastronomie et des arts de la table (“Institute for the advanced studies for flavour, gastronomy and arts de la table”) at the Cordon Bleu School in Paris, the dessert was made using helium, so that the guests had a strange duck voice for some seconds after consumption

(as sound velocity changes depending on the nature of the carrier gas, hence resulting in a modification of frequency when sounds are emitted in helium instead of air). The “water” in option 3 can be any aqueous solution, as long as the concentration is low and solutes do not noticeably change the properties of the solution. Likewise, “oil” can be any liquid fat, such as ordinary kitchen oil (olive oil, corn oil, sunflower oil, etc.), but also melted butter, melted foie gras, melted chocolate, etc. Of course, ethanol, acids and alkalis should be edible: this means that they have to be food grade, but also they can be dissolved, for example, in vodka, vinegar, baking powder, etc. Heat, finally, can be applied in many different ways, which means that more slots could be added to the table if necessary (see chapter “Uncook the Egg and Eggs at 6X °C”). In particular, it is useful to consider systematically that heat can be supplied by a hot solid, a hot liquid (water or oil) or a hot gas, or by radiation (microwaves, infrared and also all kinds of electromagnetic radiation, as they transmit energy when they are absorbed). Using these ingredients, new “products” or culinary concepts can be made, and numbers can be used to describe what they are. For example, in the third row, three numbers describe results of combinations of transformation. Now we shall consider only some cases, because, essentially, the table offers infinite numbers of possible combinations.

Some Examples As the colour indicates, the table contains old results as well as new ones. A first interesting result is given by the code 1.1.6: an egg (1) is stored, whole in the shell (1), in alcohol (6). Because the brandy can enter the egg through the pores, a result called a “baumé” is obtained, after more than one month (Figure 32.2), due to precipitation and possibly coagulation of proteins. The name “baumé” was introduced for this new product in honour of the French pharmacist Antoine Baumé (1728–​ 1804), who devised a density scale. The next code, 1.1.7, describes eggs that are stored in vinegar, for example, and that coagulate after some time (about one month), making “minus one century eggs” (the “contrary” to “one

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TABLE 32.1 A Coded System for Innovation Using Eggs (Blue Colour Indicates Traditional Applications, While Red Indicates Novel Ones) 1. Whole intact egg 1.1. Whole egg in the shell

1.2. Shell alone

1.3. Yolk and white out of the shell, unmixed

1.4. Mixed yolk and white 1.4.1. Nothing 1.4.2. Gas: Genoise before cooking 1.4.3. Water 1.4.4. Oil: Mayonnaise (one kind) 1.4.5. Solid 1.4.6. Ethanol: Thenard of white and yolk 1.4.7. Acid 1.4.8. Alkali 1.4.9. Heat: omelette, flan, flan at low temperature 1.4.3.9. Lavoisier (extreme royales), Avogadro

1.1.1. Nothing 1.1.2. Gas 1.1.3. Water 1.1.4. Oil 1.1.5. Solid 1.1.6. Ethanol:  Baumé 1.1.7. Acid: Minus one century eggs 1.1.8. Alkali: One century eggs 1.1.9. Heat: hard-​ boiled eggs, eggs at XX °C

1.2.1. Nothing 1.2.2. Gas 1.2.3. Water 1.2.4. Oil 1.2.5. Solid 1.2.6. Ethanol 1.2.7. Acid 1.2.8. Alkali 1.2.9. Heat

1.3.1. Nothing 1.3.2. Gas 1.3.3. Water 1.3.4. Oil 1.3.5. Solid 1.3.6. Ethanol 1.3.7. Acid 1.3.8. Alkali 1.3.9. Heat: Fried eggs, poached eggs, low-​ temperature poached eggs

1.1.1.1. 1.1.1.2. 1.1.1.3. 1.1.1.4. 1.1.1.5. 1.1.1.6. 1.1.1.7. 1.1.1.8. 1.1.1.9. 1.1.2.1. 1.1.2.2. ….

1.2.1.1. 1.2.1.2. 1.2.1.3. 1.2.1.4. 1.2.1.5. 1.2.1.6. 1.2.1.7. 1.2.1.8. 1.2.1.9. 1.2.2.1. 1.2.2.2. …

1.3.1.1. 1.3.1.2. 1.3.1.3. 1.3.1.4. 1.3.1.5. 1.3.1.6. 1.3.1.7. 1.3.1.8. 1.3.1.9. 1.3.2.1. 1.3.2.2. …

1.5 Yolk

1.6. White

1.5.1. Nothing 1.5.2. Gas 1.5.3. Water 1.5.4. Oil: Mayonnaise 1.5.5. Solid 1.5.6. Ethanol: Thenard of yolk 1.5.7. Acid 1.5.8. Alkali 1.5.9. Heat: Pommade egg (cooked yolk at low temperature)

1.6.1. Nothing 1.6.2. Gas: Whipped egg white 1.6.3. Water 1.6.4. Oil: Geoffroy (emulsion based on the white) 1.6.5. Solid 1.6.6. Ethanol: Thenard of white 1.6.7. Acid 1.6.8. Alkali 1.6.9. Heat: Cooked white

1.5.3.9. Royale, flans, flans at low temperature

1.6.2.2. Chaptal (raw preparation for wind crystal) 1.6.3.9. Avogadro of white 1.6.4.5.  Liebig (physically gelled emulsion) 1.6.4.9. Gibbs (chemically gelled emulsion)

1.6.2.2.9. Wind crystal 1.6.3.2.9. Vauquelin (wind crystal preparation cooked in the microwave oven)

century eggs” prepared by Asian populations by storing eggs in mixtures containing alkali such as lime or potash) (Figure 32.3). Label 1.1.9 describes in particular the hard-​ boiled egg (Figure 32.4), as the full egg in its shell (1.1) is heated (9). But considering the various denaturation temperatures of proteins from the egg, many other possibilities exist, such as “eggs at 6X °C”, i.e., eggs that are heated at 62 °C, 63 °C, 64 °C… up to 100 °C, until the temperature is constant in the egg (This, 1997a), with many different results being found at different temperatures between 61 °C and 100 °C (Figure 32.5). The code 1.3.9 corresponds to fried egg, or “oeuf cocotte”, and it is not new. The mechanism of this transformation was shown in This (1997b): for example, in egg white, the denaturation of proteins can lead to the exposure of thiol groups from cysteine residues, and the formation of disulfide bridges (oxidation) can make a three-​dimensional protein network trapping the 90% of water making up the egg white. The code 1.5.6 is a yolk coagulated by ethanol (and 1.6.6 is an ethanol-​coagulated egg white) (Figure 32.6). It was named a

“thenard” (of white, or of yolk), from the name of the French chemist Louis-​Jacques Thenard (1777–​1857), who made many discoveries (hydrogen peroxide, Thenard blue, etc.) and in particular introduced “osmazome”, i.e., the result of extraction of meat in ethanol. Label 1.6.4 describes what was considered impossible by chefs of the past, who published that “the slightest trace of egg white with the egg yolk prevents making a successful mayonnaise” (Gencé, 1900). Here, considering that proteins are much better surfactants than phospholipids, it was proposed to whip oil in an egg white in order to make an emulsion. The oil used can be ordinary plant oil (e.g., sunflower, soybean) but also melted butter (ordinary, clarified or brown) or even melted foie gras or cheese, as long as its temperature remains lower than 61 °C, i.e., the first coagulation point of a protein of egg white. The obtained emulsion was called a “geoffroy”, from the French chemist family of scientists, including Claude Joseph Geoffroy (1685–​ 1752), a botanist; Claude-​François Geoffroy (1729–​1753), who discovered bismuth; and Etienne Louis Geoffroy (1725–​1810), an

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FIGURE 32.1  An egg can be divided into parts that can be described by a code number.

FIGURE 32.2  This “beaumé” (a) was obtained by storing a whole egg in rum for one month. The name is after the French pharmacist Antoine Beaumé (b).

entomologist (Figure 32.7). Other products, differently labelled, can also be produced but have not received names up to now. When the fourth row of the table is considered, products are now labelled with codes written with four figures. Again, some of them are traditional (a few), and many new possibilities arise. The “gibbs” (1.6.4.9), for example, is obtained by heating a geoffroy: the chemical gelation of the proteins previously used for the emulsion creates a three-​dimensional network trapping the oil droplets (stabilities of more than three months were observed) (Figure 32.8). The name is from the American physical chemist Josiah Willard Gibbs (1839–​1903).

On the next line of the table, there are more possibilities, such as 1.6.3.2.9, a product to which the name of the French chemist Nicolas Vauquelin (1763–​1829) was given. This product consists of egg white, with added air, to which water is added, and the result is heated (in this case, using a microwave oven). Figure  32.9 shows a vauquelin made by the chef Denis Martin (Restaurant Denis Martin, Vevey, Switzerland). And, of course, there are many other new possibilities, because, as said, the table is infinite in length. If one wants to create a new dish, one can just select a code and turn it into food!

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FIGURE 32.3  The various steps of transformation of an egg stored in vinegar. First the shell is dissolved, then the egg expands by osmosis, and finally the proteins coagulate.

FIGURE 32.6  A “thenard” of egg white.

FIGURE  32.4  A hard-​boiled egg. When the temperature of 100  °C is applied for a long time, sulphur-​containing proteins are degraded, and they release hydrogen sulfide, as can be shown, for example, by applying a filter paper previously dipped in a lead acetate solution.

FIGURE 32.7  A “geoffroy” is an emulsion obtained by whipping a liquid fat in an egg white, as for the making of a mayonnaise.

FIGURE 32.5  An egg cooked at 65 °C. The name initially given, “perfect egg”, is still used in many restaurants of the world.

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Conclusion

FIGURE  32.8  A “gibbs” is a chemically jellified emulsion, obtained, for example, by thermal processing of a “geoffroy”.

This digital system for eggs can be applied to any other food ingredient, such as meat, vegetables, fruits, etc. Moreover, the system is indeed a “code”, an example of many that have been produced in the last decades, such as the “table of double cooking” (This, 2002), the formalism for dispersed systems (see chapter “Dispersed System Formalism”) and the “tree of doughs” (This, 2002). All of them arise from the same idea, as used by Antoine Laurent de Lavoisier for chemistry in 1791 (Lavoisier, 1791) and later formalized by Goblot (1918).

REFERENCES Gencé. 1900. Encyclopédie de la vie pratique, Librairie nationale des beaux arts, Paris, p. 476. Goblot E. 1918. Traité de logique, Armand Colin, Paris. Lavoisier AL. 1791. Considérations générales sur la dissolution des métaux dans les acides, in Oeuvres, t.  3, Imprimerie nationale, Paris. This H. 1997a. L’oeuf parfait, www.pierregagnaire.com/​ pierre_​ gagnaire/​travaux_​detail/​76 This H. 1997b. Can a cooked egg white be uncooked? The Chemical Intelligencer, 10, 51. This H. 2002. Molecular Gastronomy, Columbia University Press, New York. This H. 2006a. Cooking in schools, cooking in universities, Comprehensive Reviews in Food Science and Food Safety, 5 (3),  48–​50. This H. 2006b. Questions d’œuf, Pour la Science, 344, 4. This H. 2007. Let’s have an egg. Eggs in Cookery, Proceedings of the Oxford Food Symposium on Food and Cookery 2006, Prospect Books, Totnes, 250–​258. FIGURE  32.9  A “vauquelin” produced by Denis Martin in Vevey (Switzerland).

Emulsions: Emulsified Systems in Food Markus Ketomäki, Trivikram Nallamilli, Christine Schreiber and Thomas A. Vilgis Max–​Planck-​Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

Principles behind Emulsions and Foams In life, it is often said that opposites attract, but in the kitchen this is not necessarily the case. For example, try to mix oil (or generally speaking, lipids; the following text uses the terms “fat” and “oil” to refer to solid and liquid natural food fats, respectively; the general term “lipid” encompasses both) and water: no amount of mixing or shaking will help you to reach a stable system with oil dispersed in water. In order to understand the fundamental reasons behind this behaviour, we need to consider two things: the electrostatic forces between the individual molecules and the concept of entropy (i.e., the degree of disorder in a system) (Silverstein, 1998). One characteristic of water molecules is their polarity. Although each molecule is electrically neutral, the electron distribution within the molecule is unbalanced, making the molecule polar. This unbalance arises from the fact that oxygen has a higher electronegativity than hydrogen: the shared electron pairs are more attracted by the oxygen atom than by the hydrogen atoms (McNaught and McNaught, 1997). The oxygen atom thus carries a slight negative charge, and the two hydrogen atoms carry a corresponding positive charge. This is also the reason why water molecules “stick together” at room temperature; the “negative” oxygen attracts the “positive” hydrogen atoms from the neighbouring water molecules. With most molecules from fat, however, the situation is different, and this has something to do with the structure of the molecules. Generally speaking, natural food fats are made of triacylglycerides (Vilgis, 2011). In oil, such molecules are primarily triacylglycerols, which consist of a glycerine residue attached (esterified) to three fatty acid residues. These fatty acid residues can be of various lengths (number of carbon atoms) and they can either be saturated or unsaturated (single or double bonds between carbon atoms). These long hydrocarbon chains are electrically balanced (unlike water molecules) and are called non-​polar. Thus, upon adding triglycerides into water, the water molecules would have to rearrange (due to their mutual attraction) around the non-​polar molecules (Huque, 1989). This process, in turn, would limit the ability of water molecules to move freely, thus resulting in a decrease in entropy in the system (Alger, 1994). However, this decrease in entropy caused by the organized structure around the triacylglycerol molecules is not compensated

through any enthalpy gains upon mixing the two substances (Streitwieser et al., 1992). This leads the non-​polar molecules to be driven by entropy to “clump” together, which, in turn, again increases the entropy of water within the system. This entropy-​ driven separation of non-​ polar and water molecules is also referred to as the hydrophobic effect (“water-​fearing”, as opposed to hydrophilic or “water-​loving”) (Huque, 1989). Finally, the differences in density between the solvent (water) and the solute (triacylglycerol molecules) cause the lighter substance (fat) to rise to the top. So, in order to mix water and oil, substances that help in bringing oil and water together (emulsifiers) are needed. There are many different mechanisms by which the emulsifiers work, but this effect is primarily due to the fact that emulsifiers typically consist of a water-​soluble (hydrophilic) and a hydrophobic (also referred to as lipophilic) part. When the emulsifier is mixed into an oil–​water mixture, the emulsifying molecules tend to move toward the oil–​water interface, where the hydrophilic part tends to remain in the water phase and the hydrophobic part tends to stay in the oil phase. When the oil–​water system is given mechanical energy (e.g., by whipping), the freshly formed small oil droplets are stabilized by the layer of the emulsifier at the oil–​ water interface. Emulsions are, however, only metastable and will eventually separate when given enough time. Depending on the medium in which the emulsifier is more soluble, it can be used to make either water-​in-​oil emulsions (when the emulsifier is more soluble in/​attracted to oil) or oil-​in-​water emulsions (when the emulsifier is more soluble in/​attracted to water) (Ohshima, 2016; Griffin, 1949; Griffin, 1954). This rule (also called “Bancroft’s rule”) does not hold in all cases. There are several factors that can promote or inhibit the efficiency of an emulsifier (e.g., temperature, dissolved salts or presence of proteins). However, as a rule of thumb, it provides a good starting point when preparing emulsions. Looking at the chemistry and physics behind emulsions, there are several mechanisms that are involved in the process of stabilizing and destabilizing emulsions. Among other things, emulsifiers help in reducing surface tension (and thus the energy) in the water–​oil interface (McClements, 2015). Each time an interface is formed in a physical system (e.g., an oil droplet in water), a certain amount of energy is required to form the boundary between the substances. Consequently, large surface 227

