A structural comparison of Gelling and non-Gelling β-Glucans in barley

Abstract

β-glucans are long chain glucose polymers, with a mixture of β-1,3 and β-1,4 linkages. They are of highest concentration in both aleurone and endosperm cell walls, where they can represent 26% and 75% of the wall by weight. β-glucans are therefore thought to be involved in the strengthening of cell walls, but are also implicated as an intermediate energy reserve between glucose/sucrose and starch. This reserve has been interpreted to act as a buffer in both starch synthesis during grain maturity and starch degradation during germination. β-glucans have implications in all three of the main uses for barley: as a human food, as an animal feed, and in malting/brewing. The latter two industries generally consider β-glucans to be a negative factor, as their high viscosity can cause reduced feed and processing efficiency. Interestingly, reduced feed efficiency is of prime benefit to humans, and consumption of β-glucans have been shown to beneficially reduce blood lipid levels, lower postprandial plasma glucose, positively affect colonic bacterial populations, and may reduce colon cancer. Therefore, the extraction and incorporation of this "functional food" into the human diet is considered very beneficial. Unfortunately, processing and extraction methods can significantly affect the structure of β-glucans, resulting in changes to viscosity, solubility and health benefits. This dissertation analyses the differences in fine structure of gelling and non gelling β-glucan extractions, and relates this to the methods used to extract them. To do this, a highly specific enzyme (licheninase) was used to partially digest β-glucan extracts. Licheninase only hydrolyses specific β-1,3 bonds, so the size of the resulting oligo saccharides gives a good indication of the structure of the initial β-glucan polysaccharide. Both the initial polysaccharides and the digested oligo saccharides were analysed via HPLC. Polysaccharide analysis found that the average degree of polymerisation (DP) of each of the main samples was (in descending order): pure commercial standard (DP400), purified conventional extract (DP300), Glucagel™ (DP 150-180), Glucafilm (DP100) and the purified bulk remainder from Glucagel extraction (DP70). Oligosaccharide analysis concluded that regardless of the gelling properties and method of extraction, the fine structures (i.e. proportions of DP3, DP4, DP5 and DP6 domains) of all samples were essentially the same. There were indications that DP1, DP2 and possibly DP5 were α-linked oligosaccharides. As the fine structure (up to DP6) seemed to be uniform, the solubility and gelling properties of a β-glucan were attributed to polymer length and/or the accessibility of linear β-(1,4) domains.