ARABINOXYLANS AS CONSTITUENTS OF AGRICULTURAL CROPS

7.2 ARABINOXYLANS AS CONSTITUENTS OF AGRICULTURAL CROPS

Arabinoxylans have been identified in all major cereal grains, including wheat, barley, oats, rye, rice, sorghum, maize, and millet, 7 as well as in other plants, such

as psyllium, 8 pangola grass, bamboo shoots, and rye grass. Recent studies on the water-soluble mucilage extracted from flaxseeds have demonstrated that heteroge-

neous, high molecular weight arabinoxylans are a major component of the extracted polysaccharides. 5 Arabinoxylans have also been isolated from corn husk and banana peels. 6 In cereal grains, arabinoxylans are localized mainly in the cell walls of starchy endosperm and the aleurone layer, in the bran tissues, and in the husk of some cereals. However, depending on the genus and species, the amount of arabinoxylans in a particular tissue may vary (Table 7.1). For example, the cell walls of wheat and rye starchy endosperm and the aleurone layer are built up mainly of arabinoxylans (~60 to 70%), yet the total arabinoxylan content in bran of these grains is higher

than that of endosperm. 9 In barley, the aleurone cell walls are built up mainly of arabinoxylans (60 to 70%), whereas the starchy endosperm cell walls contain only about 20 to 40% of these polymers and a much greater amount of β-glucans. The fine molecular structure of arabinoxylans may also vary depending on the specific tissue from which these polymers are derived. The outer layers of cereal grains (husk and bran) appear to contain acidic arabinoxylans (glucuronoarabinoxylans) contain- ing glucuronic acid in addition to arabinose and xylose residues.

The level of arabinoxylans in cereals depends on genetic and environmental factors. Among the cereal grains, rye has the highest content of arabinoxylans, followed by wheat and barley (Table 7.1). It has been shown that translocation of wheat, with the short arm of the 1B chromosome of wheat replaced by the short

Arabinoxylans

TABLE 7.1 Content of Total and Water-Soluble Arabinoxylans in Various Grains and Grain Tissues

Barley Whole grain

6.11 0.35 Hashimoto et al. 147 Whole grain

Izydorczyk et al. 148 Whole grain

Oscarsson et al. 82 Pearled grain

4.45 0.27 Hashimoto et al. 147 Pearlings

14.14 0.54 Hashimoto et al. 147 Pearled flour

Dervilly-Pinel et al. 55 Wheat Whole grain

5.77 0.59 Hashimoto et al. 149 Whole grain

Saulnier et al. 46 Bran

19.38 0.88 Hashimoto et al. 149 Flour

Izydorczyk et al. 23 Durum wheat

Lempereur et al. 13 Rye Whole grain

4.07-6.02

Bengtsson and Åman 79 Whole grain

Hansen et al. 15 Bran

1.7 Figueroa-Espinoza et al. 31 Flour

Cyran et al. 80 Oats Whole grain

2.73 0.17 Hashimoto et al. 147 Hulls

8.79 0.40 Hashimoto et al. 147 Bran

3.50 0.33 Hashimoto et al. 147 Pearled grain

3.00 0.15 Hashimoto et al. 147 Rice Whole grain

2.64 0.06 Hashimoto et al. 147 Hulls

Hashimoto et al. 147 Bran

Hashimoto et al. 147 Sorghum Whole grain

1.8 0.08 Hashimoto et al. 147 Pearlings

5.4 0.35 Hashimoto et al. 147 Corn Bran

29.86 0.28 Hashimoto et al. 147 Soybean

Hulls 13.10 Hashimoto et al. 147

arm of the 1R chromosome of rye (1B/1R gene), increases the content of water- soluble arabinoxylans, but does not affect the amount of total arabinoxylans in

comparison with standard wheat. 10 Other studies have proposed that the content of water-extractable arabinoxylans in rye is controlled by many factors scattered throughout the genome, and while chromosomes 2R, 5R, and 6R are responsible

Functional Food Carbohydrates

for increased arabinoxylan content, chromosome 3R is responsible for its reduced level. 11,12 Significant genetic and environmental variations in arabinoxylan content

have been reported for durum wheat and barley. 13,14 Hansen et al. 15 examined the effect of harvest year on the content and composition of dietary fiber in seven rye

varieties grown in Denmark and found that yearly variations in the content of total and water-extractable arabinoxylans were higher (27 to 55% of total variance) than those associated with genotype effects (14 to 19% of total variance). Yearly differ- ences in the content of total arabinoxylans were also found among five rye varieties

grown in Finland. 16 It was also found that a cold and wet season resulted in small rye kernels with a high arabinoxylan content. Coles et al. 17 conducted drought studies using a mobile rain shelter and found a positive relationship between the amount of arabinoxylan accumulated in wheat and the dry conditions. Also, the exposure of plants to UV light increased the degree of cross-linking in arabinoxylans, but the effect on the total content of these polymers was not clearly established. Wakabayashi

et al. 18 investigated the changes in the amount and composition of arabinoxylans in cell walls of wheat coleoptiles grown under continuous hypergravity conditions. It was found that the amount of arabinoxylans per unit length of coleoptiles increased under hypergravity conditions. In addition, the amount of the acidic arabinoxylans (glucuronoarabinoxylans) and ferulic acid–cross-linked arabinoxylans increased as

a result of continuous hypergravity.