Explain gelatinisation, dextrinisation and caramelisation, and apply them to thickening, baking and browning

What this dot point is asking

The syllabus wants you to explain what happens to carbohydrates when they are cooked - the gelatinisation of starch, dextrinisation, and the caramelisation of sugar - and to apply these to thickening sauces and browning food. The central idea is that starch and sugar change in predictable, useful ways when heated, and cooks rely on these changes to set, thicken, brown and flavour food.

The answer

Gelatinisation of starch

Gelatinisation is what happens when starch is heated in a liquid. The steps are:

1. The starch grains absorb the surrounding liquid and swell.
2. At around $60$ to $80\ ^\circ\text{C}$ the swollen grains burst and release starch into the liquid.
3. The released starch thickens the liquid, which sets into a smooth gel as it cools.

This is how flour and cornflour thicken sauces, gravies, custards and soups. The more starch used, the thicker the result.

Avoiding lumps

If starch is added too quickly to hot liquid, the outside of each clump gelatinises and seals before the inside can spread out, forming lumps. To avoid this, mix the starch with a little cold liquid first to make a smooth paste (a "slurry"), then stir it into the hot liquid while stirring continuously.

Dextrinisation

Dextrinisation is the browning of starch by dry heat. The starch breaks down into shorter molecules called dextrins, which are brown and add flavour. It happens on the surface of starchy foods cooked with dry heat: the golden crust of bread, browned toast, and the surface of baked or fried starchy foods.

Caramelisation

Caramelisation is the browning of sugar by heat. As sugar is heated it melts, then turns golden and finally brown, developing a rich, sweet, slightly bitter flavour. It is used to make caramel, to brown the top of baked desserts, and to add colour and flavour to many sweet and savoury dishes.

Why these matter

Together, gelatinisation, dextrinisation and caramelisation let a cook thicken liquids to the right consistency, give baked foods an appetising brown crust, and add deep flavour and colour. They are mostly desirable changes, though burning (taking browning too far) is not.

Examples in context

Example 1. Cornflour-thickened sweet and sour sauce. A glossy sweet and sour sauce is thickened by gelatinising cornflour: the cornflour is mixed with cold water, then stirred into the hot sauce where it swells and bursts to thicken it. Adding the cornflour dry would give lumps, so the cold slurry is essential.

Example 2. The browned top of a creme caramel. The caramel layer of a creme caramel is made by heating sugar until it melts and turns brown, which is caramelisation. The same browning, taken too far, would burn and taste bitter, showing why careful control of heat matters.

Try this

• Cue. Describe the steps of gelatinisation. Recall starch grains absorbing liquid, swelling, then bursting at about $60$ to $80\ ^\circ\text{C}$ to thicken the liquid.
• Cue. Explain how to thicken a sauce with cornflour without lumps. Blend the cornflour with cold liquid first, then stir into the hot liquid while heating and stirring.
• Cue. State the difference between dextrinisation and caramelisation with an example of each. Recall starch browning by dry heat (toast) versus sugar browning by heat (caramel).