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FIGURE 33.1  Schematic representation of different processes that lead to destabilization of emulsions and foams.

areas require more energy and are, because of this, inherently unstable. This instability leads to a phenomenon called Ostwald ripening, which takes place in emulsions (Kabalnov et  al., 1992). In order to minimize the surface energy in the interface, triacylglycerol molecules migrate from the small oil droplets toward the larger ones, as the energy requirement for forming one large droplet is less than that required to form several small ones. Emulsifiers also limit the fusion of droplets, a phenomenon known as coalescence. Furthermore, differences in density can cause creaming in emulsions (Adams et al., 2007). The emulsified oil droplets remain separate but start floating upwards in the water, thus forming a zone of small oil droplets on top of the water. Ingredients (such as gelatine, starch granules or pectin) and certain preparation methods (such as reducing the water content by boiling or simply whipping), which increase the viscosity of the emulsion, oppose creaming. Also, increasing the number of emulsified droplets in an emulsion can lead to a phenomenon known as jamming, which in turn leads to higher viscosity (Siemens et al., 2010). However, it is also possible to break an emulsified system or even cause it to undergo a phase inversion with the wrong treatment. Making

butter by whipping cream is the best example of this. Initially, the cream is a dispersion of oil droplets in water, but continued whipping turns the cream into butter, which is essentially an emulsified system consisting of water droplets in fat. Figure 33.1 shows various types of destabilization mechanisms in emulsions and foams.

Types of Emulsifiers in Food Emulsions and Foams For food applications, a wide range of emulsifiers can be used, depending on the type of food being produced. In food, both natural and synthetic emulsifiers are used. Natural emulsifiers can be divided into three main categories: (1) phospholipids, (2) proteins and polysaccharides and (3) particulate emulsifiers. Phospholipids have small molecules containing one polar “head” group (phosphate moiety) and a nonpolar “tail” (one or two fatty acid residues). One of the most versatile and widely used phospholipid emulsifiers is lecithin, which can be obtained from egg yolks or from any plant tissues (it is the main material of biological membranes), including soya beans. Lecithin is used in a wide range of culinary applications, such as improving

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Emulsions: Emulsified Systems in Food the stability and texture of coffee cream, salad dressings and mayonnaise. Proteins are polymers whose monomers are amino acids. Many proteins have regions that are alternately hydrophobic and hydrophilic, giving them good emulsification properties. For this reason, alongside their nutritional properties, they are the most widely used emulsifiers in the food industry. Protein-​ based emulsifiers include egg whites (Drakos and Kiosseoglou, 2006), whey proteins (beta-​lactoglobulin) (Hu et al., 2003), milk proteins (caseins) (Dickinson et  al., 1998) and gelatine (Olijve et al., 2001) from animal sources. Polysaccharides are polymers whose monomers (over 100 of them by definition) are saccharides, and they exhibit thickening properties, which aid in enhancing emulsion stability. Examples of polysaccharides include xanthan and gellan gum (extracted from bacterial cultures), carrageenans (extracted from sea weeds), pectins (extracted from fruit peels), natural and modified starches (from grain sources), galactomannans (extracted from seeds of guar beans and konjac) and chitosan (from shells of crustaceans). Xanthan consists of monomers of β-​D-​glucopyranose glucan as backbone, and its side chains consist of D-​mannopyranose and β-​D-​glucuronic acid. Gellan is a tetrasaccharide, which consists of two residues of D-​glucose and one residue each of L-​rhamnose and D-​glucuronic acid. Carrageenan is a polysaccharide made up of repeating galactose units and 3,6-​ anhydrogalactose (3,6-​AG), both sulfated and non-​sulfated; the units are joined by alternating α-​1,3 and β-​1,4 glycosidic linkages. Starch is made of amylose and amylopectin, of which the monomers are D-​glucose. Polysaccharides belonging to the galactomannan family, such as guar gum, etc., consist of monomer units of galactose and mannose. Chitosan is a polysaccharide consisting of sub-​units of D-​glucosamine and N-​acetyl-​D-​glucosamine. Particulate or Pickering emulsifiers are a recent addition to the repertoire of food emulsifiers, and include microparticles of materials like starch, microcrystalline cellulose, chitin, cocoa fibres, zein particles (from corn), soy protein particles and flavonoid particles. In recent years, these have been recognized for their roles in stabilizing emulsions and foams in food products. Unlike traditional emulsifying agents, Pickering emulsifiers are much larger (in the size range of a few tens of nanometres to micrometres) and can impart better texture and mouthfeel to food products (Linke and Drusch, 2018). Figure 33.2 shows the microscopic structure of soy oil droplets stabilized by hydrophobic rice starch particles. Synthetic emulsifiers are artificially made from non-​natural sources and include sugar-​ alcohols like sorbitol, sorbitan monolaurate (Polysorbate 20), sorbitan monostearate (Polysorbate 80), sodium and calcium salts of fatty acids, sucroglycerides, etc. For example, in ice creams, Polysorbate 80 is often added up to 0.5% (v/​v) concentration to make the ice cream smoother and easier to handle, as well as increasing its resistance to melting (Goff, 1997). Adding this substance prevents milk proteins from completely coating the fat droplets. This allows them to join in chains and nets, which hold air in the mixture and provide a firmer texture that holds its shape as the ice cream melts.

FIGURE 33.2  Dark field polarization microscopy image of a Pickering particle stabilized emulsion made with soybean oil emulsified in water at pH 3 with rice starch particles modified with octenyl-​succinic anhydride. Starch particles adsorb on to oil droplets, thus stabilizing the emulsion.

Examples of Emulsified Systems in Food Milk A classic example of food emulsion is milk. As shown in Figure 33.3, the milk fat globules (in the size range 0.1 to 10 μm) are dispersed in water. Fat globules are stabilized by a thin layer (consisting of a mixture of phospholipids and proteins) called the milk fat globule membrane (MFGM). The MFGM consists of about 30% phospholipids (sphingomyelin, phosphatidylcholine and phosphatidylethanolamine) (Lopez and Ménard, 2011), and the remaining fraction consists of glycosylated and non-​glycosalated proteins (Kanno, 1990). Additional stability in some cases is provided by micelles of caseins, vital milk proteins. Caseins are found in the form of aggregates called micelles, which are negatively charged at the normal pH of milk, i.e., 6.6 (Sinaga et al., 2017). These charged casein particles repel each other, thus avoiding fat droplet coalescence and stabilizing the milk. Milk, when left to stand, separates into a visible fat layer over an aqueous one, in a process called creaming. To avoid creaming, the fat globules in milk are reduced in size by a process called homogenization: the fat globule membrane is ruptured and fat globules are stabilized by casein micelles (Cano-​Ruiz and Richter, 1997). Thus, homogenized milk may be considered as a Pickering emulsion. In contrast to stabilization of milk, in some cases it is necessary to destabilize milk, for example in cheese making. In the manufacture of cheese, acid or rennet is added to milk, reducing the pH or enzymatically hydrolysing a key stabilizing protein in the casein micelle, which destabilizes the casein micelles, resulting in coagulation of fat and protein particles and eventually separation, leading to the formation of cheese. Fermented products such as curd, buttermilk and yoghurt (Bongard, 2009) are also examples of emulsified systems consisting of oil/​fat and water. Emulsions in which the suspended phase is more than 74% by

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FIGURE 33.3  Schematic representation of microscopic structure of milk showing fat globules and casein micelles suspended in solution of whey proteins and milk sugars.

FIGURE 33.4  Picture showing (from left to right) oat-​based emulsion, coconut-​based emulsion, soy-​based emulsion and almond-​based emulsion.

volume are called high internal phase emulsions (HIPEs) (Patel et al., 2014).

“Plant-​Based Emulsions” (Non-​Dairy) Plant-​based emulsions are oil-​in-​water emulsions obtained from nuts, grains, seeds, legumes and cereals. Plants store oils (i.e., lipids) in the form of droplets (in the size range of ca. 300–​ 350 nm) encapsulated by a layer of phospholipids and proteins (Waschatko et  al., 2012). These droplets are called oil bodies or “oleosomes”. Plant-based drinks are generally extracted by blending and then straining soaked grains (barley, rice, oat and wheat), legumes (soy, lupin, pea and ground nuts), nuts (cashew, almond, hazel nut, pistachio, walnut, etc.), seeds (chia, flax, hemp, pumpkin, sesame, sunflower, safflower, etc.), coconut, etc. Figure 33.4 shows some examples of plant drinks, some of which have been used across cultures around the world for thousands of years. Just as with dairy milk, plant drinks can be used as such or further processed/​fermented to products analogous to yoghurt and cheese. Plant-​based recipes include horchata (a north African beverage from soaked and sweetened tiger nuts), Indian/​Thai

chicken curry with coconut milk, tofu-​based recipes from soy milk, and wheat milk halwa (a type of dessert from India). Plant drinks have attracted attention in the vegan community worldwide. Apart from being used in food products, plant drinks are also used in a variety of cosmetic and skin care products like skin creams, moisturizer, etc.

Butter and Margarine Butter is an example of an emulsified system that consists of more than 83% fat, 13% water, about 3% ash and 1% milk proteins (Farmer, 2011). Butter is also a rich source of fat-​ soluble vitamins like vitamins A, D and E (Delgado-​Zamarreño et  al., 1995). Butter is made by churning fresh or fermented cream to separate butterfat from buttermilk. In order to make butter, the cream recovered over cow milk is cooled to around 5–​7  °C before churning to allow some of the fat to crystallize (Gunstone, 2006). The fat crystals serve as a basis for further crystallization and help to break the milk fat globule membranes during churning (Provost et  al., 2016). The milk fat aggregates first into fat particles and later into solid masses

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FIGURE 33.5  Picture of butter (a) at the temperature of 22 °C and schematic representation of the microscopic structure of butter (b).

during churning. The fat mass collected in this process is subsequently kneaded to remove any remaining pockets of buttermilk and to achieve the desired texture. The final texture of butter depends on the size and structure of the fat crystalline network in the butter that is brought about by the kneading process (typically around 14–​16 °C) (Fuquay et al., 2011). Butter is available in varieties such as salted butter, spreadable butter, clarified butter, ghee and maitre d’hotel butter. Figure  33.5b also shows a schematic representation of the microscopic structure of butter. Margarine is another example of an emulsified system consisting of water droplets in a continuous phase of fat, which is used for flavouring, baking and cooking. Margarine was first created by the French chemist Hippolyte Mège-​Mouriès in 1869 and was originally made as a cheaper substitute for butter using beef tallow. Today, margarine is made from a variety of oils such as safflower, corn, soy bean and animal fats (Carpenter and Slover, 1973). Just as with butter, margarine consists of about 80% oil, 15–​18% water phase and 1–​2% salt and other solids. The oil is often hydrogenated to make it less susceptible to oxidation and improve shelf life. The hydrogenated oil and milk (water base) is mixed with lecithins, and the mixture is blended together in an emulsification chamber at 38  °C. This process leads to the formation of an emulsified system consisting of tiny water droplets (0.42–​2.7 μm) suspended in a crystalline oil phase (Balinov et al., 1994). Then, the contents are cooled to about 7 °C (in a scraped-​surface heat-​exchanger called a votator) to a semi-​ solid state that is further processed and packed.

Spreads, Dips, Dressings and Sauces Spreads, dips, dressings and sauces are also emulsified systems of an oil phase suspended in a water base. In the case of spreads and dips, the proportion of the oil/​fat, i.e., the suspended phase, is very high compared with the suspension medium, i.e., the watery base. Such emulsified systems with a very high percentage (>74% by volume) of oil phase are called HIPEs (Pulko and Krajnc, 2012). Hummus is a good example of a spread/​dip and is made from a mixture of chickpea (Cicer arietinum) puree, tahini

FIGURE  33.6  Picture of hummus (left) and hollandaise sauce (right). In most homemade spreads and sauces, proteins from mustard, sesame and egg yolk act as emulsifiers.

