Evaluation of structure in the formation of gels by structurally diverse (1→3)(1→4)-β-D-glucans from four cereal and one lichen species

The (1 --> 3)(1 --> 4)-beta-D-glucans from four cereal sources (oats, wheat, barley and rye) and one lichen source (Icelandic moss) were used to test two proposed structurally based hypotheses about the gelling mechanism of these polymers. Structures were evaluated using high performance anion exchange chromatography of the oligosaccharide fragments released by a (1 --> 3)(1 --> 4)-beta-D-glucan-4-glucanohydrolase. This determined the relative amounts of cellodextrin units, of different degrees of polymerisation, which are joined by beta-(1 --> 3) linkages in the intact polysaccharide chain. Oat beta-glucan had the lowest beta-(1 --> 3)-linked cellotriosyl unit content and lichenan had the highest. Strong correlations were found between the fraction of beta-(1 --> 3)-linked cellotriosyl units in the beta-glucans and the elasticity of 6% gels in water, as measured by dynamic rheometry. Differential scanning calorimetry showed that the beta-(1 --> 3)-linked cellotriosyl unit content was also correlated with the onset and peak temperatures when 6% beta-glucan gels were melted. No correlation was found between the longer (DP 6-9) beta-(1 --> 3)-linked cellodextrin oligosaccharide content and either the gel elasticity or melting characteristics. These findings are consistent with a model in which runs of consecutive beta-(1 --> 3)-linked cellotriosyl units form the junction zones in the gel network, but not with a model in which longer beta-(1 --> 3)-linked cellodextrins associate, as in cellulose fibres, to produce the gel network. Microscopic images of the beta-glucan gels from the five species revealed that the microstructure was not homogeneous in any of the samples, which may be related to the variability in the enthalpy of melting of gels. There was a coarsening of gel structure as the beta-(1 --> 3)-linked cellotriosyl unit content increased. (C) 2004 Elsevier Ltd. All rights reserved.