Carbohydrate Properties

As the mixture heats, the structure within the granules loosens, allowing them to absorb more water and swell. With further heating, granules can burst and release starch into the surrounding liquid. The swollen granules and released starch make the mixture resist flowing: its viscosity increases. This thickening process is gelatinisation.

Both heat and liquid are needed. Milk can supply the water needed in a white sauce; stock can supply it in gravy. Gelatinisation also occurs when rice, pasta and potatoes cook in water. Some starch-thickened mixtures set into a gel as they cool.

Making a smooth white sauce

A roux-based white sauce demonstrates gelatinisation. You need a saucepan, hob, weighing scales, measuring jug and wooden spoon or whisk, together with measured flour, butter and milk.

1. Melt the butter gently in the saucepan.
2. Stir in the flour to make a smooth paste, called a roux. Cook it briefly while stirring, without allowing it to brown for a white sauce.
3. Add the milk gradually, mixing each addition thoroughly into the paste before adding more.
4. Heat the mixture while stirring or whisking, including around the base and sides of the pan. As the starch absorbs liquid and swells, the sauce thickens.
5. Once thickened, continue cooking gently while stirring until the sauce is smooth and the flour is cooked.

Distributing the flour evenly before and during heating helps prevent lumps. If flour clumps together, the outside can thicken before liquid reaches the flour inside.

An all-in-one sauce offers another method: combine the fat, flour and cold liquid at the start, then heat while stirring or whisking. The preparation differs, but the thickening still comes from starch gelatinisation.

A milk-based béchamel and a stock-based velouté are examples of sauces that use this principle. Milk can also be infused with flavourings before it is used in a sauce.

Controlling the thickness of a sauce

The starch-to-liquid ratio determines how much starch is available to thicken a given amount of liquid. With the same amount of liquid, more starch produces a thicker sauce. With the same amount of starch, more liquid produces a thinner sauce.

For a practical comparison, prepare two small sauces using the same measured volume of milk but different masses of flour. Keep the method and heating conditions the same, and compare the sauces at the same serving temperature. The sauce containing more flour should flow less readily from a spoon.

This relationship lets a cook choose a consistency suited to the dish: a pouring sauce needs to flow easily, whereas a thicker sauce may be needed to coat ingredients. Adding liquid can loosen an overly thick sauce.

How heat reaches the sauce—and why it needs stirring

The metal saucepan conducts heat from the hob through its base. Heat then passes by conduction into the sauce touching the hot pan.

Within the liquid, warmer regions rise and cooler liquid moves down to replace them. These convection currents help distribute heat through the sauce. As the sauce thickens, it moves less readily, so natural circulation becomes less effective.

Stirring or whisking is agitation. It moves the sauce around, distributes heat and starch more evenly, and reduces the risk of lumps, sticking and burning at the hot base. Convection does not remove the need to stir.

Dextrinisation: starch changes under dry heat

Dextrinisation occurs when dry heat breaks starch down into smaller molecules called dextrins. It contributes to browning and can give starchy foods a crisper texture and sweeter flavour. Examples include bread, biscuits and pastry cooked by baking or toasting.

Bread provides a useful practical demonstration. Make and bake bread using a suitable recipe, then compare the browned crust with the softer interior. The outside is exposed to dry heat and loses moisture, allowing dextrinisation to occur at the surface. The moist interior does not develop the same dry, browned surface.

Longer heating increases the extent of dextrinisation, but excessive heating can burn the food. Other reactions also contribute to bread browning, so dextrinisation should not be treated as the only cause of its crust colour.

Caramelisation: sugars develop colour and flavour

Caramelisation happens when sugars are heated sufficiently to break down and undergo chemical changes. Water is released, and brown colours and characteristic caramel flavours develop. Unlike dextrinisation, its starting material is sugar rather than starch.

Vegetables contain natural sugars, so they can caramelise without added sugar. Roasting vegetables can produce browned surfaces and a richer flavour. Caramelised onions are another practical example.

To prepare them, use a chopping board and knife to slice onions, then heat a little oil in a large frying pan over a low heat. Add the onions and a pinch of salt, and cook slowly for about 30–40 minutes, stirring occasionally. They become soft and golden; if they start to catch, add a splash of water and stir. The gentle hob setting allows controlled cooking rather than rapid scorching: it does not mean caramelisation happens in cold food.

Observe the change in colour and flavour as cooking progresses. Added sugar is not necessary for this basic demonstration. Control the heat carefully, because burnt sugar produces a bitter taste rather than the desired caramel flavour.

Sauce essentials

• Starch granules absorb liquid, swell and release starch, increasing viscosity.
• More starch for the same liquid → thicker sauce.
• More liquid for the same starch → thinner sauce.
• Roux: cook flour with fat, then add liquid gradually while mixing.
• All-in-one: combine fat, flour and cold liquid, then heat while stirring or whisking.
• Conduction: heat passes through the pan into the sauce touching it.
• Convection: warmer liquid rises and cooler liquid replaces it.
• Agitation: distributes heat and starch; reduces lumps, sticking and burning.

Practical recall

• White sauce: demonstrates gelatinisation and the effect of starch-to-liquid ratio.
• Baked bread: dry heat promotes dextrinisation at the crust.
• Caramelised onions or roasted vegetables: heating natural sugars develops colour and flavour; excessive heating causes burning and bitterness.