(paste from ground sesame seeds), lemon juice (Citrus limon), olive oil (Olea europaea), cumin (Cuminum cyminum) and salt (Figure 33.6). In this mixture, chickpea puree acts as the water base, and proteins present in the tahini (sesame proteins) act as emulsifier that stabilizes it (Al-​Mahasneh et al., 2017). Hollandaise is an example of a sauce, which contains egg yolks, pepper, lemon juice, butter and salt. For the sauce, the egg yolk is mixed with water (or wine, vinegar or lemon juice), and mixed while heating. This allows the proteins present in egg yolk to denature partially and coagulate to allow the formation of a foam while mixing (Hopia et al., 2013). The melted butter is added subsequently into the mixture, which is then emulsified (together with the lipids from the egg yolk) by the proteins and lecithin from the egg yolk. The resulting sauce is a combination of an (oil-​in-​water) emulsion, a foam and a suspension, as it contains melted fat droplets, air bubbles and protein aggregates from the egg (This, 2016; Rognsa et al., 2014). The final texture of the sauce, in turn, depends on the amount of air incorporated into the sauce and the size of the fat droplets (Hopia et al., 2013; Rognsa et al., 2014).

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FIGURE 33.7  Liver and blood sausages, and a smoked bratwurst (a); schematic of microstructure of sausage emulsion (b).

Sausages Sausages, such as German frankfurter, Austrian wiener, hot dogs, mortadella, bologna, liver sausages and pâté, are also examples of emulsified dispersions of fat in water together with particulates of meat. The manufacturing process of sausages involves the preparation of an emulsified mixture of finely chopped or minced meat in water together with seasonings, salts and, where necessary, binders or extenders (Belitz et al., 2004). The exact composition of ingredients depends on the type of sausage being produced. Initially, the (muscle and fat) tissue is comminuted (i.e., chopped, flaked, ground and/​ or minced) to the desired degree (Toldra, 2010; Varnam et al., 1995). The final texture of the sausages is determined by the duration and the method of comminution (Belitz et  al., 2004). This process generates a lot of heat, however, due to friction between the sausage batter and the blades (heating the contact surface up to 80  °C) and can increase the temperature of the meat mixture by 10–​20 °C during the first 15 minutes of processing (Toldra, 2010; Pearson and Gillett, 2012). To minimize cooking losses and achieve optimum stability for the emulsified system, the comminution temperatures should be kept below 18  °C (Belitz et  al., 2004; Essien, 2003; Zayas, 2012). This is achieved by using ice or iced water. When meat is cut into fine particles, the muscle fibres and connective tissue are broken into pieces and mixed with fat droplets and particles of fatty tissue. During the processing, the fat partially melts and leads to the formation of an emulsion system with the proteins present in the system (Belitz et al., 2004). The different types of protein contribute to different degrees to the formation of a monomolecular layer (ca. 130 nm in thickness) around the fat globules (in decreasing order of importance: myosin, actomyosin, sarcoplasmic proteins and actin) (Belitz et  al., 2004). The fat droplets (together with water) in the mixture are thus suspended in a matrix of (salt-​soluble) proteins, leading to a thick paste, which is an emulsified suspension (Figure 33.7) (Zayas, 2012). To further improve the texture and mouthfeel, other components such as water binders and emulsifying agents are added to the mixture. Finally, a mixture of spices, salt and sodium nitrite is added, which also helps in curing the meat. Depending on the

type of sausage being produced, the sausage batter may be loaded into casings and sold raw (e.g., bratwurst and breakfast sausages) or smoked (e.g., kielbasa), fermented (e.g., sucuk), cooked (e.g., liver sausage, blood sausage) or dried (e.g., salamis and cervelats) (Belitz et al., 2004; Pearson and Gillett, 2012).

Anise-​Flavoured Liquors and Spirits (Microemulsions) A special type of emulsion called a microemulsion occurs when a highly hydrophobic oil (e.g., anethole in anise-​ flavoured drinks) dissolved in a water–​ethanol solution (as, for example, in ouzo) is diluted with water. In this case, a spontaneous oil-​ in-​water emulsion is formed, the mixture turning from being clear to opalescent (cloudy) (Carteau et al., 2008). This is commonly called the “ouzo effect” (also “pastis effect”), or “louche”, or “spontaneous emulsification” (Lopez-​Montilla et  al., 2002). Microemulsions form spontaneously and are highly stable even without any surfactant. The underlying physical principles are depicted in Figure 33.8 as a simplified system of three components, with ethanol being the emulsifier (with a hydrophobic hydrocarbon tail and a hydrophilic OH group). The (yellow) oily odorant molecules in ouzo surround themselves with ethanol molecules (red dots) and are thus dispersed in the continuous water phase (blue dots). However, ethanol is not a particularly strong emulsifier (with a partition coefficient of around −1.0 to −1.5) due to its short hydrophobic tail of only two carbon atoms (Leo et  al., 1971; Vilgis, 2011). Thus, the ethanol molecules are only loosely bound to the oil–​water interface and retain much of their freedom to diffuse and move around in the solution. A thermodynamic equilibrium is thus formed between the states of being bound to the oil–​water interface and being diffused in the water. This equilibrium can then, in turn, be disturbed by adding water into the drink. The ethanol molecules diffuse within a larger volume of water, thus reducing the number of available alcohol molecules near the anethole molecules. This causes the anethole molecules to come together to “share” the available ethanol molecules with each other. This process continues with the addition of water until the anethole droplets become large enough to scatter light, which

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FIGURE  33.8  A schematic representation of the ouzo effect/​pastis effect. At higher concentrations of water, molecules of essential oil (anethole, or 1-​ methoxy-​4-​[(1E)-​prop-​1-​en-​1-​yl]benzene) come together to maximize their exposure to alcohol, leading to the formation of nanoscopic oil droplets, which scatter light, resulting in the development of the characteristic cloudiness of ouzo or pastis.

causes the solution to become turbid. The droplet size of such emulsions ranges from 300 nm to about 2.4 μm and can change with composition and time (Grillo, 2003). Pastis (France), aquavit (Scandinavia), arak (Persia and the Middle East), sambuca (Italy), aguardiente (Columbia), anis (Spain), ouzo (Greece), absinthe (Europe), mastik (Balkans), raki (Turkey) and xtabentun (Mexico) are a few examples of liquors that form microemulsions when served with water. These liquors generally contain essential oils extracted into alcohol from a range of spices (like wormwood, anise and fennel), herbs (like peppermint, coriander, hyssop, etc.) and botanicals (like roots, barks, flowers and seeds). This effect is also demonstrated in Figure 33.9: it is apparent that a small amount of water is enough to disturb the delicate balance in the drink and to cause it to turn cloudy.

Beverages Beverages such as fruit/​vegetable juices and milk-​based beverages like chocolate milk, milkshakes and several types of coffee-​ based beverages fall into the category of emulsified suspensions (a suspension and also an emulsion). A simple example such as fruit juice contains crushed fruit pulp suspended in water. To this base, several different flavouring oils such as citrus peel (Citrus sinensis), lemon (Citrus limon), lavender (Lavandula angustifolia), peppermint (Mentha piperita), thyme (Thymus vulgaris), cinnamon (Cinnamomum verum), tea tree (Melaleuca

alternifolia), rose wood (Dalbergia nigra), etc. are added to improve the flavour of the juice. Further, polysaccharides such as xanthan (Mirhosseini et al., 2008), carrageenan or pectin (Ibarz et  al., 1996) may be added to improve the mouthfeel and flow properties, stability and shelf life of the final product. Another class of beverages are milk based, such as fruit milkshakes, chocolate milks and coffee-​milk-​based beverages. In all these beverages, milk forms the water base of the beverage, and most of the flavouring compounds, such as carotenoids, polyphenols and essential oils, are emulsified into fine droplets suspended in this water phase (Voilley and Etiévant, 1995). The fat component of the milk is also suspended in this complex mixture, thus forming an oil-​in-​water emulsion. Many of the final properties of these beverages, like texture, mouthfeel, flavour release, stability and shelf life, depend on droplet size distribution, viscosity and interactions between different components in the mixture.

Foams in Food Systems Foams in food systems are typically dispersions of gas bubbles in a liquid, which at first sight resemble emulsions. Instead of fat molecules (or triacylglycerols, to be exact), the dispersed phase in foams consists of a gas (e.g., air or carbon dioxide), and the dispersion medium is typically a liquid. However, incorporating air into a liquid without any additional stabilizers typically results

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FIGURE 33.9  Picture showing cloudiness (turbidity) developed in mixtures of ouzo and water at different mixing proportions. In all glasses, 80 mL ouzo was mixed with (from left to right) 0, 10, 15, 17.5, 20, 22.5, 25, 30 and 40 mL of water.

FIGURE 33.10  Examples of emulsified systems (beverages): (from left to right) latte macchiato, strawberry yoghurt (emulsified gel), mango lassi and chocolate milk.

FIGURE 33.11  Dark field polarization microscopy image of foam bubbles stabilized by a mixture of wheat gluten proteins. Microscopic foam bubbles form a dense network, giving a rich texture to the foam.

only in short-​lived bubbles. More stable culinary foams are thus created with proteins, fats or surfactants (abbreviation from “surface active agent”) (Myhrvold et  al., 2011). Different types of foams are stabilized by different underlying mechanisms. For example, a foam made with egg white is a typical foam stabilized by proteins. In this case, the protein molecules are partially denatured by whipping and subsequently coat the surface of the air bubbles. In this process, the denatured proteins rearrange at the water–​gas interface in such a way that the hydrophobic segments of the protein stick to the gas phase while the hydrophilic parts remain in the water phase (Kijowski, 1995; Brown, 2018). In this way, the proteins form a thin film around the air bubbles, thus stabilizing the foam. Whipping cream, on the other hand, is stabilized with semi-​solid fat crystals (Hasenhuettl and Hartel, 2008). Dairy triacylglycerides have melting points between −40  °C and +40  °C, so that at room temperature, whipping cream contains both solid fat crystals and melted fat (Chandan, 2011; Marshall et  al., 2012). Through whipping, the triacylglycerol molecules (in crystals and in liquid droplets) move to the edges of the gas bubbles, where they form a thin membrane around them, stabilizing the foam. Thus, as the triacylglycerol molecules have to be present both as solid crystals and in melted form in order to make whipped cream, the temperature of the cream should not be too high. According to the current consensus, the best whipped cream is achieved by using chilled/​refrigerated whipping cream at around 5–​10  °C (Hasenhuettl and Hartel, 2008; Ihara et  al., 2010; Myhrvold et al., 2011; Provost et al., 2016; Brown, 2018). Finally, foams can be formed using emulsifiers, such as lecithin from egg yolk. The underlying physics in this case are similar to those described earlier in the context of emulsions: the emulsifier molecules arrange themselves at the gas–​liquid interface in such a way that the hydrophobic part of the emulsifier remains in the gas and the hydrophilic in the water phase (Vega et al., 2012).

Emulsions: Emulsified Systems in Food Culinary foams, just like emulsions, are inherently not stable and collapse over time (with the exception of solid foams such as bread). The mechanisms of foam decay are similar to those of emulsion instability and phase separation (Hasenhuettl and Hartel, 2008; Bamforth et  al., 2011). To name some, the walls between bubbles can burst (causing the bubbles to merge), leading to foam coalescence. As gasses are (to varying degree depending on the gas and pressure) soluble in water, the gas molecules can dissolve into the water phase and escape the bubbles. This process takes place between bubbles with different Laplace pressures, which leads the bubbles to diffuse from areas of high Laplace pressure (small bubbles) into areas of lower Laplace pressure (large bubbles). For example, the Laplace pressure of an air bubble (at 25 °C) in water with a diameter of 1000 μm is around 300  Pa (Weaire and Hutzler, 2001). Halving the diameter (500  μm) leads the pressure to increase to almost 600  Pa, and a bubble of only 100 μm has a Laplace pressure of almost 3 kPa. This behaviour of gas molecules diffusing from smaller to larger bubbles is called disproportionation (the foam “version” of Ostwald ripening). Finally, as a foam consists of a dispersed gas phase separated by a thin membrane from the continuous water phase, the water slowly drains away from between the bubbles, a phenomenon called film drainage.

