Wheat. Peter R. Shewry

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СКАЧАТЬ for breadmaking, protein quality is one of the primary concerns. However, several other factors are of crucial importance in determining whether grain is acceptable.

      1.5.5.1 Alpha‐Amylase Activity

      Alpha‐amylase is a naturally occurring enzyme that breaks down starch to release sugars during seed germination. Some α‐amylase is beneficial for breadmaking because the sugars released are required for fermentation. In fact, commercially produced enzyme may be added as an improver in commercial breadmaking. However, excessive activity results in a dark crust (due to Maillard reactions) and a sticky crumb with poor resilience and texture, the latter preventing the use of mechanical loaf slicing (Chamberlain et al. 1982).

      Excessive α‐amylase can result from several syndromes, three of which come into play early in grain development (Lunn et al. 2001). These are the retention of high levels of α‐amylase in the pericarp (retained pericarp α‐amylase activity, RPAA), the deposition of specific α‐amylase isoforms in the endosperm cavity during grain development (pre‐maturity α‐amylase activity, PMAA, or late maturity α‐amylase, LMA) and, very rarely, the germination of the grain early in development (pre‐maturity sprouting, PrMS). However, by far the most common syndrome is germination (sprouting) of the mature grain before harvest, i.e. pre‐harvest sprouting (). This occurs when the crop is subject to wet conditions just prior to harvest, and when the grain has low levels of dormancy.

Levels of α‐amylase are assessed using the Hagberg Falling Number (HFN) test. This was initially developed to monitor the levels of α‐amylase in wheat growing in the field in Sweden to decide whether the crop was at risk of spoilage. It is now widely used by the grain industry as a measure of α‐amylase activity in harvested grain. HFN is a measure of the viscosity of a mixture of water and milled wheat mixed in a tube and placed in a water bath at 100 °C. Alpha‐amylase breaks down the starch and, therefore, reduces the viscosity of the water: wheat mixture. The falling number is the time in seconds required for stirring (60 seconds) plus the time taken for a stirrer to fall through the flour suspension while it is being liquefied by the enzyme (Vaidyanathan 1987).

      1.5.5.2 Seed Coat Colour

      Wheats can be described as red or white based on the intensity of red pigmentation in the seed coat (Lachman et al. 2017). Seed coat colour is largely determined by homologous genes at the R‐1 loci on the long arms of the group 3 chromosomes (i.e. R‐A1, R‐B1, and R‐D1). Red cultivars carry one or more of the red (dominant) alleles and the intensity of pigmentation increases as gene dosage increases to three (Flintham 2000). Most American and European wheats are red. This is because the red seed coat is associated with greater resistance to PHS (Mares et al. 2009; Ji et al. 2018). By contrast, white wheats are more suited to areas that are dry during ripening and harvest and are favoured for the manufacture of certain types of flat bread, steamed breads, and noodles. White wheats can also be used to produce a higher extraction rate of flour during milling without products becoming discoloured with bran flecks (Hatcher et al. 2006).

1.6 Further Chapters

      In this chapter, we have discussed the importance of wheat and briefly explained some of the reasons for that importance in terms of the diversity of wheat adaptation and of wheat use. In the next four chapters we will explore the environmental requirements and sustainability of wheat production. The chapter headings reflect what has always been known by wheat farmers: that for high yields you need a good soil, ample water, mild temperatures, and sunshine. Chapter 6 (Canopy management) evaluates farming practices associated with increasing land‐use efficiency. In subsequent chapters we return to the wheat grain and its use, with respect to grain development and composition (Chapter 7); grain processing and flour functionality (Chapter 8); and the role of wheat in diet and health (Chapter 9). We conclude with a chapter on modern breeding methods and targets for further sustaining wheat production and its use for food security.

References

      1 AHDB (2011). HGCA Grain Storage Guide for Cereals and Oilseeds, 3e. Agriculture and Horticulture Development Board.

      2 Allaby, R.G., Stevens, C., Lucas, L. et al. (2017). Geographic mosaics and changing rates of cereal domestication. Philosophical Transactions of the Royal Society B: Biological Sciences 372: 20160429. https://doi.org/10.1098/rstb.2016.0429.

      3 Allen, A.M., Wilkinson, P., Burridge, A. et al. (2021). The role of gene flow and chromosomal instability in shaping the bread wheat genome. Nature Plants 7: 172–183. https://doi.org/10.1038/s41477‐020‐00845‐2.

      4 Anderson, K., Ivanic, M., and Martin, W. (2013). Food price spikes, price insulation, and poverty. In: The Economics of Food Price Volatility (eds. J.‐P. Chavas, J. Hummels and B.D. Wright), 311–339. National Bureau of Economic Research https://www.nber.org/books‐and‐chapters/economics‐food‐price‐volatility/food‐price‐spikes‐price‐insulation‐and‐poverty.

      5 Arranz‐Otaegui, A., Carretero, L.G., Ramsey, M.N. et al. (2018). Archaebotanical evidence reveals the origins of bread 14400 years ago in northeastern Jordan. Proceedings of the National Academy of Science, USA 115: 7925–7930. https://doi.org/10.1073/pnas.1801071115.

      6 Austin, R.B., Morgan, C.L., Ford, M.A. et al. (1982). Flag leaf photosynthesis of Triticum aestivum and related diploid and tetraploid species. Annals of Botany 49 (2): 177–189. https://doi.org/10.1093/oxfordjournals.aob.a086238.

      7 Austin, R.B., Ford, M.A., and Morgan, C.L. (1989). Genetic improvement in the yield of winter‐wheat – a further evaluation. Journal of Agricultural Science 112: 295–301. https://doi.org/10.1017/S0021859600085749.

      8 Bakker, J.T. (1999). The mills‐bakeries of Ostia: description and interpretation. Dutch Monographs on Ancient History and Archaeology 21: 131.

      9 Barber, H.M., Carney, J., Alghabari, F. et al. (2015). Decimal growth stages for precision wheat production in changing environments? Annals of Applied Biology 166 (3): 355–371. https://doi.org/10.1111/aab.12207.

      10 Bates, B., Lennox, A., Prentice, A. et al. (Eds.) (2014a). National Diet and Nutrition Survey: Results from Years 1–4 (combined) of the Rolling Programme (2008/2009–2011/2012). Executive Summary. Public Health England. https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/594360/NDNS_Y1_to_4_UK_report_executive_summary_revised_February_2017.pdf

      11 Bates, B., Lennox, A., Prentice, A. et al. (Eds.) (2014b). National Diet and Nutrition Survey: Results from Years 1‐4 (combined) of the Rolling Programme (2008/2009–2011/2012). Public Health England. https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/216484/dh_128550.pdf

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