Cereal grains for the food and beverage industries
Woodhead Publishing Limited, 2013 a large number of aleurone granules. They are relatively high in ash, protein, total phosphor, phytate phosphorus, fat and niacin. In addition, concentrations of thiamine and ribofl avin are higher in the aleurone than in the other parts of the bran. Furthermore, the aleurone layer is particularly rich in enzymes, which play a vital role in the germination process (Anonymous, 2000).
Over the embryo, the aleurone cells are modifi ed, becoming thinwalled cells, and may not contain aleurone granules (Delcour and Hoseney, 2010a). The aleurone cells are also common as a storage reserve for lipid droplets. Most of the aleurone layer is removed as part of the bran during roller milling (Dexter and Wood, 1996). The wheat embryo, also called germ by millers, makes up 2.5–3.5 % of the kernel and lies on the lower dorsal side of the caryopsis.
At grain maturity, it comprises an embryonic axis (shoot or epicotyls, mesocotyl and radical) and a scutellum, which is considered to be homologous with a cotyledon (Khan and Shewry, 2009). The scutellum lies between the embryonic axis and endosperm, and its name derives from its shield-like shape. The germ is relatively high in protein (25 %), sugar (18 %) mainly sucrose and raffi nose, and ash (5 %). It also has the highest concentration of lipids (16 %) and hence lipidsoluble vitamins E of all the components of the wheat kernel, with levels of up to 500 ppm (Delcour and Hoseney, 2010a).
It also has the highest moisture content among constituents of the mature grain (Song et al., 1998), but not all water-soluble vitamins are found in their highest concentrations here (Michael, 2009). Nevertheless, the use of wheat germ is still challenging because of its poor stability and the presence of anti-nutritional factors such as: (i) raffi nose which is not digested by pancreatic enzymes but metabolized by gas-producing bacteria of the large intestine, thus causing disorders such as fl atulence (Rizzello et al., 2010); (ii) phytic acid which markedly decreases the mineral bioavailability (Febles et al., 2002); and (iii) wheat germ agglutinin (WGA) which is responsible for the hyperplastic and hypertrophic growth of the small bowel and pancreas (Matucci et al., 2004).
The starchy endosperm occurs as a solid mass occupying the centre of the kernel and represents the largest morphological component in all cereals, and it is also the component with the greatest value (Evers and Millar, 2002). It is composed of three types of cells that vary in shape, size and location in the kernel (Greer et al., 1951). Peripheral or subaleurone cells are the fi rst row of cells inside the aleurone layer, which they resemble in size (60 μm in diameter).
Next are several rows of prismatic starchy endosperm cells (130–200 μm long, 40–60 μm wide). They extended inward to about the centre of the cheeks. Central cells (2.6 μm thick, 72–144 μm long, 70–120 μm wide) occur in the centre of the cheeks (Bradbury et al., 1956b; Michael, 2009) and they are more irregular in shape and size than the other cells. The endosperm cell walls comprise about 75 % polysaccharide, the latter comprising about 70 % arabinoxylans, 20 % (1–3)(1–4)β-dglucans, 7 % β-glucomannan and 2 % cellulose (Bacic and Stone, 1980). Proteins are also presents at a level of about 15 %