Wheat. Peter R. Shewry

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Wheat - Peter R. Shewry


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consumption is coeliac disease (CD). CD has been recognised since ancient Greek times; the first modern description was given by the British paediatrician Samuel Gee in 1887. Gee also adopted the classical Greek name from koiliakós, meaning abdominal. However, the link to wheat was not made until the 1940s by Willem Karel Dicke, a Dutch scientist. The link with gluten was established by 1952. CD is an autoimmune response which results in damage to the small intestine. This leads to several symptoms, notably malabsorption of nutrients and diarrhoea. It is triggered in genetically susceptible individuals by the ingestion of wheat gluten or related proteins from barley or rye. The aetiology of CD is well understood and there is no cure except avoidance of the proteins responsible for triggering the response. It is estimated to affect about 1% of the global population but may exceed this in some countries. Small proportions of coeliac patients may also suffer from dermatitis herpatiformis or neurological symptoms (including ataxia).

      True allergy to the ingestion of wheat (or gluten proteins) is relatively rare. However, there is greater concern about a loosely defined group of symptoms known as non‐coeliac gluten sensitivity or non‐coeliac wheat sensitivity, which has been reported to affect between 0.5 and 10% of the population. This condition is still poorly understood but current work suggests that the prevalence may be higher than that of CD, with proteins other than gluten responsible for triggering the response. These conditions are discussed in detail in Chapter 9 (see also Brouns et al. 2019).

      There is no doubt that concerns about adverse effects on health, propagated particularly in the social and popular media, have affected the consumption of wheat in some counties. However, they must be considered in perspective and not allowed to overshadow the health benefits from wheat consumption or the degree of global food security based on increasing wheat production (Lillywhite and Sarrouy 2014; Peña et al. 2017).

      1.5.4.3 Dough Properties that Determine Processing Quality

Graph depicts the relationships between grain protein content, dough strength, grain texture, and the quality of bread wheat for various products.

      Source: Adapted from Peña (2002), Uthayakumaran and Wrigley (2017), and Moss (1973).

      The cooking quality of pasta made from durum wheat is mainly determined by the ability to absorb water while retaining firmness and shape and without becoming sticky (Sarrafi et al. 1989). Both the processing and cooking of pasta are, therefore, dependent on strong gluten. Tenacious doughs have high initial resistance to extension but break after only a relatively small distance; they are often described as short and their suitability is limited to some domestic uses (Guzman et al. 2016). The role of gluten proteins in determining dough rheology and end‐use functionality has led to the importance of grain protein concentration and protein quality in the marketing and classification of wheat (e.g. CGC 2020).

      1.5.4.4 Importance of Total Protein Concentration

Photo depicts gluten produced by hand washing, stretched to show the cohesive properties.

      Source: Taken from Shewry et al. (1995).

      Grain protein concentration is not usually determined directly but calculated based on the N concentration determined by chemical analysis (Kjeldahl wet chemistry or Dumas oxidative combustion) or by near‐infrared spectroscopy (NIRS) calibrated based on N analysis (Carson and Edwards 2009). It is often assumed that there is a constant relationship between the amount of N and the amount of protein in biological samples and that the crude protein concentration can be calculated by multiplying the N concentration by a constant factor, with N × 6.25 being most widely used. However, this is not the case because it wrongly assumes that all proteins have a similar N content. Gluten proteins, for example, have higher N contents than most other proteins due to the presence of between 30 and 50% of glutamine, an amino acid which contains two N atoms (as opposed to one in most other amino acids). Consequently, N × 5.7 is widely accepted as a conversion factor for wheat grain and flour (Draper and Stewart 1980). However, this factor is still imprecise as it will vary with the proportion of gluten proteins in the sample. Hence, it will be lower for high protein grain. Similarly, it will be lower for white flour than for wholemeal (with N × 5.83 having been suggested [Kent and Evers 1994]) and vary between mill streams. Nevertheless, it is clearly impractical to use a range of values and a conversion factor of N × 5.7 is almost universally used to calculate protein concentration for marketing and utilization of wheat grain.

      1.5.4.5 Importance of Protein Quality

      In addition to protein concentration, the relative proportions and subunit compositions of the gliadin and glutenin fractions also have significant effects on dough rheology and functionality for different end uses. In broad terms, gliadins contribute to the viscosity and extensibility and glutenins to the elasticity and


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