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

GENETICS AND NUTRIENT COMPOSITION 65
Unlike cellulose, β-glucans are partially soluble in aqueous solutions, a characteristic
that is attributed to the molecular structural differences of the two polysaccharides.
The presence of the interspersed β-(1,3) bonding creates a molecule with
quite different spatial arrangements compared to total β-(1,4) bonding, which no
doubt relates to the difference in solubility between the two polysaccharides.
The irregularly spaced β-(1,3)-linkages interrupt the relatively rigid ribbonlike
β-(1,4)-glucan conformation as in cellulose and confer flexibility and irregular
shape on the barley β-(1,3)(1,4)-glucan, consistent with its water solubility
(Woodward et al. 1983b). The solubility of barley β-glucans was first recognized
and utilized by the brewing industry in identifying desirable cultivars for
malting. Barley β-glucans are regarded as undesirable components in the malting
and brewing processes for three reasons: (1) high levels of β-glucans in malt
are associated with lower malt extract values; (2) β-glucans produce viscous
extracts, causing difficulty in filtration; and (3) undegraded β-glucans can create
haze or precipitates in beverages (Woodward and Fincher 1983; Bamforth 1985).
The viscosity of β-glucan extracts can be used under laboratory conditions to
indicate the level of β-glucans in barley. Thus, this characteristic was utilized
in early malt barley breeding programs to estimate the level of β-glucans in
barleys (Greenberg and Whitmore 1974; Bendelow 1975; Morgan 1977). Under
laboratory conditions, the extract viscosity is altered not only by the β-glucan
level but also by the experimental conditions (i.e., solvent pH, temperature, and
concentration) and by the instruments used in recording the viscosity measurements.
Viscosities of pure aqueous extracts are reduced rapidly due to native
β-glucanase activity and are therefore not generally used. Most often, acid or
alkaline extracts that deactivate the enzyme system are used to avoid this confounding
factor. Regardless of method utilized, genotypic differences in extract
viscosity correspond to differences in β-glucan content, with the exception of one
or two cultivars (Fastnaught 2001). The viscosity of soluble barley β-glucans
has negative implications for the malting and brewing industries and in some
instances food production, but is considered an advantage in dietary and health
applications (Bhatty 1992).
The β-glucan level in barley is under genetic control, although the concentration
is often modified by the environment, especially hot, dry conditions during
kernel maturation. Such conditions almost always produce increased levels of
β-glucans (Bendelow 1975; Anderson et al. 1978). It was suggested that the
increase in β-glucans may be caused by either a decrease in grain filling due
to impaired starch and/or protein synthesis or because β-glucan synthesis is
enhanced in such an environment. On the other hand, under moist conditions,
such as rain during ripening, kernel β-glucan levels are decreased (Bendelow
1975; Aastrup 1979). Regardless of the fact that barley β-glucan levels are
significantly influenced by environmental factors, there is general agreement
among researchers that genetic background is the most important factor of final
β-glucan content of the barley kernel (MacGregor and Fincher 1993). However,
the genetics of barley kernel β-glucan has proven to be complex and is not completely
understood (Ullrich 2002). Some specific genetic controls of β-glucan