Beer Foams The beer head is one of the first characteristics we tend to notice when we receive our drink. Too much of it, and we feel cheated out of our beer; too little of it, and the beer appears flat and stale. Not surprisingly, scientific research has also established that the quality of the beer foam is perceived as one of the most (if not even the most) important signs of overall quality in a served beer (Bamforth, 2000). Apart from the perceived quality, however, beer foam also fulfils several other important functions. To name some, the foam dampens the oscillations of the liquid when the glass is carried (about 10% in comparison to a headless beer) (Narziß et  al., 2017). It also serves as a barrier, slowing down the escape of gas out of the beer; the bitter flavour of hops tends to concentrate in it, and many aroma compounds also tend to concentrate in the foam (Bamforth, 2016). Lastly, the foam also contributes to the mouthfeel of the beer, and in part to the characteristic CO2 tingle (Langstaff and Lewis, 1993a; Langstaff and Lewis, 1993b). A lot of research effort has been put into understanding the life of beer foam. There are several substances that contribute to (or reduce) foam quantity and quality (such as proteins, polyphenols, alpha acids, oligo-​/​polysaccharides and metal ions, to name some) (Bamforth et al., 2011). Essentially, however, beer foam is a type of foam stabilized by proteins, originating mainly from the malts (Asano and Hashimoto, 1976, 1980; Bamforth, 1985). A further important factor increasing the stability of the beer foam is the so called iso-​α-​acids contributed by the hops. The iso-​α-​acid molecules are surface-​active and contribute to the head stability by cross-​linking foam proteins. An example of an alpha acid is humulone, also called α-​lupulic acid ((6S)-​3,5,6-​ trihydroxy-​2-​(3-​methylbutanoyl)-​4,6-​bis(3-​methylbut-​2-​en-​1-​ yl)cyclohexa-​2,4-​dien-​1-​one). Together, these two components

235 lay the foundations for a stable beer foam. Further components such as polyphenols and melanoidins similarly support the foam stability (Bamforth, 1998; Bamforth et  al., 2011; Lewis and Lewis, 2003). Finally, substances that increase the beer viscosity (polysaccharides such as β-​glucans, consisting of six-​ sided D-​glucose monomers) and pentosans ([(2R,3R,4S,5R)-​2-​ hydroxy-​5-​[(2S,3R,4S,5R)-​5-​hydroxy-​3,4-​disulfooxyoxan-​2-​yl] oxy-​3-​sulfooxyoxan-​4-​yl] hydrogen sulphate) originating from the malts) slow down the rise of the gas bubbles in the beer, giving them more time to collect foam-​promoting substances on their way up (Bamforth et al., 2011). However, research until now has not been able to show unambiguously that increased viscosity increases the foam stability by reducing foam drainage. Fats and detergents, on the other hand, inhibit foam formation and destabilize existing foams, which is why dirty drinking glasses tend to make themselves noticeable (Langstaff and Lewis, 1993a; Dickie et al., 2001; Bamforth et al., 2011). Apart from the main players in beer foam stability (proteins and iso-​α-​acids), factors such as alcohol content, pH, polyphenols and gas type influence the foam quality in beer.. Both ethanol and polyphenols have been shown to be foam-​promoting or foam-​inhibiting depending on the exact circumstances (Sarker et al., 1995; Brierley et al., 1996; Bamforth, 1998; Evans et  al., 1999, 2002; Fisher et  al., 1999; Lewis and Lewis, 2003). Ethanol generally lowers the surface tension in beer foam, and thus, it could be expected that high ethanol content would promote foam stability (weak correlation between ethanol content and improved foam stability has been established in some studies) (Bamforth, 1998). However, it has also been observed that a high ethanol content possibly disturbs the interactions between proteins and iso-​α-​acids and thus destabilizes the foam (akin to lipids). Polyphenols, on the other hand, are assumed to slightly promote foam stability in the same way as iso-​α-​acids do (cross-​ linking proteins in the foam) (Lewis and Lewis, 2003; Bamforth, 1998; Sarker et al., 1995). However, polyphenols are also generally associated with the precipitation of foam-​promoting proteins in different stages of the brewing process (and leading to haze formation later in the finished product) (Lewis and Serbia, 1984; Barth, 2013; Bamforth, 2016). The pH of beer has been observed to correlate with foam stability (Melm et  al., 1995; Bamforth and Kanauchi, 2003). The reason for this is assumed to be the amphiphilic nature of proteins and iso-​α-​acids, which means that a variation in pH results in a change in charge in these substances. Thus, a lower pH could lead to a greater dissociation of the proteins and iso-​α-​acids, which would in turn promote their migration to the beer–​gas interface. The change in charge is also expected to aid their interaction. Lastly, beer is typically carbonated either using carbon dioxide or nitrogen, as demonstrated in Figure 33.12. Guinness is typically nitrogenized, whereas in other types of beer (such as wheat beer), carbon dioxide is used. The advantage of using nitrogen in beer when compared with CO2 arises mainly from the substantially lower solubility of nitrogen in beer (about 100 times less than CO2) and from the fact that it is pH neutral (Bamforth et al., 2011). Due to the lower solubility of nitrogen, higher gas pressures can be used to dispense beer through narrower openings to produce a great number of small bubbles (Carroll, 1979; Barth,

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FIGURE 33.12  (a) Comparison of beer foam on wheat beer (left) and Guinness (right). (b) Fine and dense foam on Guinness beer. (c) Coarse foam on wheat beer.

2013). The foam stability is, in turn, enhanced by the lower partial pressures between the inner bubble and the atmosphere when using nitrogen. Thus, by the time the foam on a wheat beer is long gone, the foam on a Guinness remains practically unchanged.

Ice Creams Ice creams, when considering their physics and chemistry, are surprisingly complex systems. A  wide range of frozen desserts fall under the category of “ice cream”. Dairy ice creams (dairy ingredients, sugar and flavours), gelato (Italian custard-​based ice cream) and sorbet (fruit-​based ice cream that contains no fat and no milk) are a few popular varieties (Clarke, 2015).They are complex emulsified systems consisting of air (about 30–​50% of the volume), ice crystals (up to 40%), fat crystals (5–​10%) and an aqueous phase consisting of sugar, milk proteins and flavouring components (Merkus and Meesters, 2013). The proteins play a role in stabilizing fat globules and air bubbles in the ice cream. At the same time, some emulsifiers are added to ice cream mixtures to displace some of the proteins adsorbed to the fat interfaces (Marshall et al., 2003). This not only allows the fat droplets to partially coalesce and entrap air bubbles (similar to the mechanism in whipped cream) but is also very important for the right texture and to slow the meltdown (Cropper et al., 2013). Any remaining proteins that are not adsorbed to the interfaces remain dissolved in the unfrozen water phase and increase its viscosity (El-​Zeini, 2016). Finally, yet importantly, the proteins also influence the flavours of the ice cream. As the milk protein content of the ice cream increases, there is a corresponding decrease in flavour intensity due to binding interactions of flavour molecules with proteins (Hansen, 1996). The term “sugar” in ice cream manufacturing covers several different kinds of sugar compounds, including such sugars as D-​glucose, D-​fructose, sucrose and sugar alcohols (Ozdemir et al., 2008). Apart from sugar giving the ice cream its sweetness, the softness and the viscosity of the ice cream can be modified

by varying the sugar content and the composition of the added sugar mix (Clarke, 2015). Generally speaking, a higher sugar content leads to fewer ice crystals in ice cream, thus making it softer (Vega et al., 2012). Addition of sugar molecules causes a depression in the freezing point of water in ice cream. This slows down the crystallization process and thus leads to the formation of fewer ice crystals. The same effect can also be attained without increasing the sweetness of the ice cream by using different kinds of sugars. The fat in ice cream also plays several roles. As already mentioned, the fat helps to stabilize the foam part of the ice cream, but it is also responsible for the creamy texture, and any fat-​soluble flavour molecules are dissolved in it. The fat also influences the mouldability (Goff, 1997; Goff, 2008) of the ice cream and helps it to retain its shape when it is melting. Butterfat, cream and vegetable oils are commonly used in both industrial and homemade ice creams as fat sources. Water makes up about 60–​72% of ice cream’s weight (Clarke, 2015). Not all of this is frozen into crystals; 15–​27% remains liquid and contains dissolved sugars and proteins. Smaller ice crystals are preferred over large ones, as they give a softer texture, unlike larger crystals, which lead to a coarser, gritty texture. The problem, however, is the propensity of the ice crystals for recrystallization, which causes the small ice crystals to merge into larger ones over time. This is also one of the major factors limiting ice cream’s shelf life. In order to slow this down, it is necessary to use stabilizers (usually polysaccharides such as locust bean gum or carrageenans). Apart from limiting ice crystal growth, stabilizers also help to achieve a smooth texture, reduce or slow down lactose crystal growth, and increase the ice cream’s resistance against meltdown. Making a short leap from the fundamental physics to manufacturing ice cream, there are different possibilities for how to make the ice cream after mixing the ingredients. Factory-​produced ice cream is typically homogenized, pasteurized and aged before further processing (Batt and Tortorello, 2014), but for home use, these steps are skipped. In the food industry, the freezing is quite

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Emulsions: Emulsified Systems in Food often done with devices called scraped surface heat exchangers, which in their simplest form consist of a cylindrical container with a double wall and a rotating dasher in the middle. The working principle is quite similar to that of a refrigerator; a liquefied gas (typically liquid ammonia) flows through the double wall of the cylinder and cools the container (down to around −30 °C) as it evaporates. The dasher in the middle scrapes the frozen ice cream off the walls of the vessel while beating and aerating it simultaneously. After initial freezing, the ice cream is hardened at around −30 to −45 °C in a freezer with cold air. Alternatively, the ice cream can also be processed via low-​temperature extrusion, whereby the ice cream is sent through a cylindrical container with refrigerated walls and a screw extruder. This method offers several advantages in comparison to the first method, such as higher shear stress with lower shear rate, lower processing temperatures, and the formation of smaller ice crystals and air bubbles. The basic principles of ice cream manufacturing stay the same for homemade ice cream. The easiest way to do this is by using an ice cream maker, which works in the same way as the scraped surface heat exchangers used in professional manufacturing. One of the major differences in this case is the freezing temperature (around −20 °C), which is reached by using salted ice water (or an eutectic mixture of ice and salt, to be precise). Alternatively, the ice cream mixture can be whisked at regular intervals while freezing (e.g., once every hour for two or three times) to break the ice crystals. In this way, however, the consistency of the ice cream won’t necessarily be as good as by using an ice cream maker.

Pacojet Ice Cream One possible way of making ice cream in professional gastronomy is by using Pacojet. In this equipment, the ice cream mixture is frozen overnight and the frozen mixture is “blended” (or “pacotized”) the following day. We conducted our own experiments on different flavour combinations in this way. The basic recipe used in our simple experiments consisted essentially of the main flavour component (e.g., mango juice, red wine, tea-​flavoured milk or plant-based drink), an emulsifier (e.g., egg yolk or pure lecithin), fat (e.g., olive oil or butter), and sugar. The following are four examples for recipes we used (Figure 33.13).

FIGURE 33.13  Mango-​mint-​olive oil ice cream.

Earl Grey ice cream 300 g Full-​fat  milk 200 g Whipping cream 3 bags Earl Grey tea 100 g Sugar 1 Egg yolk Blueberry-​oat ice cream 400 g Oat milk 60 g Melted butter 300 g Blueberries (frozen) 60 g Oats 100 g Sugar 1 Egg yolk For the recipe with Earl Grey, the milk–​ cream mixture was warmed up and the tea bags soaked in it for about 10 minutes. Otherwise, the ingredients were simply mixed together/​blended and frozen overnight. The frozen mixture was pacotized the following day and served immediately. For better consistency, the pacotized ice cream can be stored in a freezer for about an hour before serving. The ingredients and amounts can easily be varied for different flavour combinations and consistencies. As quite often, the only limit is your imagination!

Mango-​mint-​olive oil ice cream 440 g Mango juice 60 g Olive oil 100 g Caster sugar 1 Egg yolk Handful of mint Glühwein ice cream 220 g 220 g 60 g 100 g 1

Glühwein /​mulled wine Berry juice (mixed berries, e.g., black currants, raspberries) Melted butter Caster sugar Egg yolk

Conclusion This chapter is intended to give a rough overview on the topic of emulsified systems and foams in the context of food systems. Emulsified systems play a big role in the food industry, but they can also be found in the cosmetic, pharmaceutical and chemical industries, to name a few. Among foods, there is a great variety of emulsified systems ranging from beverages to solid food. As both emulsified systems and foams are fundamentally metastable, it is important to understand the physical principles underlying this instability. This allows one to improve the shelf life, flavour, aroma retention, texture and many other aspects of food products. Understanding these principles will also help with home cooking,

