D. B. Bechtel (Retired), Bechtel Consulting, Manhattan, Kansas, U.S.A.; J. Abecassis, L'Institut National de la Recherche Agronomique—Unité Mixte de Recherche, Montpellier, France; P. R. Shewry, Centre for Crop Genetic Improvement, Rothamsted Research, Harpenden, Hertfordshire, U.K.; A. D. Evers (Retired), Ascus Ltd., Markyate, Hertfordshire, U.K.
WHEAT: Chemistry and Technology, Fourth Edition
Pages 51-95
DOI: https://doi.org/10.1094/9781891127557.003
ISBN: 978-1-891127-55-7
Abstract
The description that follows is based largely on studies of common or bread wheat (Triticum aestivum L.). This species, on which by far the most research has been conducted, shares many morphological characteristics with other wheat species that appear in commerce. This chapter therefore may be considered a useful guide but not a definitive description of species other than T. aestivum.
The structure of the wheat grain is important for all aspects of utilization, as it determines the grain's behavior during processing. During milling, the grain is mechanically separated into various components on the basis of how it is composed structurally. The miller must be able to clearly separate the outer layers of the grain and the embryo from the starchy endosperm to produce a high yield of white flour. Similarly, variation in grain hardness (which is a structural as well as a biochemical feature) affects the flour yield, amount of starch damage, and energy requirements, as well as many other factors. Other structural features, such as the extent of endosperm cell degeneration in the depleted layers adjacent to the embryo, have a profound effect on how well and efficiently the embryo is removed during milling.
Many structural elements that are established or altered during grain development and harvesting affect end-use properties. For example, a tightly adherent hull (husk) may lead to damage to the embryo and the outer portion of the caryopsis during threshing. These changes could lead to reduced germination, impaired storability, and increased susceptibility to insects and fungi. The production of excess cell wall material in the outer portions of the grain would add nonnutritive mass, whereas increases of cell wall material in the starchy endosperm could be a means of adding dietary fiber to white flour in vivo. The distribution of the components in the grain determines whether they are present in white flour produced by milling. For example, storage protein is typically much more concentrated in the subaleurone region than in the rest of the starchy endosperm. During milling, this layer sorts with the bran if it remains tightly bound to the aleurone cells.
These are just a few examples of how the structural features of the wheat caryopsis affect how it is utilized. This chapter reviews the development of the wheat grain as well as the structure of the mature grain. We have regarded the compiling of this chapter as a revision of the equivalent chapters in earlier editions of this monograph (Hlynka 1964; Pomeranz 1971, 1988). The authors of the earlier chapters called upon their own practical experiences and their comprehensive and painstaking study of the literature, to which they gave generous credit. We have not, in all cases, returned to the original sources but have cited the conclusions of our predecessors rather than the evidence by which the conclusions were deduced. For a more detailed discussion of these topics, the first and second editions are recommended (Hlynka 1964, Pomeranz 1971). Since those earlier editions, important contributions have been made from various microscopic techniques, and we have attempted to include relevant contributions from these innovative areas. Molecular and genetic aspects have been included in this revision to reflect the advances and opportunities that have arisen in recent years, while the inclusion of mechanical aspects reflects the progress also made in relating grain shape and structure to milling properties.
