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