Isolation, structural features and rheological properties of water-extractable β-glucans from different Greek barley cultivars

beta-Glucans were isolated from six Greek barley cultivars (Persefoni, Kos, Thessaloniki, Athinaida, Dimitra and Triptolemos) by water extraction at 47degreesC, enzymatic removal of starch and protein and subsequent precipitation of the water-soluble beta-glucans with 37% (w/v) ammonium sulfate saturation. The purity of barley beta-glucans was high (>93% dry basis) with some small contamination by protein (<3.84%). The molecular size of the beta-glucan isolates was determined by high-performance size-exclusion chromatography (HPSEC); the weight-average molecular weights and the intrinsic viscosities ranged between 0.45 x 10(6) and 1.32 x 10(6) and 2.77 and 4.11 dl g(-1), respectively. Structural features of barley beta-glucans were revealed by C-13 NMR spectroscopy and high-performance anion-exchange chromatography (HPAEC) of the oligomers released by the hydrolytic action of lichenase. Lichenase degradation showed that beta-glucans from all barley cultivars consisted of blocks of cellotriosyl and cellotetraosyl units, accounting for 90.6-92.3% of the total oligomers released, with a molar proportion of these units between 2.31 and 2.77. Rheological measurements of aqueous solutions/dispersions of beta-glucans showed the behaviour of non-interacting polysaccharides and a transition from the typical viscoelastic response to gel-like properties after a time period that depended on the molecular size of the polysaccharide. The lowest molecular size beta-glucans from the Triptolemos cultivar showed shorter gelation times than their higher molecular weight counterparts. The effect of sugar incorporation (glucose, fructose, sucrose, xylose and ribose), at a concentration of 30% (w/v), to the beta-glucans gels (6% w/v) on compression parameters seemed to be related to the type of sugar used; the pentose sugars substantially reduced gel firming. (C) 2004 Society of Chemical Industry.