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Carteau D, Bassani D, Pianet I. 2008. The “Ouzo effect”: Following the spontaneous emulsification of trans-​anethole in water by NMR. Comptes Rendus Chimie, 11 (4–​5), 493–​498. Chandan RC. 2011. Dairy ingredients for food processing: An overREFERENCES view. In Dairy Ingredients for Food Processing (eds Chandan Adams F, Walstra P, Brooks B, Richmond H, Zerfa M, Bibette J, RC and Kilara A). Blackwell Publishing Ltd, 3–​33. Hibberd D, Robins M, Weers J, Kabalnov A. 2007. Modern Clarke C. 2015. The science of ice cream. Royal Society of Chemistry. aspects of emulsion science. Royal Society of Chemistry. Cropper S, Kocaoglu-​Vurma N, Tharp B, Harper W. 2013. Effects of Al-​Mahasneh M, Rababah T, Alu’Datt M. 2017. Effect of palm oil locust bean gum and mono-​and diglyceride concentrations on (PO) and distilled mono-​ glycerid (dmg) on oil separation particle size and melting rates of ice cream. Journal of Food and rheological properties of sesame paste. Journal of Food Science, 78 (6), C811–​C816. Processing and Preservation, 41 (3), e12896. Delgado-​ Zamarreño M, Sanchez-​ Perez A, Gomez-​ Perez M, Alger DB. 1994. Erroneous explanations for the limited water soluHernández-​ Méndez J. 1995. Automatic determination of bility of organic liquids. Journal of Chemical Education, 71 liposoluble vitamins in butter and margarine using Triton X-​ (4), 281. 100 aqueous micellar solution by liquid chromatography with Asano K, Hashimoto N. 1976. Contribution of hop bitter substances electrochemical detection. Analytica Chimica Acta, 315 (1–​2), to head formation of beer. Report of the Research Laboratories 201–​208. of Kirin Brewery Co. Ltd, 19, 9–​16. Dickie KH, Cann C, Norman EC, Bamforth CW, Muller RE. 2001. Asano K, Hashimoto N. 1980. Isolation and characterization of Foam-​negative materials. Journal of the American Society of foaming proteins of beer. Journal of the American Society of Brewing Chemists, 59 (1), 17–​23. Brewing Chemists, 38 (4), 129–​137. Dickinson E, Semenova MG, Antipova AS. 1998. Salt stability of Balinov B, Söderman O, Wärnheim T. 1994. Determination of water casein emulsions. Food Hydrocolloids, 12 (2), 227–​235. droplet size in margarines and low-​calorie spreads by nuclear Drakos A, Kiosseoglou V. 2006. Stability of acidic egg white protein magnetic resonance self-​diffusion. Journal of the American emulsions containing xanthan gum. Journal of Agricultural Oil Chemists’ Society, 71 (5), 513–​518. and Food Chemistry, 54 (26), 10164–​10169. Bamforth C. 1985. The foaming properties of beer. Journal of the El-​Zeini H, El-​Abd M, Mostafa A, El-​Ghany F. 2016. Effect of Institute of Brewing, 91 (6), 370–​383. incorporating whey protein concentrate on chemical, rheoBamforth C. 2000. Perceptions of beer foam. Journal of the Institute logical and textural properties of ice cream. Journal of Food of Brewing, 106 (4), 229–​238. Processing and Technology, 7 (2), 1000546. Bamforth C. 2016. Brewing materials and processes: A practical Essien E. 2003. Sausage manufacture: Principles and practice. approach to beer excellence. Academic Press. Elsevier Science. Bamforth C, Kanauchi M. 2003. Interactions between polypeptides Evans DE, Sheehan M, Stewart D. 1999.The impact of malt derived derived from barley and other beer components in model foam proteins on beer foam quality. Part II: The influence of malt systems. Journal of the Science of Food and Agriculture, 83 foam-​ positive proteins and non-​ starch polysaccharides on (10), 1045–​1050. beer foam quality. Journal of the Institute of Brewing, 105 (3), Bamforth C, Russell I, Stewart G. 2011. Beer: A quality perspective. 171–​178. Academic Press. Evans DE, Sheehan MC. 2002. Don’t be fobbed off: The substance Bamforth CW. 1998. Bringing matters to a head: the status of research of beer foam –​A review. Journal of the American Society of on beer foam. In Monograph European Brewery Convention. Brewing Chemists, 60 (2), 47–​57. Verlag Hans Carl Getraenke-​Facherverlag, 10–​23. Farmer FM. 2011. The Boston Cooking School cook book. Read Barth R. 2013. The chemistry of beer: The science in the suds. Wiley. Books Ltd. Batt CA, Tortorello ML. 2014. Encyclopedia of food microbiology. Fischer S, Sommer K. 1999. Cling of beer foam to different Academic Press, Vol. 10. surfaces. In Proceedings of the Congress-​European Brewery Belitz HD, Grosch W, Schieberle P, Burghagen MM. 2004. Food Concention, 183–​190. chemistry. Springer. Fuquay JW, McSweeney PL, Fox PF. 2011. Encyclopedia of dairy Bongard S, Meynier A, Riaublanc A, Genot C. 2009. Protein-​ sciences. Academic Press. stabilized oil-​in-​water emulsions and low-​fat dairy stirred gels. Goff HD. 1997. Colloidal aspects of ice cream—​ a review. Journal of Food & Nutrition Research, 48 (1). International Dairy Journal, 7 (6–​7), 363–​373. Brierley ER, Wilde PJ, Onishi A, Hughes PS, Simpson WJ, Clark Goff HD. 2008. 65 Years of ice cream science. International Dairy DC. 1996. The influence of ethanol on the foaming properJournal, 18 (7), 754–​758. ties of beer protein fractions: A comparison of Rudin and Griffin WC. 1949. Classification of surface-​active agents by “HLB”. microconductivity methods of foam assessment. Journal of the Journal of the Society of Cosmetic Chemists, 1, 311–​326. Science of Food and Agriculture, 70 (4), 531–​537. Griffin WC. 1954. Calculation of HLB values of non-​ionic surfactants. Brown AC. 2018. Understanding food: Principles and preparation. Journal of the Society of Cosmetic Chemists, 5, 249–​256. Cengage Learning. Grillo I. 2003. Small-​angle neutron scattering study of a world-​ Cano-​Ruiz M, Richter R. 1997. Effect of homogenization pressure wide known emulsion: Le Pastis. Colloids and Surfaces on the milk fat globule membrane proteins. Journal of Dairy A: Physicochemical and Engineering Aspects, 225 (1–​ 3), Science, 80 (11), 2732–​2739. 153–​160. Carpenter D, Slover H. 1973. Lipid composition of selected Gunstone FD. 2006. Modifying lipids for use in food. Woodhead margarines. Journal of the American Oil Chemists Society, 50 Publishing. (9), 372–​376. Hansen A, Booker D. 1996. Flavor interaction with casein and whey Carroll T. 1979. The effect of dissolved nitrogen gas on beer foam protein. ACS Publications. and palate. Technical Quarterly Master Brewers Association of Hasenhuettl GL, Hartel RW. 2008. Food emulsifiers and their America, 16 (3), 116–​119. applications. Springer, Vol. 19.

whether saving a failing mayonnaise or appreciating why chilled whipping cream is better suited for whipping than warm.

Emulsions: Emulsified Systems in Food Hopia A, Sillanpää M, Tuomisto M. 2013. Hollandaise sauce and the chemistry behind old and new preparation techniques. LUMAT (2013–​2015 Issues), 1 (2), 189–​196. Hu M, McClements DJ, Decker EA. 2003. Lipid oxidation in corn oil-​in-​water emulsions stabilized by casein, whey protein isolate, and soy protein isolate. Journal of Agricultural and Food Chemistry, 51 (6), 1696–​1700. Huque EM. 1989. The hydrophobic effect. Journal of Chemical Education, 66 (7), 581. Ibarz A, Garvin A, Costa J. 1996. Rheological behaviour of sloe (Prunus spinosa) fruit juices. Journal of Food Engineering, 27 (4), 423–​430. Ihara K, Habara K, Ozaki Y, Nakamura K, Ochi H, Saito H, Asaoka H, Uozumi M, Ichihashi N, Iwatsuki K. 2010. Influence of whipping temperature on the whipping properties and rheological characteristics of whipped cream. Journal of Dairy Science, 93 (7), 2887–​2895. Kabalnov AS, Shchukin ED. 1992. Ostwald ripening theory: Applications to fluorocarbon emulsion stability. Advances in Colloid and Interface Science, 38, 69–​97. Kanno, C. 1990. Secretory membranes of the lactating mammary gland. Protoplasma, 159 (2–​3), 184–​208. Kijowski J. 1995. Muscle proteins. In Chemical and functional properties of food proteins (ed. Sikorski ZE). CRC Press, pp. 233–​239. Langstaff S, Lewis M. 1993a. Foam and the perception of beer flavor and mouthfeel. Technical quarterly  –​Master Brewers Association of the Americas (USA), 30 (1), 16–​17. Langstaff SA, Lewis M. 1993b. The mouthfeel of beer –​a review. Journal of the Institute of Brewing, 99 (1), 31–​37. Leo A, Hansch C, Elkins D. 1971. Partition coefficients and their uses. Chemical Reviews, 71 (6), 525–​616. Lewis M, Serbia J. 1984. Aggregation of protein and precipitation by polyphenol in mashing. Journal of the American Society of Brewing Chemists, 42 (1), 40–​43. Lewis MJ, Lewis AS. 2003. Correlation of beer foam with other beer properties. Technical Quarterly & the MBAA Communicator, 40 (2), 114–​124. Linke C, Drusch S. 2018. Pickering emulsions in foods –​opportunities and limitations. Critical Reviews in Food Science and Nutrition, 58 (12), 1971–​1985. Lopez C, Ménard O. 2011. Human milk fat globules: Polar lipid composition and in situ structural investigations revealing the heterogeneous distribution of proteins and the lateral segregation of sphingomyelin in the biological membrane. Colloids and Surfaces B: Biointerfaces, 83 (1), 29–​41. López-​Montilla JC, Herrera-​Morales PE, Pandey S, Shah DO. 2002. Spontaneous emulsification: Mechanisms, physicochemical aspects, modeling, and applications. Journal of Dispersion Science and Technology, 23 (1–​3), 219–​268. Marshall RT, Goff HD, Hartel RW. 2003. Ice cream ingredients. In Ice cream (eds Goff HD and Hartel RW), Springer, 55–​87. Marshall, RT, Goff HD, Hartel RW. 2012. Ice cream. Springer. McClements DJ. 2015. Food emulsions: Principles, practices, and techniques. CRC Press. McNaught AD, McNaught AD. 1997. Compendium of chemical terminology. Blackwell Science Oxford, Vol. 1669. Melm G, Tung P, Pringle A. 1995. Mathematical modeling of beer foam. Technical quarterly (Master Brewers Association of the Americas) (USA), 32 (1), 6–​10. Merkus HG, Meesters GM. 2013. Particulate products: Tailoring properties for optimal performance. Springer, Vol. 19.

239 Mirhosseini H, Tan CP, Hamid NS, Yusof S. 2008. Effect of Arabic gum, xanthan gum and orange oil on flavor release from diluted orange beverage emulsion. Food Chemistry, 107 (3), 1161–​1172. Myhrvold, N, Young C, Bilet M. 2011. Modernist cuisine: The art and science of cooking. The Cooking Lab. Narziß L, Back W, Gastl M, Zarnkow M. 2017. Abriss der Bierbrauerei. John Wiley & Sons. Ohshima H. 2016. Encyclopedia of biocolloid and biointerface science, 2 volume set. John Wiley & Sons, Vol. 2. Olijve J, Mori F, Toda Y. 2001. Influence of the molecular-​weight distribution of gelatin on emulsion stability. Journal of Colloid and Interface Science, 243 (2), 476–​482. Ozdemir C, Dagdemir E, Ozdemir S, Sagdic O. 2008. The effects of using alternative sweeteners to sucrose on ice cream quality. Journal of Food Quality, 31 (4), 415–​428. Patel AR, Rodriguez Y, Lesaffer A, Dewettinck K. 2014. High internal phase emulsion gels (HIPE-​gels) prepared using food-​ grade components. RSC Advances, 4 (35), 18136–​18140. Pearson AM, Gillett TA. 2012. Processed meats. Springer US. Provost JJ, Colabroy KL, Kelly BS, Wallert MA. 2016. The science of cooking: Understanding the biology and chemistry behind food and cooking. John Wiley & Sons. Pulko I, Krajnc P. 2012. High internal phase emulsion templating –​ a path to hierarchically porous functional polymers. Macromolecular Rapid Communications, 33 (20), 1731–​1746. Rognså GH, Rathe M, Paulsen MT, Petersen MA, Brüggemann DA, Sivertsvik M, Risbo J. 2014. Preparation methods influence gastronomical outcome of hollandaise sauce. International Journal of Gastronomy and Food Science, 2 (1), 32–​45. Sarker DK, Wilde PJ, Clark DC. 1995. Control of surfactant-​induced destabilization of foams through polyphenol-​ mediated protein-​protein interactions. Journal of Agricultural and Food Chemistry, 43 (2), 295–​300. Siemens AO, Van Hecke M. 2010. Jamming: A simple introduction. Physica A: Statistical Mechanics and Its Applications, 389 (20), 4255–​4264. Silverstein TP. 1998. The real reason why oil and water don’t mix. Journal of Chemical Education, 75 (1), 116. Sinaga H, Bansal N, Bhandari B. 2017. Effects of milk pH alteration on casein micelle size and gelation properties of milk. International Journal of Food Properties, 20 (1), 179–​197. Streitwieser A, Heathcock CH, Kosower EM, Corfield PJ. 1992. Introduction to organic chemistry. Macmillan. This H. 2016. Solution to Hollandaise challenge. Analytical and Bioanalytical Chemistry, 408 (27), 7543. Toldrá F. 2010. Handbook of meat processing. Wiley. Varnam A, Sutherland JM, Sutherland JP. 1995. Meat and meat products: Technology, chemistry and microbiology. Springer US. Vega C, Ubbink J, Van der Linden E. 2012. The kitchen as laboratory: Reflections on the science of food and cooking. Columbia University Press. Vilgis TA. 2011. Das Molekül-​Menü S. Hirzel Verlag. Voilley A, Etiévant P. 2006. Flavour in food. Elsevier Science. Waschatko G, Junghans A, Vilgis TA. 2012. Soy milk oleosome behaviour at the air–​ water interface. Faraday Discussions, 158, 157–​169. Weaire DL, Hutzler S. 2001. The physics of foams. Clarendon Press. Zayas JF. 2012. Functionality of proteins in food. Springer.

Emulsions and Foams: Ostwald Ripening and Disproportionation in Practice Hervé This vo Kientza1,2 INRAE, AgroParisTech, UMR 0782 SayFood, 75005, Paris, France 2 Group of Molecular Gastronomy, INRAE-​ AgroParisTech International Centre for Molecular Gastronomy, F-​75005, Paris, France

1

The physical phenomena called “Ostwald ripening” (particularly applied to emulsions) and “disproportionation” (for foams) are often discussed in molecular gastronomy circles, because dispersed systems such as foams or emulsions are frequently part of dishes. Much has been done since the discovery of these two destabilizing mechanisms (among others) at the very beginning of the 20th century (Ostwald, 1901), but in a handbook such as this one, an important question is to know their real, quantitative importance in culinary practice. Here, we give only elementary data about what was made about the theory of these mechanisms, and we add some comments about how the theory can be applied.

