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
As discussed, the gluten network is largely responsible for the unique biophysical properties of dough, a balance between extensibility and elasticity. Highly viscous doughs are readily extended when stretched whereas elastic doughs resist extension and exhibit elastic recoil when the stress is relaxed (Chapter 8). Highly elastic doughs are referred to as strong and are preferred for making leavened bread (Figure 1.22). This is because the gas released during fermentation (proofing) is captured in bubbles, allowing the dough to rise. The protein network is then denatured by baking, giving a light porous structure to the bread. By contrast, biscuit production requires weak but extensible doughs that can be spread out into an even layer without recoil after rolling. Intermediate strength doughs can be used to produce steamed bread, chapatis, and noodles (Figure 1.22), but puffed breakfast cereals require strong wheat to prevent disintegration when pressure is released after cooking. Flaked and shredded products are made from wheats with weak gluten similar to those used for biscuits (Blackman and Payne 1987; Lin et al. 1990).
Figure 1.22 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
Gluten proteins account for up to 80% of the total grain protein, and their proportion becomes larger with increasing protein concentration. The quantity of wet gluten derived from a flour is, therefore, highly correlated with grain protein concentration. Grain protein concentration is partly determined by genetic factors, with cultivars bred for breadmaking often containing about two percentage points more protein than cultivars bred for livestock feed when grown under the same conditions (Snape et al. 1993). However, there are also strong environmental effects, with protein content ranging between about 6 and 20% due to variation in, for example, temperature, and availabilities of water, N, and S for grain filling (Carson and Edwards 2009; Gooding 2017).
Figure 1.23 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.
Although protein concentration is not the same as protein quality, a minimum protein concentration is required for breadmaking and increasing protein concentration can, to some extent, compensate for lower quality. Hence, higher protein concentrations are required for pasta and breadmaking than for other uses (Figure 1.22). The minimum requirement for the CBP is around 13% DM. Products requiring a weak, extensible dough, such as biscuits, cakes, and pastries, usually require a protein content of less than 10.5% DM and levels down to 9% can be tolerated for some products (Carson and Edwards 2009). Low protein concentrations, and therefore high starch contents, are associated with higher alcohol yields in distilling and bioethanol industries (Taylor et al. 1993).
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