All-purpose flour is one of the kitchen staples virtually everyone has, whether they cook much or not. Unfortunately, there's no great drop-in replacement for flour for people who can't eat wheat (whether due to wheat allergies or celiac disease). You simply have to choose the best wheat-free flour for a given cooking purpose. Read on to learn about the different categories of wheat-free flours on the market and how to use them, as well as some more products you might find useful in baking and cooking without wheat.
Grains are made up of two basic components: protein and starch. The protein component of wheat is gluten, and it's what makes baked goods stick together so well. The other main component of flour is starch. There are four major wheat-free culinary starches: corn, arrowroot, tapioca, and potato. From a culinary standpoint, the four are interchangeable, so feel free to mix and match based on your dietary needs and what's in your pantry.
On its own, starch isn't sufficient to hold baked goods together, so it's not a good one-to-one substitute for flour in baking. Starch is good for thickening soups, sauces, and gravies, although it doesn't act exactly the same as wheat flour in doing so. Being virtually tasteless, it can blend with almost any cuisine. It has a tendency to create gooey lumps when heated; you can combat this by creating a slurry of starch and liquid rather than adding starch directly to a hot pan. Starch-thickened liquids tend to congeal a bit as they cool, but heating brings them back to their original state. Another quirk about starch: while flour-thickened gravies and sauces are opaque, starch-thickened sauces are shiny and translucent.
Starch is also used in a traditional Chinese cooking technique called velveting, in which egg whites and starch are used to coat chicken pieces before stir-frying over high heat. The combination of starch and egg protects the meat and keeps it from drying out.
Low-Protein, Neutral-Taste Flours
Low-protein flours include both the protein and the starch from the grain, but come from grains that are naturally low in protein. In general, these flours are the best substitutes for wheat in situations where gluten isn't important -- that is, where flour isn't holding together a baked good. They are ideally suited for thickening sauces, dredging meats, making tempura batters, and making flatbreads. They can also be used in baking in combination with other flours and baking aids.
White rice flour is the first flour many people experiment with when first diagnosed with a wheat allergy. Rice flour has a few wonderful traits: it's got a neutral flavor, it's available at many markets that carry few other special wheat-free baking aids, it's reasonably flexible from a culinary standpoint, and it stores well. My biggest objection to rice flour is the texture; I find it gritty. Brown rice flour has similar advantages and disadvantages to white rice flour but should be stored in the refrigerator.
For most non-baking applications, I prefer to use millet flour, which has a slightly stronger flavor than rice flour but a much softer texture. I find millet makes the closest replacement for wheat flour in roux-thickened sauces (like Béchamel). Another useful low-protein flour is corn flour, which is nice for dredging and often combined with other flours in baking. Corn flour is much finer and softer than cornmeal -- the two can't be used interchangeably.
Low-Protein, Stronger Tasting Flours
Amaranth, quinoa, sorghum, teff, and buckwheat flour have similar protein contents to millet, corn, and rice (with rice having the least protein of the group). These flours have similar cooking properties to other low-protein flours, and they are generally unsuitable for baking on their own.
These flours, however, taste somewhat stronger than the flours in the preceding group. Because of that, they are less suitable for sauces, where flour is generally not used for its taste. They are primarily used in recipes where their unique flavors are the focal point -- buckwheat flour, for example, is the traditional flour used to make Breton galettes -- or to combine with high-protein flours, as they can meld nicely and help mellow the flavor of the mixture.