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.
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).
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.
Because starches swell and gelatinize at warm temperatures, it’s not a good idea to sprinkle them directly into hot liquids. The outer edges of the powdered starch will gelatinize instantly, virtually sealing off the rest of the starch and resulting in lumps. In essence, the outer surface of the starch cooks before the starch has a chance to disperse and dissolve in the liquid. To prevent lumps, stir the starch into a small of amount of cool water, then stir this starch slurry into the hot liquid you want to thicken.
|Flour is the most common thickener used in recipes, from turkey gravy to apple pie, and for good reason: It’s versatile, and in most kitchens, it’s always on hand. But flour isn’t a pure starch (it contains protein and other components), so it has only about half the thickening power of other starches. The best flour to use as a thickener is all-purpose flour because it’s higher in starch than other wheat flours.||Cornstarch is a pure starch derived from corn. It can withstand a good amount of cooking and stirring before it begins to break down. That’s why it’s frequently used for thickening vanilla pastry cream, banana-cream pie filling, as well as butterscotch and chocolate puddings, all of which are cooked on the stove and involve prolonged heating and stirring.||Tapioca is a pure starch derived from the root of the cassava plant, and it comes in many forms. The small granules of pearl tapioca, labeled instant or quick cooking (Minute Tapioca is a common brand), are widely available and work well as a thickener. There’s also a powdered variety, which dissolves more smoothly than the granules but is hard to find; I get it at health-food stores.|
|APPEARANCE||The proteins in flour make flour-thickened sauces and pie fillings look cloudy.||Cornstarch-thickened sauces have a translucent shimmer.||Tapioca-thickened fillings are crystal clear and have a more jelly-like consistency than those thickened with other starches. Instant tapioca granules don’t completely dissolve; they may linger in pie fillings as soft, clear beads.|
|WHEN TO USE||Flour works best for foods that don’t suffer visually from opacity: white sauces such as béchamel, simple pan gravies, beef stew, chicken fricassée, and apple or pear pies.||Cornstarch is great for delicate sauces and gravies that you want to be translucent, like stir-fry sauces. It’s also a good choice for berry and stone-fruit pies, because it won’t cloud the jewel-like colors of the fruit juices. Since it can handle a good amount of heat, it’s fine for stove-top puddings and sauces that will be reheated.||It thickens juices faster than flour or cornstarch, so tapioca is great for all fruit pies, especially berry, peach, and rhubarb, which throw off a lot of juice. It’s also great for pies that will be frozen and reheated, because tapioca holds liquid, so the pie filling won’t weep when frozen and thawed.|
|WHEN NOT TO USE||Berry pies or sauces where clarity is important||Don’t sprinkle it directly into hot pan juices for gravy because it will clump.||Don’t use instant tapioca for pies with open lattices or large steam vents because the granules will be exposed directly to the hot air of the oven and won’t dissolve. It’s also not ideal for pan sauces or stovetop custards because it can’t withstand a lot of stirring and boiling.|
|HOW TO USE||
There are lots of ways to use flour as a thickener—that’s the great thing about it: You can cook it with aromatic vegetables in the pot before adding a braising liquid. You can dredge stew meat in flour before browning it, and the flour will later thicken the stew. You can mix it with a little cool liquid to form a paste and then whisk it into a simmering pan sauce. One thing you shouldn’t do with flour is toss it directly into hot liquid—the dry granules will likely clump together.
To achieve full thickening power and eliminate raw flour taste, flour-thickened mixtures must be brought to a boil and then cooked for about 3 minutes. But don’t go overboard with the cooking, because flour thickens more as it cools; as a rule, stop cooking gravies and sauces when they’re a bit thinner than their ideal consistency.
|More than any other starch, cornstarch is prone to clumping when exposed to hot liquids. To avoid lumps, mix cornstarch with something that will help separate the granules from one another. For pies and custards, combine it with the sugar for better dispersion. For sauces, mix the cornstarch with a tablespoon or two of the liquid called for in the recipe—the liquid should be cool. Simmer the sauce for at least a minute to eliminate the pasty flavor of raw starch.||For best results, let pearl tapioca sit with the fruit for 5 to 10 minutes before you bake the pie so that the fruit juices can begin to soften the granules. And before you remove a pie from the oven, make sure the juices at the center are bubbling, even if it seems the juices at the edge have been fully cooked for quite a while.|
Writing this article gave me an excuse to experiment with a couple starches that I hadn’t used much before, potato starch and arrowroot. They’re both wonderful thickeners, and I hope you’ll give them a try, too.
Potato starch is most commonly called for in European recipes. It’s easy to find in the baking ingredient aisle of East Coast markets, but in the rest of the country, look for it in the kosher section of the store. Potato starch thickens quickly without a pronounced flavor that needs to be cooked off, which makes it great as a lastminute fix for too-thin sauces.
Arrowroot powder comes from the root of a tropical plant of the same name. Look for it in gourmet or health-food stores. Arrowroot starch granules are very small and make sauces exceptionally smooth. Like flour and cornstarch, it can withstand long cooking and higher temperatures, and like tapioca, it is remarkable for its clarity. It’s a great choice for stir-fry sauces and any kind of fruit pie filling.
When I started researching starch-substitution guidelines, I discovered so many inconsistencies from one book to the next that I decided to conduct my own experiment, and here’s what I found:
2 Tbs. flour = 1 Tbs. cornstarch = 1 Tbs. tapioca = 1 Tbs. + 1-1/2 tsp. arrowroot = 1-1/2 tsp. potato starch
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