Wheat. Peter R. Shewry

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      1.3.5 Increases in Harvest Index

In addition to the characteristics of the classic domestication syndrome, modern cultivated wheats now differ in many ways from their wild progenitors. The increase in harvest index for grain has been of great significance, i.e. the increase in the ratio of the grain mass to the total crop biomass (but often neglecting the roots and fallen leaves) (Donald 1962). Austin et al. (1982) reported harvest indices of only 0.15, 0.05, and 0.16 for T. uratu, Ae. speltoides, and T. tauschii, respectively, while the harvest index of tetraploid wild emmer was a little higher at 0.28. Even at this level, however, it would be necessary to produce more than 3.5 t of crop biomass to yield each tonne of grain. Although farmers have long recognized that shorter wheats have higher harvest indices (Roberts 1847; Garnett 1883), cultivars released before 1900 still had indices below 0.35 (Austin et al. 1989). During the twentieth century, significant increases in harvest index were achieved, particularly through breeding programmes that incorporated dwarfing alleles from Japanese wheats, reducing crop heights by 10–20% (Borojevic and Borojevic 2005). By the mid‐1980s, the harvest index of UK wheats approached 0.55 (Austin et al. 1989), i.e. at harvest most of the crop biomass in the field was present in the grain. This progression was seen in wheats throughout the world and across ploidy levels (Evans 1993). The considerable implications of the adoption of high harvest index, shorter wheats on wheat agronomy, quality, and sustainability are covered in Chapter 6.

1.4 Wheat as Food

      Although wheat still accounts for half of the total intake of calories in some countries, the contribution to nutrition in Europe in historical times was even greater. For example, the cost of bread was estimated to account for between a third and two thirds of the total budgets of working families (as opposed to landowners and gentry) in the UK in the period between 1760 and 1836, with the highest estimate being almost 90% for one labourer's family (Peterson 1995). Apparently, therefore, the coarse bread that was consumed at the time was able to meet most of the nutritional requirements of adults engaged in physical work.

      The wider contributions of wheat and bread to health are often ignored, with both being widely regarded as little more than a source of energy. In fact, even in modern Europe wheat, and particularly bread, still provide surprisingly high contributions of a range of essential nutrients. For example, although bread only contributes 10–13% (depending on age and gender) of the daily intake of energy in the UK, it also contributes 10–12% of the daily intake of protein and between 10 and 20% of the daily intakes of minerals (iron, zinc, copper, magnesium, selenium, and calcium) and B vitamins (B1 thiamine, B3 niacin, B9 folate). More importantly, it is also a major source of dietary fibre, contributing about 20% of the daily intake (Bates et al. 2014a, b). These contributions are higher in Poland, with bread providing 21.9% of energy, 16.5% of protein, 35.4% of fibre, 24.9% of iron, and 20.7% of folate (vitamin B9) (Laskowski et al. 2019). The wide consumption and low price of bread therefore make it an excellent vehicle for delivering improved nutrition to large populations at low cost.

      The nutritional and health impacts of wheat are discussed in detail in Chapter 9, with the following sections focusing on more practical aspects of the processing of wheat for food, particularly bread.

      1.4.1 The Development of Milling and Baking

The remains of grains of wild cereals and goat grasses from c. 50 000 to 60 000 BP have been found in cave dwellings at Kebrara in modern‐day Israel (Lev et al. 2005), while remains of c. 23 000 years old wild emmer have been identified from the Ohalo II hunter‐gatherer camp on the shores of the Sea of Galilee (Weiss et al. 2004). The seed collections from both sites are diverse. Kebrara is dominated by the remains of legume seeds; at Ohalo II, the emmer is a rarity amongst greater amounts of wild barley ( Hordeum spontaneum ) and an extensive range of smaller seeded grasses. Nonetheless, there is good evidence that grains of wild wheats were eaten at least 13 000 years before domestication, and a much earlier date is probable.

      Raw wheat seeds are tough, unpalatable, and relatively indigestible unless milled or crushed. At Ohalo II, there is evidence that seeds were crushed between grindstones, long before the advent of agriculture. Crushing would have exposed the starchy endosperm and dramatically increased the bioavailability of nutrients, particularly that of carbohydrate. It would also have speeded the rate with which blood sugar was increased, i.e. the glycaemic index (Rubel 2011).

The ancient process of milling involved manually grinding the grain between stones; a lower, usually concave, static quern stone (called a saddle quern) and an upper hemispherical or cylindrical (like a rolling pin) handstone pushed over the grain. A fine example from Norway is shown in Figure 1.16a.

      Of particular importance for the adoption of wheat as a food crop were the properties of the dough formed by mixing the flour released from crushed or milled grain with water. Wheat doughs have unique visco‐elastic properties that confer significant functional and organoleptic qualities on wheat products, particularly bread and other baked goods. Baking dates back at least 14 000 years (Henry et al. 2011, 2014; Arranz‐Otaegui et al. 2018), and possibly much earlier (Rubel 2011). In particular, doughs made from wheat, far more than from other cereals, are able to trap carbon dioxide released during fermentation by yeast, and hence can produce baked foods with low density. This is the basis of the diverse forms of leavened bread. Yeast spores may occur naturally on the surface of cereal grains and some fermentation occurs readily in wheat dough left to rest. However, it is probable that early bakers used sourdough systems (mixtures of lactobacilli and yeasts), with starters being carried over from batch to batch as in artisan bakeries today. Bread production was clearly important in the Uruk culture (6000–5100 BP) of lower Mesopotamia, and cuneiform writings from around 4000 BP include Sumerian poems and myths about the invention of bread, along with recipes. The Egyptians appear to have perfected leavened bread production by 4000 BP.

Figure 1.16 Stone querns on display at the Archaeological Museum, University of Stavanger, Norway. a, neolithic (possibly bronze age) saddle quern; b, pair of rotary quern stones (46–47 cm diameter) from about 550 CE.

      Source: Photographs kindly provided by Per Storemyr (Per Storemyr Archaeology and Conservation).

From around 2500 BP the laborious use of saddle querns had been replaced by rotary querns. These comprised two circular stones with the upper being rotated by hand and the grain fed through a hole in the upper stone and the flour emerging from the edge (Figure 1.16b). Further refinement of this basic system led to mechanized stone milling. In Roman times, grain was ground as it passed down between a lower meta stone and the upper, rotated, catillus (Figure 1.17). The Romans prized wheat as the empire's principal staple food. In Rome, at any one time, up to 320 000 of the poorer population were supported by free wheat grain and bread (Rickman 1980).

By the 1800s CE stone milling had evolved into a highly sophisticated process, with the mill stones being carved (dressed) to improve efficiency. Dressing divides the surface of the stone into flat areas, called lands, which are separated by furrows. Further small feathering grooves are then carved from the furrows onto the lands. These provide cutting edges, СКАЧАТЬ