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essentially seeds, and are surrounded by a hard covering material designed to protect the would-be plant from the external elements.The original or wild wheat seed that grew many thousands of years ago had a tough, hard-to-break husk which made it difficult to use. However, the development of crude hybrids (the mixing of different wheat varieties) led to the "domestication" of wheat with more manageable properties. Moreover, the wheat kernel develops in the husk, but the husk is removed during harvest, unlike barley or oats, where the husk remains attached to the kernel.

The modern day wheat kernel consists of three main constituents: the germ, bran, and endosperm (Figure 8-3). The germ portion contains the embryonic plant, along with oils, vitamins, and other nutrients. It represents 2% to 3% of the total kernel weight.The bran portion, or coat, represents 12% to 13% of the kernel, and is comprised of multiple distinct layers.The bran contains mostly cellulose and other fi brous carbohydrates, along with proteins, minerals, and vitamins. The remaining portion, about 85%, is called the endosperm and is the main constituent of wheat flour. It consists of mostly protein and starch, along with some water (12% to 14%) and a small amount of lipid (1%). Kernels that are crushed or milled without regard to separation of the individual fractions yield a "whole" wheat flour containing the germ, bran, and endosperm. Usually, however, these three constituents are separated, either partially or nearly completely, in the milling step, yielding refined flours of endosperm, with little or no germ or bran (Box 8-2).

Flour Composition

The specific composition of flour is critically important because it has a major influence on the fermentation as well as the physical structure of the dough and finished bread. Wheat flour, after all, is the primary ingredient in most

266 Microbiology and Technology of Fermented Foods Box 8-2. Principles of Milling Wheat

The object of wheat milling is to extract the flour from the wheat kernel.This is such an integral step in the manufacture of bread that an entire milling industry has developed whose purpose is to do just that—convert wheat into flour. Originally, some thousands of years ago, milling was done in a single step, as the kernels were ground or pounded manually by a crude mortar and pestle or between stones.

Even today, single-pass milling is performed to make whole wheat flour, although the milling technologies have obviously improved and are now mechanized and automated. Still, stone-ground flours are widely available and are prized for their quality. Functionally, however, stone-grinding has no advantages over other size reduction techniques, such as a hammer mill.

The modern milling operation begins with various wheat handling steps, including sampling, inspection, and testing; screening; and conditioning (Catterall, 1998). Following these steps, which ensure that the wheat has the appropriate composition and quality and is free of debris and impurities, the wheat is ready for milling.Although the whole wheat flours produced by a single-pass grinding step have wide appeal, most breads are made using flours that contain reduced levels (or even none) of the germ and bran. It is the job of the wheat miller to separate these wheat constituents from the main component, the endosperm.

In principle, this can be done simply by successive grinding and separation steps, such that the resulting fractions can either be removed or further processed. In actual practice, the wheat kernels are passed through a series of paired-rollers or stones whose gap distances between each pair is successively more narrow. Modern grinding devices consist of cast iron break rollers, constructed with flutes or ridges to enhance the grinding or shearing action (Figure 1). The rolls run at different speeds to give a shearing, rather than a crushing, action. In between each so-called break steps the ground wheat is passed through sieves that retains the coarse material (containing bran still stuck to the endosperm), while permitting the fine flour to pass. Along the way, an air purifying device is also used to further separate the different fraction. Eventually, the germ and bran-free white flour that is the most common type used for bread manufacture is produced.

Figure 1. A simplified diagram of wheat milling. Adapted from www.fabflour.co.uk.

Box 8-2. Principles of Milling Wheat (Continued)

The milling operation yields dozens of different product streams (Calvel, 2001). Flour removed from the early and late separations is called clear or common flour. It is generally less refined, higher in protein (>14%), and somewhat darker in color. Patent flour is obtained from the intermediate separation steps. It has the least amount of germ and bran, and, therefore has the whitest appearance. Patent flour is still high in protein (13%) and is considered to have the best bread-making quality (at least in the United States and Canada).

In many parts of the world, it is a standard practice to re-combine the different flour fractions to give flours of varying composition, ranging from a straight flour containing all of the separated fractions, to different patent flours containing 60% to 80% of the original wheat.This allows baker to choose a flour based on desired strength (i.e., protein content) or functional properties (i.e., dough elasticity, bread flavor, and oven spring).

It is important that a high yield be achieved during the milling process.The yield is given as the extraction rate, and is expressed as the amount of flour that is obtained from the wheat. In general, extraction rates of about 72% are achieved, meaning that 72 Kg of straight flour can be obtained from 100 Kg of wheat.The balance (about 28%) includes mainly bran and germ. However, since 85% of the wheat is endosperm, some of the remaining material consists of endosperm that adheres to the bran layers (Pyler, 1988).

References

Calvel, R. 2001. The Taste of Bread. (R.L.Wirtz, translator and J.J. MacGuire, technical editor). Aspen Publishers, Inc. Gaithersburg, Maryland. Catterall, P., 1998. Flour milling, p. 296-329. In S.I? Cauvin and L.S.Young, (ed.), Technology of breadmak-

ing. Blackie Academic and Professional (Chapman and Hall). London, UK. Pyler, E.J., 1988. Wheat flour, p. 300-377. In Baking Science and Technology. Volume 1, 3rd edition, Sosland Publishing Co. Merriam, Kansas.

breads. As noted above, however, most of the flour used for bread manufacture in the United States is refined white flour, containing only the endosperm portion. It consists of two main components—protein and starch—with a small amount of hemicellulose and lipid.

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