Food science: what happens when we cook

Why does a sauce thicken, an egg set and toast turn brown? Cooking is science you can eat.

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Changes to starch, protein and sugar

Gelatinisation: when starch is heated in a liquid, the starch grains absorb liquid, swell and burst, thickening the liquid. This is how a white sauce or custard thickens.
Coagulation: when proteins are heated, they set. A runny egg becomes solid when cooked; this cannot be reversed.
Caramelisation: when sugar is heated, it melts and turns brown, giving a sweet, nutty flavour.
Dextrinisation: when starch is heated with dry heat, it turns brown, as in toast and the crust of bread.

Raising agents

Raising agents make mixtures rise by adding gas, which expands when heated:
TypeExampleHow it works
MechanicalWhisking, creaming, sievingTraps air in the mixture
ChemicalBaking powder, bicarbonate of sodaGives off carbon dioxide gas when wet and heated
BiologicalYeastFerments sugar, giving off carbon dioxide gas
SteamBatters such as Yorkshire puddingsWater in the mixture turns to steam in a hot oven

Taste, texture and browning

Use your senses: taste, smell, texture (mouthfeel) and appearance all affect how much we enjoy food.
The basic tastes are sweet, sour, salty, bitter and umami (savoury).
Season food by tasting as you cook, using a clean spoon each time, and adding small amounts of seasoning.
Enzymic browning: cut apples, bananas and avocados turn brown when exposed to oxygen. An acid, such as lemon juice, slows this down.
Worked example

Explain why a white sauce thickens when it is heated and stirred.

  1. The flour in the sauce contains starch.
  2. When it is heated in milk, the starch grains absorb liquid, swell and burst.
  3. This is gelatinisation, which thickens the sauce. Stirring stops lumps forming.

Answer: Starch in the flour gelatinises: the grains absorb liquid, swell and burst, thickening the sauce.

Key idea

Gelatinisation thickens sauces (starch and liquid), coagulation sets proteins (eggs), caramelisation browns sugar, dextrinisation browns starch with dry heat (toast). Raising agents: mechanical (air), chemical (baking powder), biological (yeast) and steam.

Key words
Gelatinisation
Starch grains absorbing liquid, swelling and bursting when heated, which thickens sauces.
Coagulation
The setting of proteins when they are heated, such as an egg becoming solid.
Caramelisation
The browning of sugar when it is heated.
Dextrinisation
The browning of starch with dry heat, as in toast.

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Extra practice

2 more questions

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  1. Which ingredient thickens a white sauce by gelatinisation?

    1. Egg white
    2. Sugar
    3. Flour
    4. Salt
    Show the answer

    Answer: Flour

    Starch in flour gelatinises.

  2. Which nutrient coagulates when heated?

    1. Starch
    2. Sugar
    3. Protein
    4. Fibre
    Show the answer

    Answer: Protein

    Proteins set when heated.

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Sources and references

Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.