Barley for Food and Health: Science, Technology, and Products

64 BARLEY: GENETICS AND NUTRIENT COMPOSITION
In the endosperm they comprise 20 to 25% of the cell wall material and 86% in
the aleurone. Barley arabinoxylans vary considerably with respect to the ratio of
xylose to arabinose. In hulls, ratios of about 1 : 9 have been observed (Aspinall
and Ferrier 1957), whereas in starchy endosperm or aleurone tissues, ratios of
1 : 3 were reported in a number of studies reviewed by MacGregor and Fincher
(1993). From the literature it appears that the ratios reported are very dependent
on the solvent used to solubilize the molecules. Arabinoxylans found in
barley generally have a (1-4)-β-xylopyranosyl backbone that supports one or
two α-l-arabinofuranosyl residues. Most arabinose is found as monomeric substituents
but with a small proportion of oligomeric sidechains, consisting of two
or more arabinosyl residues or an arabinosyl residue with a terminal xylosyl
residue (Voragen et al. 1992; Viëtor et al. 1993). Galactosyl, glucosyl, and xylosyl
can be present as terminal residues, but are quantitatively minor; however,
glucuronopryanosyl terminal residues may constitute up to 4% by weight of the
arabinoxylan in the hull (Aspinall and Ross 1963).
Estimates of arabinoxylan contents of barley grain range from 4 to 7% by
weight, being concentrated in the outer layers of the caryopsis and the hull
(Hashimoto et al. 1987). It was reported by Henry (1987) that less than 25%
of total barley arabinoxylan is found in the endosperm, constituting about 1.5%
by weight. Henry (1986) concluded that total arabinoxylan content and xylose–
arabinose ratios varied somewhat with genotype but that content varied more,
due to environmental factors. For example, thin or small kernels with low starch
content contain higher amounts of arabinoxylans than large or plump kernels
containing high levels of starch. This is presumed to be because most arabinoxylan
is located in the outer layers of the grain, and small thin grains contain
relatively less endosperm than plump large grains. According to MacGregor and
Fincher (1993), there have been no comprehensive studies on the genetic basis
for arabinoxylan levels in barley, nor is there information on the number of genes
involved or their location in the genome.
Of all the components of TDF in barley, β-glucans are probably the most
important in terms of human diet and health benefits. β-Glucans consist of
high-molecular-weight linear chains of β-glucosyl residues polymerized through
both β-(1,3) and β-(1,4) linkages. Barley β-glucans are members of a family
of polysaccharides that is heterogeneous with respect to molecular size, solubility,
and molecular structure (MacGregor and Fincher 1993). Although it is not
possible to show a single structure for β-glucan, it has been demonstrated that
blocks of two or three contiguous (1,4)-linked β-glucosyl residues are separated
by single β-(1,3) linkages along the polysaccharide chain (Parrish et al. 1960;
Woodward et al. 1983a). The β-glucosyl units are referred to as cellotriosyls
(trisaccharides) and cellotetraosyls (tetrasaccharides). A schematic representation
of barley β-glucan is shown in Figure 4.1.
1,3 : 1,4 - Beta - GLUCAN
FIGURE 4.1
β-Glucan structure.