Wheat. Peter R. Shewry

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Wheat - Peter R. Shewry


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between grain weight and bran yield for 150 wheat genotypes from a single field experiment. Bran yields are determined using a small‐scale laboratory mill and are therefore higher than those achieved by large‐scale commercial mills.

      Source: Redrawn from Ward et al. (2008) with permission.

      1.5.2 Endosperm Texture

      The classification of wheat into hard and soft types (Chapter 8) is determined largely by the degree of cohesion between the protein and starch granules within the starchy endosperm, with hard wheats requiring greater force to break the grain and reduce the starchy endosperm to flour. Hardness is strongly but not entirely genetically determined, particularly by variation at the Hardness (Ha) locus on the short arm of chromosome 5D (Turnbull and Rahman 2002). When milled, hard wheats yield coarser but more evenly shaped flour particles. The flour is also more easily separated from the bran, and the particles flow easily over surfaces and through sieves (Kent and Evers 1994; Carson and Edwards 2009).

      Endosperm texture can be determined by standardized grinding and sieving to weigh the proportion of material that will pass through a mesh. This is the basis of the Particle Size Index (PSI) determination with soft wheats having a greater proportion of throughs (and therefore greater PSI) than hard wheats (Carson and Edwards 2009). Near‐infrared reflectance (NIR) calibrated to PSI is routinely used for rapid determination of grain hardness at mill intake. Automated determination of hardness can also be made on a single grain basis using the Perten Single Kernel Characterization System (SKCS) (Pearson et al. 2007). This measures the weight, moisture content, and crush profiles of individual seeds. The standard sample size is 300 kernels and the software provides data on the distributions of the parameters.

      1.5.3 Water Absorption

      Determining the amount of water absorbed by flour is a crucial test used by bakers to optimise the mixing conditions for baking. Bakers therefore specify the level of water absorption (WA) for the flours they purchase, and millers adjust their milling conditions to achieve this level.

      WA is usually determined using the Brabender Farinograph. This automated mixer measures the resistance of flour and water while they are being mixed to form a dough. The level of maximum resistance can be adjusted to a predetermined optimum by altering the amount of water added, the ideal amount of water being a measure of WA (Kent and Evers 1994).

      1.5.4 Gluten

      In Section 1.4.1 we introduced the unique properties of wheat doughs, pointing out that they have been significant factors behind the spread and adoption of wheat. Those unique properties are, in large part, due to the gluten proteins of wheat flour. There is a conundrum that the very component of wheat flour that contributes to its pre‐eminence as a foodstuff is also the proposed source of several adverse reactions to wheat consumption (Sapone et al. 2012). We therefore provide a brief introduction here to the gluten fraction and its role in processing and health. These properties, impacts, and potential mitigations are discussed in greater detail in Chapters 710.

      1.5.4.1 The Origin and Properties of Gluten

      1.5.4.2 Gluten and Health

      The most widely known and the most well‐characterised adverse


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