An ingredient used in many foods, flour is a fine powder made from cereals or other starchy food sources. It is most commonly made from wheat, but also maize (now called corn in many parts of the Western Hemisphere), rye, barley, and rice, amongst many other grasses and non-grain plants (including buckwheat, grain amaranths and many Australian species of acacia). Flour is the key ingredient of bread, which is the staple food in many countries, and therefore the availability of adequate supplies of flour has often been a major economic and political issue. Flour can also be made from legumes and nuts, such as soy, peanuts, almonds, and other tree nuts.
Flour is always based on the presence of starches, which are complex carbohydrates (aka polysaccharides).
Usually, the word "flour" used alone refers to wheat flour, which is one of the most important foods in European and American culture and it usually is enriched. Wheat flour is the main ingredient in most types of breads and pastries. Wheat flour was often called corn before the introduction of corn from the Western Hemisphere.
Wheat is so widely used because of an important property: when wheat flour is mixed with water, a complex protein called gluten develops. The gluten development is what gives wheat dough an elastic structure that allows it to be worked in a variety of ways, and which allows the retention of gas bubbles in an intact structure, resulting in a sponge-like texture to the final product. This is highly desired for breads, cakes and other baked products. However, certain individuals suffer from an intolerance to wheat gluten known as coeliac or celiac disease. Increased awareness of this disorder, as well as a rising belief in the benefits of a gluten-free diet for persons suffering certain other conditions, has led to an increased demand for bread, pasta, and other products made with flours that do not contain gluten.
A coarser, somewhat granular preparation, rather than a fine dust, is often called meal.
 Types of flour
 Wheat flour
The vast majority of today's flour consumption is wheat flour.
Wheat varieties are typically known as, variously, "white" or "brown" if they have high gluten content, and "soft" or "weak flour" if gluten content is low. Hard flour, or "bread" flour, is high in gluten and so forms a certain toughness that holds its shape well once baked. Soft flour is comparatively low in gluten and so results in a finer texture. Soft flour is usually divided into cake flour, which is the lowest in gluten, and pastry flour, which has slightly more gluten than cake flour.
All-purpose or plain flour
This flour is a blended wheat flour with an intermediate gluten level which is marketed as an acceptable compromise for most household baking needs.
In terms of the parts of the grain (the grass seed) used in flour—the endosperm or starchy part, the germ or protein part, and the bran or fiber part—there are three general types of flour. White flour is made from the endosperm only. Whole grain flour is made from the entire grain including bran, endosperm, and germ. A germ flour is made from the endosperm and germ, excluding the bran.
This flour is whole-grain wheat flour.
This is a flour that was subjected to flour bleaching agents for health purposes, to whiten it (freshly milled flour is yellowish), and to give it more gluten-producing potential. Oxidizing agents are usually employed, most commonly organic peroxides like acetone peroxide or benzoyl peroxide, nitrogen dioxide, or chlorine. A similar effect can be achieved by letting the flour slowly oxidize with oxygen in the air ("natural aging") for approximately 10 days; however, this process is more expensive due to the time required.
This is a flour with a maturing agent added. The agent's role is to help with developing gluten, a role similar to the flour bleaching agents. Bromate is usually used. Other choices are phosphates, ascorbic acid, and malted barley. Bromated flour has been banned in much of the world, but remains available in the United States.
This is a finely milled flour made from soft wheat. It has very low gluten content, making it suitable for soft-textured cakes and cookies. The higher gluten content of other flours would make the cakes tough.
This is a special type of whole-wheat flour. The endosperm is finely ground, as in white flour, while the bran and germ are coarsely ground. Graham flour is uncommon outside of the USA and the cities of Romania. It is the basis of true graham crackers. Many graham crackers on the market are actually imitation grahams because they do not contain graham flour or even whole-wheat flour.
Pastry flour or cookie flour or cracker flour
This flour has slightly higher gluten content than cake flour, but lower than all-purpose flour. It is suitable for fine, light-textured pastries.
