Gelatinization

Gelatinization is what happens when starch is heated in water: the starch grains soak up water, swell and burst, and the liquid thickens. It is how flour thickens a roux-based sauce, cornstarch sets a pudding, rice turns soft, and pasta cooks. Despite the name, it has nothing to do with gelatin, which is an animal protein.

Why it works

Starch is stored in plants as tight, tiny grains. In cold water they barely change, which is why you can mix cornstarch into cold water as a slurry without lumps. As the water heats, usually somewhere between 55 and 85°C (130 and 185°F) depending on the starch, the grains take in water and swell to many times their size. Long starch molecules leak out and tangle with each other, which thickens the liquid. Cornstarch needs to come close to a boil to reach its full thickness.

Sugar, fat and acid all slow gelatinization. That is why a sweet pastry cream needs more cooking than a plain cornstarch sauce, and why acidic sauces can stay thinner.

How to use it

• Bring starch-thickened sauces back to a simmer after adding the starch, then cook for a minute or two.
• In crème pâtissière, boil for 1 to 2 minutes while whisking. Undercooked pastry cream can turn runny later.
• When cooking rice and pasta, use enough water and time for the starch to fully soften.

Common mistakes

• Stopping before the sauce simmers, which leaves it thin and tasting of raw starch.
• Stirring very hard or boiling for a long time after thickening, which breaks the swollen grains and thins the sauce again.
• Adding a lot of acid early, which weakens the thickening.
• Using a starch that does not suit the dish. Cornstarch turns cloudy; arrowroot and tapioca stay clearer.

In recipes

Recipes do not use the word, but "cook until thickened", "bring back to a boil" and "until it no longer tastes floury" describe it. The opposite process, when cooled starch firms and stales, is called retrogradation.