Describe the functional properties of carbohydrates, including gelatinisation, dextrinisation and caramelisation, and their uses in cooking

What this dot point is asking

SEAB wants you to describe the functional properties of carbohydrates, the useful ways starch and sugar behave when used and heated in cooking, and how cooks make use of them. The three to know are gelatinisation (how starch thickens a sauce), dextrinisation (how starch browns under dry heat) and caramelisation (how sugar browns when heated). The marks come from naming the right process, describing what happens, and giving a kitchen use.

The answer

Gelatinisation: how starch thickens

Gelatinisation is how a starch-and-water mixture thickens when heated:

1. When the mixture is heated, the starch grains absorb water and swell.
2. As heating continues, the swollen grains burst and release starch into the liquid.
3. The liquid thickens, and on cooling it sets to a thicker gel.

This is how cornflour thickens a sauce, how a roux thickens gravy, and how a custard sets. To avoid lumps, the starch is first mixed with a little cold liquid into a smooth slurry, and the sauce is stirred constantly while it heats.

Dextrinisation: how starch browns

Dextrinisation is the browning of starch under dry heat. When starchy food is grilled, toasted or baked, the starch on the surface breaks down and turns golden-brown, adding colour and a toasted flavour. This is what gives toast, the crust of bread, and the surface of baked goods their colour.

Caramelisation: how sugar browns

Caramelisation is the browning of sugar when it is heated strongly. The sugar melts, then turns from clear to golden and then brown, developing a rich, sweet-bitter flavour. It is used to make caramel and toffee, to brown the top of dishes, and to add colour and flavour to roasted and grilled foods.

Why these properties matter

Cooks use these properties on purpose: gelatinisation to thicken sauces, soups and custards, and dextrinisation and caramelisation to brown food for an appetising colour, flavour and aroma. Knowing what causes each helps a cook control the result and avoid faults such as a lumpy sauce or burnt sugar.

Examples in context

Example 1. Thickening the gravy for a local stew. When making the gravy for a dish such as mee rebus or a Chinese braised dish, a cornflour slurry is stirred in and heated until the starch gelatinises, turning a thin liquid into a thick, glossy gravy that coats the noodles. This is gelatinisation put to work to give the dish its body.

Example 2. The golden top of a baked dish. The golden-brown crust on a loaf of bread or a baked pastry comes from dextrinisation of the starch on the surface under the dry heat of the oven, while any sugar present caramelises to deepen the colour and flavour. Together these browning processes give baked goods their appetising appearance and smell.

Try this

Q1. Name the process that thickens a starch-based sauce and describe what happens to the starch. [3 marks]

• Cue. Gelatinisation; the starch grains absorb water and swell, then burst to thicken the liquid, which sets on cooling.

Q2. Name the process that browns sugar when it is heated, and give one use. [2 marks]

• Cue. Caramelisation; used to make caramel or toffee, or to brown and flavour food.

Q3. Suggest two ways to stop a cornflour sauce going lumpy. [2 marks]

• Cue. Any two of: make a cold slurry first, stir constantly while heating, add gradually, heat gently.