Carbohydrate changes help explain why foods thicken, brown, crisp and develop new flavours during cooking. Three important changes for GCSE food science are gelatinisation, dextrinisation and caramelisation. They do not all involve the same type of carbohydrate or happen in the same conditions. Understanding the differences will help you explain practical results, such as why a sauce thickens, why toast turns brown and why heated sugar becomes golden. This guide is useful for pupils studying the shared food science content in WJEC Level 1/2 Hospitality and Catering and AQA GCSE Food Preparation and Nutrition.
What are carbohydrates?
Carbohydrates are nutrients that include sugars and starches. Sugars are usually smaller, simpler carbohydrate molecules, while starch is a complex carbohydrate made from many sugar units joined together. Both can change when food is heated, but the changes depend on the food, the presence of water and the cooking method.
In an exam, name the carbohydrate involved, describe the conditions needed and link the change to the final food product. Saying only that food “goes brown” is not usually enough.
Gelatinisation
Gelatinisation is the thickening of a liquid when starch is heated with moisture. Starch granules absorb water and swell. As heating continues, the granules lose their organised structure and release some starch into the liquid. This increases the liquid’s viscosity, meaning it becomes thicker and flows less easily.
Examples of gelatinisation
- A white sauce thickens when flour or another starch is heated with milk.
- Custard becomes thicker as its starch is heated in liquid.
- Rice, pasta and other starchy foods soften as their starch absorbs water during cooking.
For a smooth sauce, the starch needs to be distributed through the liquid. A roux spreads flour through fat before liquid is added, while a cornflour slurry mixes starch with cold liquid first. Stirring helps reduce lumps and spreads heat through the mixture. The amount of liquid, the type of starch, heating and stirring all affect the result.
Gelatinisation is useful because it gives sauces, gravies, soups, custards and other products a thicker texture. If there is not enough liquid, the starch may not hydrate properly. If the mixture is overheated or left unstirred, it may catch, become lumpy or develop an unwanted texture.
Dextrinisation
Dextrinisation is the breakdown of starch into shorter chains called dextrins when dry heat is applied. Dextrins are smaller than starch molecules. This change can produce a golden-brown colour, a toasted flavour and a crisper surface.
Examples of dextrinisation
- Bread browns and becomes crisp when toasted.
- Breadcrumbs develop colour and a toasted flavour when cooked in a dry oven or pan.
- Flour can develop a deeper colour and flavour when heated with little or no moisture.
Dextrinisation is associated with dry heat, so it is different from gelatinisation. A slice of bread can undergo gelatinisation during baking because starch is heated with water in the dough, then dextrinisation at the surface as dry heat causes browning and crisping. The same product can therefore show more than one carbohydrate change at different stages.
Caramelisation
Caramelisation is the breakdown of sugars caused by heating. It produces new flavour compounds and changes the colour from pale to golden brown and, if heating continues, darker brown. The result can include sweet, toasted or slightly bitter flavours. Caramelisation does not require starch, although a food may contain both starch and sugar.
Examples of caramelisation
- Sugar heated in a pan melts and develops a caramel flavour and colour.
- A sugar topping can become crisp and brown when heated.
- Foods containing natural sugars may develop a deeper colour and flavour during cooking, depending on the method and conditions.
Control is important. Gentle, even heating can help produce an attractive colour, while excessive heating can make caramel taste burnt and bitter. Moisture, the amount of sugar and the length of heating affect the result. Caramelisation should not be confused with the Maillard reaction, another browning reaction involving reducing sugars and amino acids from proteins. If a question asks about caramelisation, focus on the heating and breakdown of sugars.
Comparing the three changes
| Change | Main carbohydrate | Conditions | Typical effect | Practical example |
|---|---|---|---|---|
| Gelatinisation | Starch | Heat plus liquid | Thickening and softening | White sauce or cooked rice |
| Dextrinisation | Starch | Dry heat | Browning, crispness and toasted flavour | Toast or breadcrumbs |
| Caramelisation | Sugars | Heating sugar | Brown colour and caramel flavour | Heated sugar topping |
How to answer a practical food science question
Use a clear cause-and-effect explanation. For example: “The flour contains starch. When it is heated with liquid, the starch granules absorb water and swell. The sauce becomes more viscous, so it thickens.” This is stronger than writing, “The flour makes the sauce thick.”
When describing browning, identify whether the question is about starch or sugar. Toast is a useful example of dextrinisation because dry heat changes starch at the surface. Heated sugar is a useful example of caramelisation because sugar breaks down and produces new colour and flavour.
Exam-ready retrieval checklist
- Define gelatinisation as the thickening of a liquid when starch is heated with moisture.
- Explain that starch granules absorb water and swell during gelatinisation.
- Link dextrinisation with the dry heating of starch.
- Link caramelisation with the heating and breakdown of sugars.
- Give a relevant practical example for each change.
- Use precise results: viscosity, thickening, crispness, browning and flavour.
- Do not treat all browning as caramelisation.
This article covers shared food science knowledge, but assessment wording and qualification requirements are not necessarily identical. Check the current WJEC specification or AQA specification for the qualification you are studying. Official guidance and specifications can change, so use the latest board information when revising.
Frequently asked questions
Is gelatinisation the same as caramelisation?
No. Gelatinisation involves starch, liquid and heat, and usually causes thickening. Caramelisation involves the breakdown of sugars during heating and causes changes in colour and flavour.
Why does toast become crisp and brown?
The dry surface heat encourages dextrinisation of starch, contributing to browning, crispness and toasted flavour. Other browning reactions may also contribute, depending on the food and cooking conditions.
Can one food show more than one carbohydrate change?
Yes. Bread dough can undergo gelatinisation during baking as starch is heated with moisture, while the surface can later become crisp and brown through dry-heat changes including dextrinisation.