Section Revision: Food Science

This lesson provides a comprehensive revision of food science content for the AQA GCSE Food Preparation and Nutrition exam (8585), covering cooking methods, heat transfer, and the functional and chemical properties of food.

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Cooking Methods

Why We Cook Food

Reason	Explanation
Safety	Kills pathogenic bacteria (core temperature 75°C)
Palatability	Improves taste, texture and appearance
Digestibility	Breaks down cell walls and proteins, making nutrients more accessible
Preservation	Extends shelf life by destroying microorganisms and enzymes
Development of flavour	Maillard reaction and caramelisation create new flavours

Heat Transfer Methods

Method	Description	Examples
Conduction	Heat transfers through direct contact from molecule to molecule; occurs in solids and from pan to food	Frying in a pan; heat passing through a metal baking tray
Convection	Heat transfers through the movement of heated particles in a fluid (liquid or gas); hot particles rise, cool particles fall, creating a current	Boiling water; oven (hot air circulates); deep-fat frying
Radiation	Heat transfers as infrared waves; does not need a medium (can travel through a vacuum)	Grilling; toasting; microwave radiation

flowchart LR
    A["CONDUCTION<br/>Direct contact<br/>Molecule to molecule"] --- B["CONVECTION<br/>Hot fluid rises<br/>Cool fluid falls"]
    B --- C["RADIATION<br/>Infrared waves<br/>No contact needed"]

    style A fill:#e74c3c,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#f1c40f,color:#000

Cooking Methods Summary

Method	Heat Transfer	Medium	Effect on Food
Boiling	Convection	Water (100°C)	Can lose water-soluble vitamins (B, C); softens vegetables
Steaming	Convection	Steam	Retains more vitamins than boiling; no added fat
Poaching	Convection	Water (below boiling, ~80°C)	Gentle; retains shape; low-fat method
Frying (shallow)	Conduction	Oil	Quick; adds fat; produces crispy exterior
Frying (deep)	Convection + Conduction	Oil (170–190°C)	Fast; adds significant fat; crispy coating
Stir-frying	Conduction	Small amount of oil; high heat	Fast; retains nutrients; vibrant colours
Baking	Convection + Radiation	Hot air in oven	Dry heat; crust formation; browning
Roasting	Convection + Radiation	Hot air + fat	Higher temperature than baking; crispy exterior
Grilling	Radiation	Direct radiant heat	Quick; less fat; charred surface; browning
Microwaving	Radiation (microwave)	Microwave radiation	Very fast; retains nutrients; no browning
Braising/Stewing	Convection	Liquid, low temperature, long time	Tenderises tough cuts of meat; develops flavour

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Functional Properties of Proteins

Denaturation

Denaturation is the change in the structure of a protein caused by heat, acid, or mechanical action. The protein's shape changes irreversibly.

Cause	Example
Heat	Egg white turns from translucent liquid to opaque white solid when cooked
Acid	Lemon juice added to milk causes it to curdle (acid denatures casein)
Mechanical action	Whisking egg whites traps air and creates foam (protein strands unfold and form a network)

Coagulation

Coagulation is when denatured proteins set and become firm:

• Egg proteins coagulate at approximately 60–70°C (white) and 68–71°C (yolk)
• Used in: quiche setting, custard thickening, egg-based binding of burgers

Gluten Formation

When wheat flour is mixed with water and kneaded:

• Two proteins (glutenin and gliadin) combine to form gluten
• Gluten is an elastic protein network that:
  • Stretches to trap gas (CO₂ from yeast)
  • Gives bread its structure and chewy texture
  • Develops with kneading (more kneading = stronger gluten)
• Strong (bread) flour has more protein = more gluten = better bread structure
• Shortcrust pastry needs minimal gluten (less mixing = crumblier texture)

Foam Formation

Whisking egg whites creates a protein foam:

• Mechanical action denatures the egg white proteins
• Proteins unfold and form a network around air bubbles
• Creates a stable foam used in meringues, soufflés and mousses
• Over-whisking can cause the foam to collapse (proteins become too rigid)

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Functional Properties of Carbohydrates

Gelatinisation

Gelatinisation occurs when starch is heated in liquid:

Stage	What Happens	Temperature
1	Starch granules absorb water and swell	From ~60°C
2	Granules continue to swell and burst	~80°C
3	Starch molecules are released, thickening the liquid	~85–100°C
4	A gel forms as the mixture cools	On cooling

Used in: sauces (roux sauce, white sauce), custard, gravy, blancmange.

Dextrinisation

Dextrinisation occurs when dry starch is heated:

• Starch breaks down into dextrins (shorter sugar chains)
• Produces a golden-brown colour and toasted flavour
• Examples: toast, bread crust, roasted potatoes, Yorkshire pudding crust

Caramelisation

Caramelisation occurs when sugar is heated:

• Sugar melts and breaks down when heated above approximately 160°C
• Produces a golden to dark brown colour and characteristic caramel flavour
• Used in: caramel, crème brûlée, toffee, caramelised onions

Maillard Reaction

The Maillard reaction occurs between amino acids (proteins) and reducing sugars when heated:

• Produces brown colour and complex flavours
• Responsible for the flavour and colour of: bread crust, grilled meat, roasted coffee, biscuits, fried onions
• Different from caramelisation (which involves only sugar)

Exam Tip: Students often confuse dextrinisation (dry starch heated), caramelisation (sugar heated) and the Maillard reaction (protein + sugar heated). Know the difference and give correct examples for each.

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Functional Properties of Fats

Property	How It Works	Example
Shortening	Fat coats flour particles, preventing gluten formation; produces a crumbly, "short" texture	Shortcrust pastry; biscuits
Aeration	Fat traps air when creamed with sugar; air bubbles expand during baking	Creamed cakes (Victoria sponge)
Plasticity	Fat can be shaped and spread at different temperatures; solid fats can be rubbed into flour	Rubbing-in method for pastry; spreading butter on bread
Emulsification	Lecithin in egg yolk acts as an emulsifier, holding oil and water together	Mayonnaise; cake batters; salad dressings

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Raising Agents

Raising agents introduce gas into a mixture to make it rise:

Type	Agent	How It Works	Gas Produced
Chemical	Baking powder (bicarbonate of soda + cream of tartar)	Reacts with moisture and heat to produce gas	CO₂
Chemical	Bicarbonate of soda (+ acid ingredient)	Reacts with acid (lemon juice, buttermilk, yoghurt)	CO₂
Biological	Yeast	Ferments sugars to produce gas and alcohol	CO₂
Mechanical	Whisking, folding, sieving, creaming	Traps air physically in the mixture	Air
Steam	Water in mixture	Water evaporates during baking, expanding as steam	Steam

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Key Exam Topics in Food Science

Topic	Key Points
Gelatinisation	Starch + liquid + heat = thickening; starch granules swell, burst and release starch
Dextrinisation	Dry starch + heat = golden brown (toast, bread crust)
Caramelisation	Sugar + heat = brown colour and flavour (crème brûlée, toffee)
Maillard reaction	Protein + sugar + heat = brown colour and complex flavour (bread crust, grilled meat)
Gluten formation	Flour + water + kneading = elastic gluten network (bread structure)
Coagulation	Protein + heat/acid = sets firm (eggs, quiche, custard)
Shortening	Fat + flour = crumbly texture (pastry, biscuits)
Aeration	Creaming, whisking, folding = air trapped = lighter texture
Emulsification	Oil + water + emulsifier (lecithin) = stable mixture (mayonnaise)
Raising agents	Chemical (baking powder), biological (yeast), mechanical (whisking), steam

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