Self-rising or self-raising flour
This is "white" wheat flour or wholemeal flour that is sold premixed with chemical leavening agents. It was invented by Henry Jones. It can also be substituted by Maida when cooking under the Indian Cuisine. Typical ratios are:
one cup flour
Wheat flour is highly explosive when airborne. In medieval flour mills, candles, lamps, or other sources of fire were forbidden.
In Britain, many flours go by names different than those from America. Some American flours and British equivalents include:
 Other flours
 Flour type numbers
In some markets, the different available flour varieties are labeled according to the ash mass ("mineral content") that remains after a sample was incinerated in a laboratory oven (typically at 550 °C or 900 °C, see international standards ISO 2171 and ICC 104/1). This is an easy to verify indicator for the fraction of the whole grain that ended up in the flour, because the mineral content of the starchy endosperm is much lower than that of the outer parts of the grain. Flour made from all parts of the grain (extraction rate: 100%) leaves about 2 g ash or more per 100 g dry flour. Plain white flour (extraction rate: 50-60%) leaves only about 0.4 g. German flour type numbers (Mehltype) indicate the amount of ash (measured in milligrams) obtained from 100 g of the dry mass of this flour. Standard wheat flours (defined in DIN 10355) range from type 405 for normal white wheat flour for baking, to strong bread flour types 550, 650, 812, and the darker types 1050 and 1600 for wholegrain breads. French flour type numbers (type de farine) are a factor 10 smaller than those used in Germany, because they indicate the ash content (in milligrams) per 10 g flour. Type 55 is the standard, hard-wheat white flour for baking, including puff pastries ("pâte feuilletée"). Type 45 is often called pastry flour, but is generally from a softer wheat. Types 65, 80, and 110 are strong bread flours of increasing darkness, and type 150 is a wholemeal flour.
In the United States and the United Kingdom, no numbered standardized flour types are defined, and the ash mass is only rarely given on the label by flour manufacturers. However, the legally required standard nutrition label specifies the protein content of the flour, which is also a suitable way for comparing the extraction rates of different available flour types.
It is possible to find out ash content from some US manufacturers. However, US measurements are based on wheat with a 14% moisture content. Thus, a US flour with .48 ash would approximate a French Type 55.
In general, as the extraction rate of the flour increases, so do both the protein and the ash content. However, as the extraction rate approaches 100% (whole meal), the protein content drops slightly, while the ash content continues to rise.
The following table shows some typical examples of how protein and ash content relate to each other in wheat flour: This table is only a rough guideline for converting bread recipes. Since the American flour types are not standardized, the numbers may differ between manufacturers.
 Flour production
Milling of flour is accomplished by grinding grain between stones or steel wheels. Today, "stone-ground" usually means that the grain has been ground in a mill in which a revolving stone wheel turns over a stationary stone wheel, vertically or horizontially with the grain in between. Many small appliance mills are available, both hand-cranked and electric.
Flour dust suspended in air is explosive, as is any mixture of a finely powdered flammable substance with air, see Lycopodium. Some devastating and fatal explosions have occurred at flour mills, including an explosion in 1878 at the Washburn "A" Mill in Minneapolis, the largest flour mill in the United States at the time.
In history, both large and hand mills were operated. Until modern times, much flour contained minute amounts of grit, either the result of poor sifting of the grain or of grinding stones together. This grit strongly abraded teeth.
One of the most ancient methods of grinding to produce flour was by using a pair of quern-stones. These were made out of rock, and were ground together by hand. They were generally replaced by millstones once mechanised forms of milling appeared, particularly the water mill and the windmill, although animals were also used to operate the millstones.
 Flour products
Bread, pasta, crackers, many cakes, and many other foods are made using flour. Wheat flour is also used to make a roux as a base for gravy and sauces. White wheat flour is the traditional base for wallpaper paste. It is also the base for papier-mâché. Cornstarch is a principal ingredient of many puddings or desserts.