Blended White Sauces and Starch Gelatinisation

How starch turns a liquid into a sauce

A white sauce can coat pasta, hold ingredients together or provide a smooth base for other flavours. Its success depends on both the method used and the proportions of ingredients. Flour supplies starch, milk supplies liquid, and fat is used in the roux and all-in-one methods.

Gelatinisation is the thickening of a liquid when starch is heated with it. As the mixture heats, starch granules absorb liquid and swell. They eventually burst, releasing starch into the surrounding liquid and making the sauce thicker. Both heat and liquid are needed: simply mixing flour into cold milk does not produce a cooked, thickened sauce.

The starch must be distributed evenly before and during thickening. If it collects in clumps, the outside can thicken while dry flour remains trapped inside, producing lumps rather than a smooth sauce.

Choosing a thickness for the dish

Viscosity means resistance to flowing. A thin sauce has low viscosity and pours readily; a thick sauce has higher viscosity and flows less easily.

With the amount of liquid kept the same, increasing the starch produces a thicker sauce. With the amount of starch kept the same, adding more liquid produces a thinner sauce. The right consistency therefore depends on what the sauce needs to do.

These white-sauce proportions show how the same volume of milk can produce different consistencies:

These are practical guides, not one recipe for every sauce. Notice that the milk volume stays constant while the flour increases. It is the starch-to-liquid proportion that explains the increase in thickness.

Making a white sauce by the roux method

A roux is a cooked mixture of flour and fat. Preparing it first helps distribute the flour before the liquid is added. For a coating white sauce, use 50 g fat, 50 g flour and 500 ml milk.

Before starting, wash your hands, use clean equipment and measure the ingredients. Keep the saucepan handle turned away from the edge of the hob. Hot sauce can splash and cause burns, so stir carefully and use a dry oven glove when handling hot cookware.

1. Melt the fat gently in a saucepan. Do not let the butter brown: the aim is a pale sauce.
2. Add the flour and stir with a wooden spoon to form the roux. Cook gently for 1–2 minutes, stirring throughout and keeping it pale.
3. Take the pan off the heat. Add the milk gradually, beating each addition into the roux until smooth before adding more. This helps prevent lumps.
4. Return the pan to the heat. Bring the sauce to the boil while stirring continuously, including across the base and into the corners of the pan.
5. Reduce the heat and simmer for about 2 minutes. The sauce should become smooth, thickened and glossy. A coating sauce should coat the back of a spoon.
6. Remove from the heat, then add seasoning or other chosen ingredients. Adding cheese turns the white sauce into a cheese sauce for a dish such as macaroni cheese.

The roux stage prepares the flour and fat mixture; the sauce thickens through gelatinisation after the milk has been added and the mixture is heated.

The all-in-one and blended methods

The all-in-one method makes a white sauce without preparing a roux separately. Put the measured fat, flour and cold milk into the saucepan together. Heat gently, whisking continuously to keep the flour evenly dispersed and prevent lumps. Bring to the boil, then lower the heat and simmer for about 3 minutes until the sauce is thickened and glossy. Remove from the heat before adding seasoning or cheese.

A separate blended method is commonly used with cornflour or arrowroot, often for sweet sauces. First mix the starch with a small amount of cold liquid to make a thin, smooth paste. Heat the remaining liquid to boiling, then carefully pour it over the paste while stirring. Return the mixture to the saucepan and bring it to the boil, stirring continuously. Remove it from the heat once it has thickened. Sugar and flavouring can then be added to a cornflour sauce.

These methods use different ways of distributing starch, but all rely on heating starch with liquid to thicken the sauce.

How heat reaches the sauce—and why stirring matters

Conduction transfers heat through direct contact. On a solid hotplate, heat passes from the hotplate into the metal saucepan base touching it, then through the metal and into the sauce touching the hot base and sides.

Convection transfers heat through movement within the liquid. Warmer liquid rises, while cooler liquid moves down to replace it, setting up circulating currents that carry heat through the sauce.

As the sauce becomes more viscous, it circulates less readily. Agitation, through stirring or whisking, helps move the sauce and distribute heat evenly. It also keeps starch dispersed and moves sauce away from the hot base, reducing lumps, sticking and burning. Stirring therefore supports both a smooth texture and even cooking; it is not a substitute for controlling the heat.

Infused sauces, béchamel and velouté

An infusion flavours a liquid by leaving ingredients in it before straining them out. This changes flavour without leaving pieces in the finished smooth sauce.

For an infused béchamel, put a small piece of carrot, a small onion, six whole peppercorns and one bay leaf into the milk. Bring it very slowly to the boil, then leave it to infuse for about 20 minutes. Carefully strain out the flavouring ingredients and use the milk to make the sauce by the roux method. Handle the hot milk and strainer carefully to avoid splashes.

Velouté uses stock instead of milk, thickened by the roux method. It is often simmered for longer to allow flavours to develop and infuse. Béchamel and velouté can be used in fillings for vol-au-vents. Changing the liquid or flavourings changes the sauce’s character, while starch gelatinisation remains the thickening process.

Thickening and consistency

• Gelatinisation: heat + liquid → starch granules absorb liquid, swell and burst → sauce thickens.
• More starch with the same liquid → higher viscosity.
• More liquid with the same starch → lower viscosity.
• Choose pouring, coating or binding consistency to suit the dish.

Method recall

• Roux: melt fat → stir in flour and cook gently → remove from heat → beat in milk gradually → boil while stirring → simmer until glossy.