Emulsions and Foams Dispersed systems are ubiquitous in food, because plant and animal tissues used as culinary ingredients are formally gels (see the Gels chapter in this book) and also because energy is added through culinary processes, making emulsions from fat and water, and foams from air and liquid. Even some raw plant tissues are aerated systems; for example, apples contain up to 25% air (Verboven et al., 2010). For dishes composed by assembling traditional food ingredients (as opposed to ingredients used in note by note cooking; see Note by Note Cooking and Note by Note Cuisine chapter, by Hervé This vo Kientza and Róisín Burke), most systems are dispersed, such as breads, cakes, desserts, etc. In particular, sauces have complex structures, comprising emulsions, foams, suspensions and more complex structures based on the elementary (two phases) dispersed systems (This, 2012). If a minimum of 25% of dispersed fat or air is arbitrarily used for deciding the physical state of sauces, 56% of the traditional French savoury sauces from the Guide culinaire (Escoffier et al., 1903) are emulsions, and 2% of them are foamy (also called “aerated”) systems. Regarding food emulsions, one the simplest (in terms of dispersed system formalism (DSF) complexity) is mayonnaise (Depree and Savage, 2001; This, 2016a). It is often said to be “an O/​ W emulsion made from egg, vinegar, and oil, containing typically 70 to 80% oil” (Ishibashi et al., 2016), but food science and technology has been too reliant on the data from the food industry for

these proportions instead of looking at culinary books, which contain different and often more balanced information. In particular, for mayonnaise, culinary books give different oil and water proportions. For example, in the recipes given by Jules Gouffé (1867) or by a “Group of Professionals” (Réunion de professionnels, 1946), the mass proportions are: water 18%, oil (including both triglycerides and phospholipids) 66% and proteins 4%. For Carême (1828), who was closer to the time when mayonnaise appeared (he called it “magnonnaise”), the oil proportion is even lower, with 51% and 49 % of water and oil, respectively. However, according to the Guide culinaire, which has been used by many chefs since its publication, the proportion of oil can reach 90%. Of the hot savoury sauces that are said to be “emulsified”, bearnaise, hollandaise, bechamel and white sauce are so popular that they appear in the Codex Alimentarius (FAO, 2018), again with definitions that do not correspond to culinary practices (and they are more legitimate than the Codex Alimentarius, which was designed by the industry, from traditional cooking). For these sauces, the oil/​water proportions (w/​w) are respectively 65/​30, 65/​30, 18/​64 and 37/​65. However, it should be observed that these sauces are not simple O/​W emulsions; bechamel and white sauce owe much of their rheological behaviour to swollen starch granules rather than packing of oil droplets; hollandaise, bearnaise and custard (Figure 34.1) (This, 2012) owe their character to the coagulation of egg yolk proteins; they are sometimes “suspensions” first, before being emulsified, contrary to what has sometimes been published (Perram, 1977). Indeed, even for the 17% of French savoury sauces that are said to have the biphasic O/​W organization because of the use of butter or oil a correct description should go along with an indication of temperature, as butter is often used for savoury sauces; since the melting of milk fat begins at −10 °C and ends at 60 °C (Lopez and Ollivon, 2009; Bouteille et al., 2012), an important proportion of fat is in the solid state at room temperature, and systems containing butter have a more complex DSF formula than simply O/​W. Also, for many systems, the Ramsden (also mistakenly called Pickering) description should replace the simplistic idea of stabilization by molecular surfactants adsorbed at the oil–​water interface (Binks, 2002; Rayner et  al., 2014). For 241

242

Hervé This vo Kientza destabilization, lowering their energy. The Gibbs free energy of formation of an emulsion can be written simply:

FIGURE  34.1  Microscopic picture of bearnaise before (a)  and after (b)  cooking. The proteins aggregate in solids dispersed in water, making a (S+W)/​O system rather than a pure O/​W emulsion.

sauces in which the oil/​water ratio is reduced (e.g., less than 10%), other stabilization mechanisms can be observed, such as an increase of the viscosity of the aqueous phase through the concentration of various compounds such as gelatin or saccharides (mono-​, oligo-​or polysaccharides) or of aggregates obtained through the thermal/​mechanical modifications of animal or plant tissues (see the chapter on applesauce in this book). For culinary foams as well, the issue of stability is more difficult than for the simple dispersion of air bubbles in a solution of surfactants, because very few aerated food systems are limited to only one liquid and one gaseous phase, as in whipped egg white. Solid foams (continuous solid networks with dispersed gas bubbles) are considered as outside the realm of this chapter, even if many solid foams in foods, e.g., cakes, pastries, bread, meringues, soufflés, etc., begin life as liquid foams, and during this period instability may occur before their stabilization via formation of a continuous solid network.

Let Us Remember That Metastable Systems Are Not Thermodynamically Stable One important question about culinary foams and emulsions concerns their stability, because, if it is recognized that energy is needed to make them, this means that there is a possibility of

∆G = γ∆A − T ∆S f (34.1)

where γ is the surface tension, ΔA the area increase, T the absolute temperature and ΔSf the variation of entropy. During the making of a dispersed system, the total interface area of the dispersed phase increases (the ratio is equal to n for one object divided into n smaller ones), which means that energy is needed in the process. Of course, the entropy increases, because there are more microscopic configurations having the same energy after division than before; however, the energy term outweighs the entropy ΔSf associated with the formation of the dispersed structures from the bulk constituents. All this results in ΔG ≥ 0, leading to the thermodynamic instability of the emulsions. The instability of dispersed systems is not due only to the immiscibility of the hydrophobic phase and water; the difference in density of the various phases is a main factor driving the physical evolution. However, the physical description of dispersed systems is not enough to understand the evolution of such systems, as the particular molecular constitution of the phases (with solutes) and of the surfactants (phospholipids do not behave in the same way as proteins, for example, in terms of auto-​diffusion, steric hindrance, interactions, etc.) determine their metastability. Moreover, it has to be observed that, for emulsified or foamed culinary systems, which generally have more than two phases, there are reasons to predict more complex changes than for simple O/​W or G/​W systems. The volume fraction of the dispersed phase (very concentrated emulsions can behave rheologically as gels) and the size of structures have to be considered (Robins et al., 2002).

Destabilization Mechanisms Various destabilization mechanisms are known in which small, dispersed structures (oil droplets or air bubbles) disappear while large corresponding structures grow, reducing the stability of the whole system (Wang et al., 2016). As is shown schematically in Figure 34.2, these include: 1. creaming with or without aggregation and increase in the droplet diameter; 2. aggregation with or without creaming; 3. increases in the droplet or bubble diameter through Ostwald ripening; 4. droplet or bubble coalescence, leading to the production of a separated oil phase; 5. gas diffusion from air bubbles. As a result of their thermodynamic instability, emulsions and foams will generally tend to reduce their total free energy through an increase in droplet or bubble diameter, thereby reducing their total interfacial area. Young (1805) and Laplace (1880) recognized

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Emulsions: Ostwald Ripening

FIGURE 34.2  Various destabilization mechanisms for dispersed systems.

that the pressure in small, dispersed structures increases when the radius decreases:



Pinside = Pmedium + 2

γ R

(34.2)

 ∂F  where γ is the surface tension  and R the radius of the  ∂A  T,V,n bubble. The consequence is easily drawn for foams using Henry’s law, which states that the concentration in a compound making up the gas of bubbles, in the liquid continuous phase, can be described as being proportional to the partial pressure of this compound in the bubble (McQuarrie and Simon, 1997). This creates a concentration difference between small and large bubbles, triggering the net diffusion of the compound toward the large bubbles. As a whole, this all results in an overall increase in average bubble size of the foam; this is the process of disproportionation, which contributes to coarsening of foams. In the literature, this has also been referred to as isothermal distillation or molecular diffusion (Kabalnov et al., 1987; Varescon et al., 1990). The same analysis can be done for emulsions, with the chemical potential including a variation with concentration in the liquid phase instead of pressure in gas. However, one should observe immediately that, whereas Ostwald ripening and disproportionation have often been compared, the actual effects are different due to huge solubility differences of air (8  mg/​L for oxygen at 20 °C) and triglycerides in water (less than 10–​9 mol/​L for tristearin) (Funasaki et al., 1976).

Changes in Emulsions In simple terms, Ostwald ripening is the growth of the largest droplets at the expense of the smallest ones as a result of

the difference in chemical potential of the material within the droplets (which is an obvious way of saying that energy is driving the phenomena of the world). Because of its importance in food quality, Ostwald ripening in emulsions has been much studied (Lifshitz and Slyozov, 1961; Wagner, 1961; Enomoto et al., 1987; Kabalnov and Shuchkin, 1992; De Smet et al., 1997; Yarranton and Masliyah, 1997; Dalgliesh, 1997). A comprehensive review of Ostwald ripening in emulsions was published by Taylor (1998), and a recent article studied how to distinguish Ostwald ripening from coalescence (Santos et al., 2017). Theoretically, a calculation of the rate of Ostwald ripening (i.e., the rate at which the radius of the droplets in an emulsion increases with time) has been proposed. However, this calculation holds only with important simplifications. The process is assumed to be diffusion-​controlled, the rate-​limiting step of the growth rate being the diffusion of the dissolved droplet phase (solute) through the bulk phase (Meinders et al., 2002; Novales et al., 2003). This excludes any other effects at the interface that would dominate the rate. In particular, it is assumed that there is no barrier to the passage of the solute across the interface: in emulsions stabilized by small surfactants, this is a reasonable assumption, but it may not be true in systems stabilized by thick polymeric species such as proteins (present in most culinary systems), as in the simple culinary emulsion that is mayonnaise. In order to test such theoretical analysis, many experimental studies have been performed at the limit of highly dilute emulsions (volume fraction lower than 1%), i.e., in conditions where the droplets are not in permanent contact (Schmitt et al., 2004). In the kitchen, this can occur, for example, when an emulsion is diluted, such as in soups from Gascony that are made by adding a vegetable stock to aiolli or mayonnaise (Daguin, 1981). But it does not hold for mayonnaise and other cousins. The models used to describe Ostwald ripening are based on and similar to the models used to describe the dissolution of an air bubble and disproportionation of a foam. For a single droplet with radius R in an infinite medium, the transport of molecules over the surface per unit time can be written using Fick’s diffusion law, in the steady state solution, as:



d n = 4 π RD (Cm − Cs ) dt R

(34.3)

with D the diffusion coefficient, Cm the concentration of the dispersed phase in the medium, and Cs the solubility of the dispersed droplet, which is given by Kelvin’s equation:



α  C s = C∞  1 +   R

(34.4)

with C∞ the bulk solubility and α the so-​called capillary length, given by:

α=2

Vm σ RgT

(34.5)

where Vm is the molar volume of the dispersed phase, Rg the gas constant, T the temperature and σ the interfacial tension.

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Hervé This vo Kientza

Now, for a entire emulsion, with many droplets, the most complete theory was proposed independently by Lifshitz and Slyozov (1961) and by Wagner (1961). This so-​called “LSW theory” assumes that the system has been ripening for a sufficiently long time for quasi-​steady state conditions to apply (asymptotic solution). For a full discussion of the LSW theory and its derivation, see Ramsey (1947); Dunning (1973); Kahlweit (1975). One result is that the coarsening rate, defined as the rate of increase of the cube of the number-​averaged particle size, is equal to:

ω=

R3 dRC 3 4α DC∞ = = dt dt 9ρ

(34.6)

where ρ is the density of the dispersed phase. Because of the importance of interfacial elasticity, Ostwald ripening has been studied by means of numerical simulations in which time-​dependent (elastic) interfacial behaviour is taken into account (Meinders and van Vliet, 2004). Calculations on the dissolution of a single droplet in an infinite medium under saturated conditions show that the dissolution process can be stopped only when the interfacial tension goes to zero; when interfacial stress relaxation is included, which prevents a continuous zero interfacial tension, no stabilization of the dissolution process is observed, and the droplet dissolves completely. In the case of a group of droplets, using the assumptions of LSW theory, numerical calculations on the coarsening of emulsion droplets with finite interfacial elasticity show that a stable situation occurs at finite interfacial tensions of the droplets. The coarsening behaviour strongly depends on the saturation of the dispersed phase in the continuous phase. Stabilization can only be accomplished by adding insoluble species to the dispersed phase, by using particles as stabilizers, such as in Ramsden systems (Rayner et al., 2014), or by micro-​encapsulation of the emulsion droplets by thick insoluble interfacial layers, which have a thickness that is in the order of the radius of the droplet. Vincent (1984) has suggested a criterion to determine whether Ostwald ripening will occur in an emulsion, and suggested that the presence of surfactant molecules may form interfacial tension gradients, which may oppose Ostwald ripening. A droplet may be considered to be in equilibrium if dP/​dr > 0, where P is the Laplace pressure of the droplet. Consequently, taking into consideration the area of the droplet and the Laplace equation, 2 dγ/​d ln(A) > γ. The left hand side of this inequality is just the dilational modulus, ϵ, of the interface, and so, for mechanical equilibrium:



2 ϵ > γ

(34.7)

If this inequality is satisfied, then the droplet should be in equilibrium and show no Ostwald ripening. The control of Ostwald ripening has also been the subject of many studies (Webster and Cates, 1998; Webster and Cates, 2001). It may be slowed down or halted by the addition of components that are poorly soluble or insoluble in the continuous phase. However, phenomena can change completely depending on particular systems. Taisne and Cabane (1998) examined the coarsening of concentrated alkane-​in-​water droplets (ϕ ≈ 20–​40%)

stabilized by a non-​ionic poly(ethoxylated) surfactant, following a temperature quench. Since the rate of ripening does not depend on the alkane chain length, they concluded that the transfer of oil from the smaller drops to the larger ones does not occur by diffusion across the continuous phase but rather, through the direct contact of the droplets (permeation). Finally, the traditional view of emulsions is not enough, and the analysis of culinary emulsions should consider systems such as nano-​ emulsions, micro-​ emulsions and Ramsden emulsions (Berton-​ Carabin and Schroën, 2015; Solans et  al., 2005; McClements, 2012). For nano-​emulsions, with droplet sizes ranging from 50 to 200 nm, Ostwald ripening is the main destabilizing mechanism (Izquierdo et al., 2002). For micro-​emulsions, coalescence is considered to be the major mechanism of destabilization, but stabilizers can often prevent it.

Changes in Foams In food systems, there are very few pure G/​L (where G is a gas, and L a liquid) foams. With molecular cooking, more G/​W (W stands for an aqueous solution) foams were introduced, using surfactants such as milk or egg proteins, or various additives such as lecithins (This, 2016b). In liquid foams, drainage of the continuous phase between bubbles, followed by the close approach of bubble surfaces and bubble coalescence, can occur, leading to foam collapse, loss of gas, and loss of desired foam structure and consistency. In addition, even in the absence of drainage and coalescence, diffusion of gas can occur, as said earlier, between bubbles that possess different internal pressures (and therefore concentrations of gas). The net diffusion flux is usually greatest from small to large bubbles, due to the greater Laplace pressure of the former, although the pressure within bubbles at any given time also depends on the interfacial elasticity of the adsorbed film around the bubbles and the extent of their shrinkage. More rapid shrinkage against a high interfacial dilatational elasticity reduces the net internal pressure more quickly (Dickinson et al., 2002; Ettelaie et al., 2003). The migration of gas between bubbles eventually leads to coarsening of the foam. As said before, the effect is called disproportionation and is analogous to Ostwald ripening in emulsions; it can accelerate drainage and coalescence (Murray and Ettelaie, 2004). De Vries (1958a; 1958b), Princen and Mason (1965) and Princen et al. (1967) provided the basic framework for studying gas diffusion to an atmosphere from a single bubble. Prins (1987) showed that the viscoelasticity of the bubble surface would reduce the rate of gas diffusion. Lucassen (1981) and Wijnen and Prins (1995) confirmed that the dynamic surface properties, such as interfacial elasticity, have a significant effect on the rate of Ostwald ripening: disproportionation can be stopped if the bubble surface is purely elastic and the surface dilatational elasticity is larger than half the surface tension.

