What happens to starch and sugar when they are heated with or without water?
The functional and chemical properties of carbohydrate: gelatinisation of starch, dextrinisation, caramelisation and the use of sugar and starch in cooking, with the conditions that cause each and food examples.
A focused answer on the functional and chemical properties of carbohydrate for Eduqas GCSE Food Preparation and Nutrition (C560), covering gelatinisation of starch, dextrinisation and caramelisation, the conditions that cause each, and their food examples in sauces, baking and sweets.
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What this dot point is asking
Eduqas wants you to explain what happens to carbohydrates, starch and sugar, when they are heated, using the correct terms (gelatinisation, dextrinisation, caramelisation) with the conditions that cause each and a food example. These are common Food Investigation topics.
Gelatinisation of starch
This is how starch-thickened sauces (white sauce, gravy, custard) and fillings are made. The thickness depends on the ratio of starch to liquid: more starch gives a thicker sauce. Stirring keeps the sauce smooth and stops lumps, because it spreads the starch grains so they swell evenly. Starches such as cornflour, wheat flour and arrowroot all gelatinise, at slightly different temperatures, which is why a recipe may specify one over another. Once gelatinised and cooled, the gel can be firm enough to slice (as in a set custard or a lemon meringue pie filling), because the burst starch and released liquid form a network that holds its shape.
Dextrinisation
Dextrinisation needs dry heat (no added liquid) and a high enough temperature, which is why the crust of a loaf browns while the moist crumb inside stays pale. It improves the colour, flavour and appeal of baked starchy foods.
Caramelisation
Caramelisation gives the topping on a creme caramel, the colour of caramelised onions (their natural sugars brown), and the snap of hard caramel. If sugar is heated too far it burns and tastes bitter. Caramelisation and dextrinisation, along with the Maillard reaction (browning between sugars and proteins), together explain most of the brown colour and rich flavour that develop when food is cooked.
Try this
Q1. At roughly what temperature do starch grains burst and thicken a sauce? [1 mark]
- Cue. About 80 to 85 degrees C.
Q2. Name the process that browns the starchy crust of toast. [1 mark]
- Cue. Dextrinisation (browning of starch by dry heat).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20186 marksExplain what happens during gelatinisation when a starch-thickened sauce, such as a white sauce, is made.Show worked answer →
A 6-mark extended-response question on a single process.
A white sauce is made from a starch (flour) and a liquid (milk). When the starch and liquid are heated together and stirred, the starch grains absorb the liquid and swell. At about 60 degrees C they begin to absorb water; at around 80 to 85 degrees C the swollen grains burst and release starch into the liquid, which thickens it. This is gelatinisation.
As the mixture is heated further it thickens more and, on cooling, sets to a gel. Stirring keeps the sauce smooth and stops lumps forming, and the ratio of starch to liquid controls the final thickness (more starch gives a thicker sauce).
Top-band answers (5 to 6 marks) describe the starch grains absorbing liquid, swelling and bursting to thicken the sauce, with the approximate temperature and the role of stirring.
Eduqas 20214 marksExplain the difference between dextrinisation and caramelisation, giving a food example of each.Show worked answer →
A 4-mark question contrasting two browning processes.
Dextrinisation is the browning of starch by dry heat: when starchy foods are baked, grilled or toasted, the starch breaks down into dextrins, which brown the surface. An example is the golden-brown crust of bread or toast.
Caramelisation is the browning of sugar by heat: when sugar is heated to a high temperature it melts and breaks down, turning golden then brown and developing a sweet, slightly bitter flavour. An example is the caramel topping on a creme caramel or the golden colour of caramelised onions.
Markers reward the correct cause for each (dry heat on starch versus heat on sugar) and a suitable food example for each.
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Sources & how we know this
- WJEC Eduqas GCSE Food Preparation and Nutrition specification (C560) — WJEC Eduqas (2016)