What happens to starch and sugar when food is heated?
The functional and chemical properties of carbohydrate: gelatinisation of starch, dextrinisation and caramelisation, with the temperatures, conditions and food examples for each.
A focused answer on the functional and chemical properties of carbohydrate for OCR GCSE Food Preparation and Nutrition (J309), covering gelatinisation, dextrinisation and caramelisation, the temperatures and conditions for each, and their food examples.
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What this dot point is asking
OCR wants you to explain what happens to carbohydrates when food is prepared and cooked, using the correct terms (gelatinisation, dextrinisation, caramelisation) with the conditions, approximate temperatures and food examples.
Gelatinisation
Gelatinisation begins at around degrees C and is complete by about to degrees C, which is why a sauce thickens as it nears the boil. The ratio of starch to liquid sets how thick the sauce becomes: more starch makes a thicker sauce. A roux (fat and flour cooked together) coats the starch granules so they disperse evenly and the sauce stays smooth without lumps. Gelatinisation thickens white sauce, gravy, custard and lemon meringue filling.
Dextrinisation
Dextrinisation needs dry heat (no added water), which is why toasting bread or baking pastry browns the surface. It works alongside the Maillard reaction (a browning reaction between proteins and sugars) to give baked goods their golden colour and flavour.
Caramelisation
Caramelisation is a change to sugar alone (no protein needed), unlike the Maillard reaction, which needs proteins and sugars together. Overheating sugar past caramel makes it bitter and then burnt, so the temperature must be watched.
Starch as a setting agent
As well as thickening, gelatinised starch can set a mixture as it cools. A cornflour-thickened sauce or a blancmange thickens when hot and then sets to a soft gel when cold, because the swollen starch holds the liquid in a network. The type of starch matters: cornflour gives a clear, glossy gel suited to fruit sauces and pie fillings, while wheat flour gives an opaque sauce suited to a savoury white sauce. Waxy (modified) starches are used in commercial products such as instant desserts and pie fillings because they thicken without lumping, stay smooth when frozen and thawed, and hold their texture, which is why convenience foods often list modified starch.
Why these changes matter in cooking
Knowing these properties lets you explain and control real dishes. You use gelatinisation to thicken a gravy, custard or lemon meringue filling to the right consistency by choosing the starch-to-liquid ratio; you rely on dextrinisation and the Maillard reaction to brown and flavour bread crust, pastry and biscuits; and you use caramelisation to make toffee, caramel sauce and the crisp top of a creme brulee. Controlling temperature and time is the key skill: too little heat leaves a sauce thin or a crust pale, while too much burns the sugar or scorches the starch.
Try this
Q1. Name the process by which starch granules swell and thicken a sauce when heated with liquid. [1 mark]
- Cue. Gelatinisation.
Q2. Give a food example of caramelisation. [1 mark]
- Cue. Toffee, caramel, fudge or the top of a creme brulee.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20194 marksExplain how starch thickens a sauce when a roux-based white sauce is made and heated.Show worked answer →
A 4-mark structured question.
The starch granules from the flour are suspended in the liquid (the roux of fat and flour helps them disperse without lumping). When the sauce is heated, the starch granules absorb the liquid and swell. This is gelatinisation. As heating continues, the granules burst and release starch into the liquid, which thickens the sauce. Stirring keeps it smooth.
Markers reward naming gelatinisation and explaining that the granules absorb liquid, swell and burst to thicken, ideally noting the roux coats the starch so it disperses without lumping.
OCR 20214 marksExplain the difference between dextrinisation and caramelisation, giving a food example of each.Show worked answer →
A 4-mark structured question.
Dextrinisation is the browning of starch by dry heat. When starchy foods are toasted or baked, the dry heat breaks the starch into dextrins, which brown and add flavour, for example the golden crust of toast or pastry.
Caramelisation is the browning of sugar when it is heated. As sugar is heated above about 160 degrees C it melts and turns golden brown, changing flavour, for example making toffee, caramel or the top of a creme brulee.
Markers reward dextrinisation as the dry-heat browning of starch (toast) and caramelisation as the browning of sugar by heat (toffee), with a correct example for each.
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