How Does This Apply? For sure, the discovery of new phenomena and mechanisms is a goal of the natural sciences in general, and of molecular gastronomy in particular, but as said in the Introduction, knowing

245

Emulsions: Ostwald Ripening their real effect is an important step for improving theories, as new mechanisms can appear when the known ones do not correctly explain the phenomena. Dispersed systems are thermodynamically unstable, but they are metastable due to the presence of the adsorbed layers at the O/​W or the G/​W interface; barriers may be electrostatic in nature (adsorption of an ionic surface active agent), or steric (adsorption of a non-​ionic surface active agent or polymer), and the two effects can come into play, for proteins at certain pH values, for example. These barriers not only prevent dispersed particles from coming into direct contact but also serve to stabilize the thin film of liquid between two adjacent droplets or bubbles. Coalescence occurs when this thin film thins further and ruptures. The presence of adsorbed surfactants can reduce the likelihood of rupture through the Gibbs–​Marangoni effect (Firouzi and Nguyen, 2017). Reduction in interfacial tension may also play a significant role in stability. In the case of polymers, other factors may also come into play, such as the viscoelastic nature of the adsorbed layer. There have been many attempts to correlate the viscoelastic properties of adsorbed layers with emulsion stability, but with only limited success.

Application to Emulsions In the kitchen, emulsions are never of the kinds that were theoretically described, because either they are far from the dilute regime, or the high viscosity of the continuous liquid phase is too complex for simple diffusion, or particles of different kinds adsorb at the interfaces. This is sometimes recognized, as when Tcholakova observed that “the Ostwald ripening is usually negligible for protein-​ stabilized triglyceride-​ in-​ water emulsions” (Tcholakova et al., 2006). Moreover, culinary systems usually contain a lot of different surfactants that protect emulsions against aging factors such as coagulation and coalescence. If added in sufficiently large amounts and after complete coverage of the oil–​ water interface, surfactants spontaneously form micelles in the continuous aqueous phase: the presence of micelles drastically increases the “solubility” of the oil. Therefore, an effect of micelles on the Ostwald ripening may be anticipated. This effect can even be enhanced dramatically by adding micellar surfactant solutions to ripening emulsions. If enough surfactant is added, a competition can occur between droplet size increase by Ostwald ripening, on the one hand, and droplet size decrease by solubilization of the oil in the added micelles, on the other hand. Also, theoretical analysis of the distribution of radiuses should be considered with care, because whereas food science considers typically highly polydisperse emulsions, with droplet radii easily spanning two orders of magnitude (Robins et  al., 2002), it has been shown that, for finished mayonnaise, only one order of magnitude is considered for droplet radiuses, and their droplet size is so big (up to 0.1 mm) that the droplets are not subject to Brownian motion, which would resist gravity-​induced creaming. For emulsions in which egg yolk is used, the emulsifying properties of egg yolk are regulated by phospholipids, lipoproteins, hydrophobic and hydrophilic proteins (livetin and phosvitin), and cholesterol (This, 2009). However, it is not clear whether the

interface is covered by low-​density lipoprotein (LDL) (Mizutani and Nakamura,1985) or high-​density lipoprotein (HDL). Recently, Ariizumi et  al. (2017) reported that mayonnaise prepared with lower egg yolk concentrations was destabilized faster, and they showed that long-​term stability (for five months) was affected by the protein load and protein composition in the adsorbed layer at the interface. Usually, proteins stabilize emulsions through steric repulsion, which is highly dependent on pH and ionic strength (Guilmineau and Kulozik, 2006a; 2006b). Clearly, this observation is not relevant for culinary practice, because it would be microbiologically dangerous to keep a sauce for such a long time in the usual home conditions, and also, cooks do not try to reduce the yolk concentration (the yolk is used in kitchens for flavour). In terms of salt, laboratory conditions also do not match culinary ones. Salt can certainly neutralize any charge found in proteins (McClements, 1999), which aids protein adsorption onto oil droplets, and it can be calculated that there are about 100 neutralizing ions for each charge on a protein, in culinary conditions, so that this effect could hold. On the other hand, it has been written (Prost and Rondelez, 1991) that salt could destabilize emulsions, but this is not true for mayonnaise; in our experiments, the emulsified structure remained after the addition of 200 g of sodium chloride to a mayonnaise made from 1 egg yolk, 20 g of an 8% acetic acid solution and 200 g of sunflower oil. Clearly, culinary emulsions are different from poly(dimethylsiloxane) O/​W emulsions, which were destabilized through coalescence at 1 M NaCl concentration (Koh et al., 2000). Of course, it has to be observed that such a huge quantity of salt is never used. More generally, the food science literature contains strange information as compared with culinary practice. For example, it has been reported that high mayonnaise stability is generally associated with a fine and uniform oil droplet size (Harrison and Cunningham, 1985), whereas mayonnaise/​spoonable salad dressing droplets are smaller, with sizes of about 2–​3 μm (Tung and Jones, 1981). However, in our experiments, we measured oil droplet diameters between 1 and 100 μm. In the last few years, micro-​and nano-​particles of biological origins have been extensively explored as colloid stabilizers (i.e., stabilization of liquid/​liquid and/​or liquid/​vapour interfaces) for application in bio-​related fields, such as foods and nutraceuticals. Interestingly, this concept has been applied for centuries in food products without being scientifically researched (Tavernier et al., 2016). For instance, in remoulade (a precursor of mayonnaise, known at least since 1419) (Tirel, 1392), the fine mustard particles contribute to physical and colloidal stability of the emulsion by preventing the coalescence of oil droplets (Binks, 2002). Indeed, the flourishing field of Ramsden emulsions (also called Pickering emulsions) is discovering outstanding stabilizations against coalescence and Ostwald ripening compared with emulsions stabilized with low-​molecular-​weight emulsifiers (Timgren et al., 2011; Kargar et al., 2012).

Application to Foams Surface-​active macromolecules, such as most proteins, tend to form adsorbed layers resembling thin three-​dimensional gels,

246 which essentially have little effect on the mean free path of gas molecules through them. The interfacial rheology of adsorbed films can modify the rate and extent of shrinkage of bubbles due to disproportionation. In order to shrink, bubbles have to do work against the interfacial elasticity and viscosity, which provide an energy barrier that could inhibit bubble shrinkage. In a key article, Kloek et  al. (2001) reviewed the potential effects of both bulk rheology and interfacial dilatational rheology on the shrinkage kinetics of bubbles. Some estimates of the range of effects in real food systems are discussed. For the most part, the behaviour of adsorbed protein films during disproportionation seems to fit very well to a simple theoretical model of gas diffusion and bubble shrinkage, where a zero or finite value of the dilatational elasticity is included (McClements, 1999; Robins, 2000). This also indicates that, at the typical rates of interfacial deformation due to bubble shrinkage that are observable, dilatational viscosities are not sufficiently high to exert a significant effect. Loss moduli are expected to fall with decreasing deformation rate (or frequency) of deformation. So far, only one notable exception to this good fit of experiment to theory has been noted (Al-​Malah et al., 2000). This anomaly occurred with the egg white protein ovalbumin. Results with ovalbumin were often not very reproducible, and occasionally very much longer shrinkage times than usual were observed, which did not fit the simple theory. Ovalbumin is one of those proteins that microscopically appear to form protein particles around a bubble as it shrinks, but in fact, it does this far more readily than other proteins. Aggregates are often visible before significant shrinkage has taken place, and the aggregates sometimes appear to be part of some sort of semi-​soluble interfacial protein network before shrinkage has even begun. However, here again, food science departs strangely from what is really observed in the kitchen. For example, Dutta et al. (2004) wrote that “some products that benefit from air incorporation are ice cream, confectionery, salad dressings and mayonnaise” … but in fact, microscopic analysis of mayonnaise does not show any air bubbles; mayonnaise is not an aerated system.

Hervé This vo Kientza

Berton-​Carabin CC, Schröen K. 2015. Pickering emulsions for food applications: background, trends, and challenges, Annual Review in Food Science and Technology, 6, 263–​297. Binks BP. 2002. Particles as surfactants –​similarities and differences, Current Opinion in Colloid and Interface Science, 7, 21. Bouteille R, Perez J, Khifer F, Jouan-​ Rimbaud-​ Bouveresse D, Lecanu B, This H. 2013. In situ quantitative proton nuclear magnetic resonance spectroscopy analysis of milk fat fusion, Journal of Dairy Science, 96, 2071–​2080. Carême MA. 1828. L’Art de la cuisine française au XIXe siècle, Chez l’auteur, Paris. Daguin A. 1981. Le nouveau cuisinier gascon, Stock, Paris. Dalgliesh DG. 1997. Adsorption of protein and the stability of emulsions, Trends in Food Science and Technology, 8, 1–​6. De Smet Y, Deriemaeker L, Finsy R. 1997. A simple computer simulation of Ostwald ripening, Langmuir, 13, 6884. De Vries AJ. 1958a. Foam stability Part I: Structure and stability of foams, Recueil des travaux chimiques des Pays-​ Bas, 77(1),  81–​91. De Vries AJ. 1958b. Foam stability Part II: Gas diffusion in foams, Recueil des travaux chimiques des Pays-​Bas, 77(3), 209–​223. Depree JA, Savage GP. 2001. Physical and flavour stability of mayonnaise, Trends in Food Science and Technology, 12(5–​6), 157–​163. Dickinson E, Ettelaie R, Murray BS, Du Z. 2002. Kinetics of disproportionation of air bubbles beneath a planar air–​water interface stabilised by food proteins, Journal of Colloid and Interface Science, 252, 202–​214. Dunning WJ. 1973. In Smith AL (Ed.), Particle Growth in Suspensions, SCI Monograph No. 38, Academic Press, London. Dutta A, Chengara A, Nikolov AD, Wasan DT, Chen K, Campbell B. 2004. Texture and stability of aerated food emulsions  –​ effects of buoyancy and Ostwald ripening, Journal of Food Engineering, 62, 169–​175. Enomoto Y, Kawasaki K, Tokuyama M. 1987. Computer modeling of Ostwald ripening, Acta Metallurgica, 35, 907–​913. Escoffier A, Gilbert P, Fetu E. 1903. Guide culinaire, Emile Colin, Paris. Ettelaie E, Dickinson E, Du Z, Murray BS. 2003. Disproportionation of clustered protein-​ stabilized bubbles at planar air-​ water interfaces, Journal of Colloid and Interface Science, 263, 47–​58. FAO. 2018. Codex Alimentarius, www.fao.org/​fao-​who-​codexalimen tarius/​fr/​ Firouzi M, Nguyen AV. 2017. The Gibbs–​ Marangoni stress and non DLVO forces are equally important for modeling bubble Conclusions and Perspectives coalescence in salt solutions, Colloids and Surfaces A: After decades of work on very simple models that do not corresPhysicochemal and Engineering Aspects, 515, 62–​68. pond to culinary practices, recent results on Ramsden emulsions Funasaki N, Hada S, Suzuki K. 1976. The dissolution state of a triare now getting closer to the description of culinary systems of glyceride molecule in water and its orientation state at the air–​ the O/​W and G/​W kinds, but a lot remains to be done to investiwater interface, Chemical and Pharmaceutical Bulletin, 24(4), 731–​735. gate all possible microstructures that can be met in sauces. And Gouffé J. 1867. Le livre de cuisine : comprenant la cuisine de ménage new mechanisms could be discovered during this analysis! et la grande cuisine. Hachette, Paris. Guilmineau F, Kulozik U. 2006a. Impact of a thermal treament on the REFERENCES emulsifying properties of egg yolk. Part 1: Effect of the heating Al-​Malah KI, Azzam MOJ, Omari RM. 2000. Emulsifying propertime, Food Hydrocolloids, 20(8), 1105–​1113. ties of BSA in different vegetable oil emulsions using conduct- Guilmineau F, Kulozik U. 2006b. Impact of a thermal treament on the ivity technique, Food Hydrocolloids, 14, 485. emulsifying properties of egg yolk. Part 2: effect of the envirAriizumi M, Kubo M, Handa A, Hayakawa T, Matsumiya K, onmental conditions, Food Hydrocolloids, 20(8), 1114–​1123. Matsumura Y. 2017. Influence of processing factors on the Ishibashi C, Hondoh H, Ueno S. 2016. Influence of morphology and stabilityof model mayonnaise with whole egg during long-​ polymorphic transformation of fat crystals on the freeze-​thaw term storage, Bioscience, Biotechnology, and Biochemistry, stability of mayonnaise-​ type oil-​ in-​ water emulsions. Food 81(4), 803–​811. Research International, 89, 604–​613.

