Starches as Thickeners

Starches thicken by gelatinization, which, as discussed in Chapter 6, is the process by which starch granules absorb water and swell to many times their original size, and starch molecules uncoil into long threads.

Another important point made in Chapter 6 is that acids inhibit gelatinization. Whenever possible, do not add acid ingredients to sauces until the starch has fully gelatinized.

Starch granules must be separated before heating in liquid to avoid lumping. If granules are not separated, lumping occurs because the starch on the outside of the lump quickly gelatinizes into a coating that prevents the liquid from reaching the starch inside.

Starch granules are separated in two ways:

• Mixing the starch with fat. This is the principle of the roux, which we discuss now, and of beurre manié, which is discussed in the next section.
• Mixing the starch with a cold liquid. This is the principle used for starches such as cornstarch. It can also be used with flour, but, as we note later, the result is an inferior sauce. A mixture of raw starch and cold liquid is called a slurry.