Wheat. Peter R. Shewry

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СКАЧАТЬ processes (Carson and Edwards 2009). The large amounts of water absorbed by hard flours also contribute to the longer shelf life of bread (shelf life being determined by drying and starch retrogradation). Hence, hard wheats are preferred for breadmaking (Figure 1.21). By contrast, flours from soft wheats absorb little water and are preferred for the manufacture of most types of biscuit (cookies) where the low water content reduces the required baking time, cost, and the risk of cracking during cooling after baking. Soft wheats with low WA are also favoured for making cakes and pastries (Figure 1.21).

      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 7–10.

      1.5.4.1 The Origin and Properties of Gluten

Gluten is a continuous proteinaceous network that is formed when wheat flour is mixed with water to form dough. It can be prepared at home by gently washing a ball of dough under running tap water or with dilute salt solution. This removes most of the soluble components, including the albumin and globulin proteins, plus the starch and other particulate matter, leaving a cohesive mass that can be extended but also shows elastic recoil (Figure 1.23). Gluten consists predominantly (75–80%) of proteins, most of which can be classified into two related groups called gliadins and glutenins. These two groups form the major storage protein fraction of the grain. However, they differ in that whereas the gliadins are present as monomers, the glutenins form high molecular mass polymers stabilised by inter‐chain disulphide bonds. Both groups can also be defined as prolamins in that the individual subunits (but not necessarily the polymers) are insoluble in water but soluble in alcohol‐water mixtures.

As storage proteins, the biological role of the gliadins and glutenins is to provide nitrogen (N), sulphur (S), and carbon skeletons to support seed germination and seedling growth. It is therefore serendipitous that the interactions of gliadins and glutenins with each other and with other components in wheat doughs have such dramatic effects on wheat utilization. This is because they largely define the rheological properties of doughs, including the balance between dough viscosity and elasticity that determines the quality for different end uses.

      1.5.4.2 Gluten and Health

      The most widely known and the most well‐characterised adverse reaction to wheat 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 СКАЧАТЬ