Flour is the very finely ground meal of edible grains, such as wheat, rye and rice. Most grain is ground with huge high-speed steel rollers. This process strips away the nutritious bran and germ and destroys vitamins. Therefore, federal standards in the United States require that steel-ground flour be enriched with some of these lost vitamins. Historically, in America, grain was ground between two rotating stones to produce stone-ground flour. This process, which does not waste the wheat germ and vitamins, results in a more nutritious product. Stone-ground flours are available in some large supermarkets, health food stores and by mail order.

Flour contains protein. The most important is glutenin (commonly known as gluten), the substance that provides the structure in baked goods. Flours are categorized by the grain from which they come and, in the case of wheat flour, by the variety of the grain and the processing method used. Listed below are the most common flours available.

To produce a flour with a moderate amount of gluten, high-gluten hard wheat and low-gluten soft wheat are combined. This creates a product that can be used for most baking. It contains neither the germ nor the bran. Bleached flour is whitened with chemical agents. Wheat flour naturally whitens through oxidation if allowed to age for a month or two. Bleached and unbleached flour can be used interchangeably. Most all-purpose flour comes presifted, eliminating the need to sift unless specified in a recipe.

This flour is ground entirely from high-gluten hard wheat. Bakers prefer bread flour for yeast breads, because it produces breads with the best taste, volume and texture.

This flour is made from low-gluten soft wheat. It has a fine texture and is a good choice for cakes, pastries and quick breads.

Milled from rye grain, this flour contains less gluten than wheat flour. It needs to be mixed with wheat flour (all-purpose or bread) to produce a loaf that rises well and is not too dense. Rye flour is available in light, medium and dark varieties. The medium variety is the most commonly available. Rye flour is used to make rye and pumpernickel bread.

Made from all-purpose wheat flour to which baking powder (a leavener) and salt have been added, self-rising flour is popular in southern states. Use it only when a recipe calls for it.

Milled from the entire wheat kernel, whole wheat flour retains all of the grain's natural flavor, color and nutrients. It is generally used in combination with all-purpose or bread flour to avoid overly dense or poorly risen loaves.

Store all-purpose, bread, cake, rye and self-rising flour in an airtight container in a cool, dry place for up to six months. Temperatures above 70°F encourage bug infestations and mold. For longer storage, refrigerate or freeze flour in moistureproof wrapping. Whole wheat flour is more perishable than other types of flour, so purchase it in small amounts and store it in the refrigerator for up to three months. Allow chilled flour to return to room temperature before using.