As a food scientist, I’m always on call to handle my friends’ and family’s recipe emergencies, but this time of year the phone rings more than usual: “My grandmother is from Germany, and her recipes use potato starch to thicken everything—can I use flour instead?” Or, “One pie calls for tapioca and another flour— does it really make a difference which I use?”
The most commonly used starches (in this country at least) for thickening pan sauces, gravies, puddings, and pie fillings are flour, cornstarch, and tapioca. They don’t all behave in quite the same way, though, so before you substitute one for the other, it’s helpful to know a bit about starches in general and also about each one in specific.
What are starches?
Starches are long chains of sugar molecules—thousands of molecules long, in fact—that are found, in the form of tiny, dry granules, in all plants. The starches we commonly use in the kitchen come from grains (wheat and corn) and roots (potato, cassava, and arrowroot).
How do starches thicken?
Starch granules don’t dissolve in cool or tepid liquid—stir some cornstarch into cold water and you’ll see what I mean—but when heated in a liquid, the granules swell, absorb water, and burst, emptying more starch molecules into the liquid. (Every starch granule is jam-packed with starch molecules.) The liquid then thickens because of the traffic jam of tangled molecules and also because the starch molecules sop up water.
All starches begin to thicken at around 140°F. But to achieve full thickening power, flour and cornstarch, which have a high percentage of a starch molecule called amylose, must come all the way to a boil and be held just below the boiling point for several minutes to cook off the raw starch flavor. Tapioca and other root starches, which are rich in a different kind of starch called amylopectin, thicken well before the boiling point.
Prolonged cooking and stirring as well as exposure to acids like lemon juice, wine, and vinegar weaken all starches’ thickening power. Different starches, however, can endure different amounts of heat, agitation, and acidity before they start to break down and lose their thickening power. And there are a few other differences worth learning about, such as clarity, cooking characteristics, and how well they freeze and thaw.