What chemical changes happen to proteins, carbohydrates and fats when we prepare and cook food?
The functional and chemical properties of food, including protein denaturation, coagulation, gluten formation and foam formation, carbohydrate gelatinisation, dextrinisation and caramelisation, and the role of fats and oils in shortening, aeration and plasticity.
A focused answer to AQA GCSE Food Preparation and Nutrition on the functional and chemical properties of food, including denaturation and coagulation of protein, gluten and foam formation, gelatinisation, dextrinisation and caramelisation, and the properties of fats.
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What this dot point is asking
AQA wants you to explain the science behind what happens to proteins, carbohydrates and fats when food is prepared and cooked, using the correct terms and giving food examples. This underpins why recipes work.
Protein properties
- Foam formation - whisking egg white denatures the protein, which stretches around trapped air to form a stable foam (meringue, mousse).
- Gluten formation - when wheat flour is mixed with water and kneaded, the proteins gliadin and glutenin form gluten, an elastic network that gives bread its stretchy structure and traps gas to make it rise.
Carbohydrate properties
Gelatinisation begins at around 60 degrees C and is complete by about 80 to 100 degrees C, which is why a sauce thickens as it nears the boil; the ratio of starch to liquid sets how thick it becomes. Dextrinisation and caramelisation both contribute to the golden colour and flavour of baked goods, alongside the Maillard reaction, a browning reaction between proteins and sugars (not just sugar alone) that gives bread crust, toast and roasted meat their savoury colour and aroma. These changes are why a roux thickens a sauce, why bread crust browns, and why heated sugar turns golden and sweet-smelling.
Fat and oil properties
Fats and oils have three key functions in baking:
- Shortening - fat coats flour particles and stops long gluten strands forming, giving a short, crumbly texture (shortbread, pastry).
- Aeration - creaming fat with sugar traps air bubbles, giving a light cake.
- Plasticity - fats soften over a range of temperatures, so they can be spread, creamed and shaped (useful for icing and pastry).
These properties explain real recipes: shortcrust pastry is "short" and crumbly because the fat coats the flour and limits gluten, a creamed Victoria sponge is light because creaming traps air, and choosing the right fat (firm butter for flaky pastry, soft margarine for an all-in-one sponge) changes the result. Linking a named property to a food outcome is what lifts an answer.
Try this
Q1. Name the elastic substance formed when wheat flour is mixed with water. [1 mark]
- Cue. Gluten (from gliadin and glutenin).
Q2. Explain what happens when starch gelatinises in a sauce. [2 marks]
- Cue. Starch granules absorb the liquid and swell on heating, then burst to thicken the sauce.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20196 marksExplain the chemical changes that take place when an egg is whisked and then cooked to make a meringue. (Paper 1, Section B)Show worked answer →
When egg white is whisked, air is trapped and the proteins denature (their structure unfolds) and stretch around the air bubbles to form a stable foam. This is foam formation, a functional property of egg protein.
When the meringue is baked in a low oven, the proteins coagulate (set permanently) as they are heated, holding the structure rigid, while water evaporates to leave a light, crisp meringue.
Top-band answers (5 to 6 marks) cover denaturation and foam formation on whisking, and coagulation on heating, linked to a firm, set structure.
AQA 20214 marksExplain how starch thickens a sauce when a roux-based white sauce is made and heated. (Paper 1, Section A)Show worked answer →
For 4 marks, name and explain the process.
The starch granules from the flour are suspended in the liquid. When the sauce is heated, the granules absorb the liquid and swell (gelatinisation), and as heating continues they burst and release starch, which thickens the sauce. Stirring keeps it smooth and prevents lumps.
Markers reward naming gelatinisation and explaining that granules absorb liquid, swell and burst to thicken, ideally noting the roux (fat and flour) coats the starch so it disperses without lumping.
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Sources & how we know this
- AQA GCSE Food Preparation and Nutrition (8585) specification — AQA (2016)