Wheat flour is a powder made from the grinding of common wheat used for human consumption. Wheat varieties are called "soft" or "weak" if gluten content is low, and are called "hard" or "strong" if they have high gluten content. Hard flour, or bread flour, is high in gluten, with 12% to 14% gluten content, and its dough has elastic toughness that holds its shape well once baked. Soft flour is comparatively low in gluten and thus results in a loaf with a finer, crumbly texture.[1] 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.
In terms of the parts of the grain (the grass fruit) used in flour—the endosperm or protein/starchy part, the germ or protein/fat/vitamin-rich part, and the bran or fiber part—there are three general types of flour. White flour is made from the endosperm only. Brown flour includes some of the grain's germ and bran, while whole grain or wholemeal flour is made from the entire grain, including the bran, endosperm, and germ. Germ flour is made from the endosperm and germ, excluding the bran.
To produce refined (white) wheat flour,[2] grain is usually tempered, i.e. moisture added to the grain, before milling, to optimize milling efficiency. This softens the starchy "endosperm" portion of the wheat kernel, which will be separated out in the milling process to produce what is known to consumers as white flour. The addition of moisture also stiffens the bran and ultimately reduces the energy input required to shatter the kernel, while at the same time avoiding the shattering of bran and germ particles to be separated out in this milling process by sieving or sifting.
The endosperm portion of the kernel makes up about 80% of the volume and is desirable because the products made from this white flour are often considered to have a milder flavor, smoother texture, and, in the case of bread, greater volume. The balance of the kernel is composed of the bran and the germ which tend to be coarser. With the invention of the roller milling system in the late 19th century, the bran and the germ were able to be removed, dramatically improving the appeal of baked products to the public.
The moistened grain is first passed through the series of break rollers, then sieved to separate out the fine particles that make up white flour. The balance are intermediate particles of endosperm (otherwise known as product middling or farina) and coarse particles of bran and germ. The middling then makes multiple passes through the reduction rolls, and is again sieved after each pass to maximize extraction of white flour from the endosperm, while removing coarser bran and germ particles.
To produce whole wheat flour, 100% of the bran and germ must be reintroduced to the white flour that the roller milling system was originally designed to separate it from. Therefore, these elements are first ground on another mill (usually a pin mill). These finer bran and germ fractions are then reintroduced to the endosperm (white flour) to produce whole wheat flour made of 100% of the kernel of wheat.
Flour manufacturers commonly add malted barley flour or fungal amylase to commercially-produced wheat flour. The enzymes in these additives increase the rate of conversion of starch to sugars when water is added to the flour, thus making more sugars available for yeast to metabolize during the bread-making process.[3] [4] Millers use the falling number to determine the quality of the wheat grains that are ground into flour.[4]
Wheat flour is available in many varieties; the categorization is regional, and the same name may have several different regional meanings.
Indian flours are generally categorized by how much of the grain is stripped away.
American flours are categorized by gluten/protein content, processing, and use.
The W index is an indicator of a flour's strength which is commonly used by professional bakers. It is measured using a Chopin Alveograph, and it takes into account both the toughness and elasticity of the flour's gluten. The W index is often not listed on commercial flour packaging, but it can be approximated based the flour's protein content.[9]
| Name | W Index | Protein Content (by Weight) | Example Usage | |
|---|---|---|---|---|
| Weak flour | 90 - 160 | 9% - 10% | Biscuits, cakes. | |
| Medium strength flour | 160 - 250 | 10% - 12.5% | Pizza, focaccia, baguettes, pugliese bread. | |
| Strong flour | above 300 | above 13% | Sourdough bread. |
In general, a higher W index flour will require a longer rising time for leavened bread products. Higher W index flour better retains the CO2 produced during the fermentation process, because the gluten traps the produced gas.[10]
The Zeleny sedimentation value is a measurement of how much of the flour sediments during a set period of time in a lactic acid solution. It is used to predict the baking characteristics of a particular cultivar. Since this test has costs associated with it, the Zeleny value may itself be estimated by EM irradiation and spectral analysis of the dry material. Where present, damage by insect pests and fungal diseases are the primary determinants of the sedimentation value, overwhelming the underlying genetic characteristics.
During World War II, the British government promoted National Flour; it was adopted in 1942 as a healthy and economic alternative to importing wheat.[11] The flour is described as being of 85% extraction, i.e. containing more of the whole wheat grain than refined flour, generally described as 70% extraction at the time. Parliamentary questions on the exact constitution of National Flour in 1943 reveal that it was "milled from a grist consisting of 90 per cent wheat and 10 per cent diluent grains. Authorised additions are calcium at the rate of 7oz per 280lb of flour and dried milk at the rate of 2-1NaN-1 per 280lb of flour and customary improvers in normal proportions." The diluent grains were barley, oats and rye and customary improvers were "certain oxidising agents which improve the quality of the bread baked from the flour, and their nature depends on the kind of grain used, whether hard or soft".[12] A survey of the composition of National Flour was conducted for the period 1946–1950.[13] National Flour was discontinued in 1956 against the recommendations of the Medical Research Council[11] as the government considered that the addition of nutritional supplements to refined flour removed the necessity for using National Flour on health grounds.
National Flour was also a term for a flour introduced in Kenya by the colonial government which contained 70% wheat flour and 30% maize flour.[14]