Emulsions: Ostwald Ripening Izquierdo, P, Esquena J, Tadros TF, Dederen C, Garcia MJ, Azemar N. 2002. Formation and stability of nano-​emulsions prepared using the phase inversion temperature method, Langmuir, 18,  26–​30. Kabalnov AS, Pertzov AV, Shchukin ED. 1987. Ostwald ripening in emulsions: I. Direct observations of Ostwald ripening in emulsions, Colloids Surfaces, 118(2), 590–​597. Kabalnov AS, Shchukin ED. 1992. Ostwald ripening theory: Applications to fluorocarbon emulsion stability, Advances in Colloid and Interface Science, 389, 69–​97. Kabalnov AS. 2001. Ostwald ripening and related phenomena, Journal of Dispersion Science and Technology, 22, 1–​12. Kahlweit M. 1975. Ostwald ripening of precipitates, Advances in Colloid and Interface Science, 5, 1–​35. Kargar M, Fayazmanesh K, Alavi M, Spyropoulos F, Norton IT. 2012. Investigation into the potential ability of Pickering emulsions (food-​grade particles) to enhance the oxidative stability of oil-​ in-​water emulsions, Journal of Colloid and Interface Science, 366(1), 209–​215. Laplace PS. 1880. Oeuvres complètes de Laplace, Gauthier Villars, Paris, ­chapter 4, 385. Lifshitz IM, Slyozov VV. 1961. The kinetics of precipitation from supersaturated solid solutions, Journal of Physics and Chemistry of Solids, 19, 35–​50. Lifshitz IM, Slezov VV. 1959. Kinetics of diffusive decomposition of supersaturated solid solutions, Soviet Physics Journal of Experimental and Theoretical Physics, 35(8), 331–​ 339 (English translation). Lopez C, Ollivon M. 2009. Triglycerides obtained by dry fractionation of milk fat 2. Thermal properties and polymorphic evolutions on heating, Chemistry and Physics of Lipids, 159, 1–​12. Lucassen J. 1981. Dynamic properties of free liquid films and foams. In Lucassen-​Reynders EH (Ed.), Anionic Surfactants: Physical Chemistry of Surfactant Action, Dekker, New York, 217. Lucassen-​Reynders EH. 1981. Surface elasticity and viscosity in compression dilatation. In Lucassen-​ Reynders EH (Ed.), Anionic Surfactants: Physical Chemistry of Surfactant Action, Dekker, New York, 173. Martin AH, Grolle K, Bos MA, Cohen Stuart MA, van Vliet T. 2002. Network forming properties of various proteins adsorbed at the air/​water interface in relation to foam stability, Journal of Colloid and Interface Science, 254, 175–​183. McClements DJ. 1999. Food Emulsions: Principles, Practice and Techniques, CRC Press, Boca Raton, FL. McClements DJ. 2012. Nanoemulsions versus microemulsions: Terminology, differences, and similarities, Soft Matter, 8, 1719–​1729. McQuarrie DA, Simon JD. 1997. Physical Chemistry, University Science Books, Sausalito, California. Meinders MBJ, Kloek W, van Vliet T. 2001. Effect of surface elasticity on ostwald ripening in emulsions, Langmuir, 17, 3923–​3929. Meinders MBJ, van Vliet T. 2004. The role of interfacial rheological properties on Ostwald ripening in emulsions, Advances in Colloid and Interface Science, 108–​109, 119–​126. Meinders MBJ, Bos MA, Lichtendonk WJ, van Vliet T. 2002. In Dickinson E, van Vliet T (Eds.), Food Colloids: Biopolymers and Materials, Royal Society of Chemistry, Cambridge. Mizutani R, Nakamura R. 1985. Emulsifying properties of egg yolk low density lipoprotein (LDL): comparison with bovine serum albumin and egg lecithin, Lebensmittel Wissenschaft und Technology, 17, 213–​216.

247 Murray BS, Ettelaie R. 2004. Foam stability: proteins and nanoparticles, Current Opinion in Colloid & Interface Science, 9(5), 314–​320. Nguyen Hoang TK, Deriemaeker L, La V, Finsy R. 2004. Monitoring the simultaneous Ostwald ripening and solubilization of emulsions, Langmuir, 20(21), 8966–​8969. Novales B, Papineau P, Sire A, Axelos MAV. 2003. Characterization of emulsions and suspensions by video image analysis, Colloids and Surfaces A : Physicochemical and Engineering Aspects, 221, 81–​89. Ostwald W. 1901. Analytische Chemie, 3rd ed., Engelmann, Leipzig, 23. Perram CM, Nicolau C, Perram JC. 1977. Interparticle forces in multiphase colloid systems: the resurrection of coagulated sauce béarnaise. Nature, 270, 572. Princen HM, Mason SG. 1965. The permeability of soap films to gases, Journal of Colloid Science, 20, 353–​375. Princen HM, Oberbeek JT, Mason SG, 1967. The permeability of soap films to gases: II. A  simple mechanism of monolayer permeability, Journal of Colloid and Interface Science, 24(1), 125–​130. Prins A. 1987. Theory and practice of formation and stability of food foams. In Dickinson E (Ed.) Food Emulsions and Foams, Spec public, The Royal Society of Chemistry, Cambridge, UK,  30–​39. Prost J, Rondelez F. 1991. Structures in colloidal physical chemistry, Nature Supplement, 350, 11–​23. Ramsey AS. 1947. A Treatise on Hydrodynamics II, G. Bell, London. Rayner R, Marku D, Eriksson M, Sjöö M, Dejmek P, Wahlgren M. 2014. Biomass-​based particles for the formulation of Pickering type emulsions in food and topical applications, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 458 (2014),  48–​62. Réunion de professionnels. 1946. La cuisine moderne, Aristide Quillet, Paris. Robins MM. 2000. Emulsions  –​creaming phenomena, Current Opinion in Colloid and Interface Science, 5, 265. Robins MM, Watson AD, Wilde PJ. 2002. Emulsions –​creaming and rheology, Current Opinion in Colloid and Interface Science, 7, 419–​425. Santos J, Calero N, Trujillo-​Cayado LA, Garcia MC, Munoz J. 2017. Assessing differences between Ostwald ripening and coalescence by rheology, laser diffraction and mutiple light scattering, Colloids and Surfaces B: Biointerfaces, 159, 405–​441. Schmitt V, Arditty S, Leal-​Calderon F. 2004. Stability of concentrated emulsions. I. In Emulsions: Structure Stability and Interactions (Petsev DN Ed.), Academic Press, Cambridge (USA), ­chapter 4, 607–​639. Skinner LM, Sambles JR. 1972. The Kelvin equation, a review, Journal of Aerosol Science, 3, 199. Solans C, Izquierdo P, Nolla J, Azemar N, Garcia-​Celma MJ. 2005. Nano-​emulsions, Current Opinion in Colloid and Interface Science, 10 (2005), 102–​110. Tavernier I, Wijaya W, Van der Meeren P, Dewettinck K, Patel A. 2016. Food-​grade particles for emulsion stabilization, Food Science & Technology, 50, 159–​174. Taylor P. 1998. Ostwald ripening in emulsions, Advances in Colloid and Interface Science, 75, 107. Tcholakova S, Denkov ND, Ivanov IB, Campbell B. 2006. Coalescence stability of emulsions containing globular milk proteins, Advances in Colloid and Interface Science, 123, 259–​293.

248 Tcholakova S, Denkov ND, Lips A. 2008. Comparison of solid particles, globular proteins and surfactants as emulsifiers, Physical Chemistry Chemical Physics, 10, 1608–​1627. This H. 2009. Molecular Gastronomy, a chemical look to cooking, Accounts of Chemical Research, 42(5), 575–​583. This H. 2012. Solutions are solutions, and gels are almost solutions, Pure and Applied Chemistry, 2012, 1–​20. This H. 2016a. Statgels and dynagels, Notes Académiques de l’Académie d’agriculture de France /​Academic Notes from the French Academy of Agriculture, 12, 1–​12. This H. 2016b. Mon histoire de cuisine, Belin, Paris. Tirel G. 1392. Viandier of Taillevent: An Edition of All Extant Manuscripts, University of Ottawa Press, Ottawa, Canada, 1988. Tung MA, Jones IJ. 1981. Microstructure of mayonnaise and salad dressing, In Holcomb DN, Kalab M (eds.), Studies of Food Microstructure, Scanning Electron Microscopy, AMF, O’Hare, Illinois, 231. Varescon C, Manfredi A, Le Blanc M, Reiss JG. 1990. Deviation from molecular diffusion aging model in fluorocarbon emulsions stabilized by perfluoroalkylated surfactants, Journal of Colloid and Interface Science, 137, 373–​379. Verboven P, Kerckhofs G, Mebatsion HK, Ho QT, Temst K, Wevers M, Cloetens P, Nicolai BM. 2018. Three-​ dimensional gas exchange pathways in pome fruit characterized by synchrotron x-​ray computed tomography, Plant Physiology, 147, 518–​527.

Hervé This vo Kientza Vincent B. 1984. Introduction to colloidal dispersions, In Tadros ThF (Ed.), Surfactants, Academic Press, London. Voorhees PW. 1985. The theory of Ostwald ripening, Journal of Statistical Physics, 38(1/​2). Wagner C. 1961. Theorie der Alterung von Niederschlägen durch Umlösen (Ostwald-​Reifung), Zeitschrift für Electrochemie, 65, 581. Wang J, Nguyen AV, Farrokhpay S. 2016. A critical review of the growth, drainage and collapse of foams, Advances in Colloid and Interface Science, 228, 55–​70. Webster AJ, Cates ME. 1998. Stabilization of emulsions by trapped species, Langmuir, 14, 2068–​2079. Webster AJ, Cates MZ. 2001. Osmotic stabilization of concentrated emulsions and foams, Langmuir, 17, 595–​608. Wijnen ME, Prins A. 1995. Disproportionation in aerosol whipped cream, In Food. Macromolecules and Colloids, Dickinson, E. and Lorient, D. (eds.), The Royal Society of Chemistry, Cambridge, UK, 309–​311. Yarranton HW, Masliyah JH. 1977. Numerical simulations of Ostwald ripening in emulsions, Journal of Colloid and Interface Science, 196(2), 157–​169. Young, T. 1805. An essay on the cohesion of fluids, Philosophical Transactions of the Royal Society of London, 95, 65–​87.

Emulsions: Lecithin Elzbieta Kozakiewicz1 and Daniel Cossuta2 Quality & Food Safety Coordinator, Bunge 2 Head of Specialty Ingredients – Global Innovation at Bunge Loders Croklaan

1

Lecithin has been described with several different definitions in the scientific literature, and there is a simple reason for these differences: the studies of phospholipids started in the early 1700s, but the sales of lecithin products only started in the 1930s. The research and development of these products took more than 200 years to reach commercialization, and throughout this time different definitions appeared, based on the current knowledge and objectives. For example, in his book, Szuhaj (1989) mentions three different definitions. The first definition is for commercially used lecithin, which is interpreted as a natural mixture that contains polar lipids (glycolipids, phospholipids) and neutral lipids (triglycerides) and is obtained from plant or animal sources. Lecithin has also been defined as a group of phosphorus-​ containing lipids extracted from eggs or brain tissues. The third definition, called “scientific” by Szuhaj, is that the lecithin is virtually phosphatidylcholine (PC) itself. According to the International Lecithin & Phospholipids Society (ILPS, 2020), lecithin is defined as: “A complex mixture of glycerophospholipids of animal, plant, or microbiological origin, containing varying amounts of triglycerides, fatty acids, glycolipids, sterols, and sphingophospholipids.” Leonard (2017) defines lecithin relatively simply. Lecithin is a group of different fatty substances found in plant and animal tissues that are essential for the proper functioning of cells. Besides books and journals, lecithin is defined in many encyclopedias available on the Internet as well. One of these is the Encyclopedia Britannica (2020), where two definitions can be found. The first one is the same as the scientific description by Szuhaj, with the addition of the importance of lecithin in the cell structure and cellular metabolism. The other is very similar to the ILPS definition, where they describe lecithin as a natural mixture containing significant amounts of the following components: phosphatidylcholine (PC), cephalin (phosphatidylethanolamine, PE) and phosphatidylinositol (PI). The various definitions all state that lecithin is a natural mixture of neutral and polar lipids, where the main compounds are PC, PE, phosphatidylserine (PS), PI, phosphatidic acid (PA) and triglycerides. It also contains a smaller proportion of glycolipids and different sugars. Lecithin can be found in all living cells, where it plays an important biological role in the structure of cell membranes.

Lecithin Lecithin was discovered in 1845 by the French chemist Theodore Nicolas Gobley (1811–​1876), who was able to extract it from egg yolk. It was named after the ancient Greek word lekithos, which means egg yolk. This naming convention has been used since 1850. The full chemical formula of lecithin was unknown until 1874 (Shurtleff and Aoyagi, 2016). The first lecithin products were obtained solely from egg yolk, but the process was expensive. This driving force and the limited availability opened the door for plant-​sourced lecithin products. Nowadays, the most commonly used lecithin is soy lecithin (genetically modified (GM) and non-​GM), followed by sunflower and rapeseed lecithin. Lecithin has many forms, such as liquid lecithin, de-​ oiled powdered lecithin, granules, compounded lecithin (lecithin on a carrier), and various modified lecithins (enzyme modified  –​ hydrolysed, acetylated, hydroxylated). Liquid lecithins from vegetable sources are brownish, dense, oily products that are generally sold in a standardized form. In this way, it can be guaranteed that the customer receives consistent product quality (Table 35.1). Table 35.2 shows the typical chemical and physical parameters of sunflower lecithin tested and set during standardization. In addition, organoleptic and microbiological tests are usually carried out before the product is released to the market.

Composition of Lecithin The composition of lecithin is given in Figure 35.1. In Figure 35.1a, the boxes are, from bottop to top: moisture, triacylglycerol (TAG), complexed sugars, glycolypids, minor phospholipids, PI, PA, PC and PE in absolute value (g/100 g lecithin). In Figure 35.1b, they are C16:0, C18:0, C18:1, C18:2, C18:3 and other fatty acid residues in relative value (g/100g fatty acid).

Phospholipids As we can see from Table 35.3, lecithin contains several different phospholipids. What is common in their molecular structure is that they have a glycerol backbone, which has two fatty acid residues attached in the first two positions, while a phosphoric

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Elzbieta Kozakiewicz, Daniel Cossuta

TABLE 35.1 Limits for Food-​Grade Liquid Lecithin Parameter

Unit

Acetone insoluble Hexane insoluble Toluene insoluble Moisture Loss on drying Acid value Peroxide value Arsenic Lead Mercury

% % % % % mg KOH/​kg meq/​kg ppm ppm ppm

FAO/​WHO Codex Alimentarius

European Union E322

>60 -​