Solution flow behavior and gelling properties of water-soluble barley (1 -> 3,1 -> 4)-beta-glucans varying in molecular size

Purified water-soluble barley (1 --> 3,1 --> 4)-beta-glucans were isolated from a composite whole barley flour and partially hydrolyzed with acid to obtain fractions of a wide range of molecular weights (70-250 x 10(3) Da). Another barley preparation of different origin was also purified, exhibiting the lowest molecular weight of 40 x 10(3). All samples were analyzed by C-13 NMR spectroscopy and the profile of oligosaccharides released by (1 --> 3,1 --> 4)-beta-glucanase treatment, as assayed by high performance anion exchange chromatography, revealed the typical fine structure of (1 --> 3,1 --> 4)-beta-D-glucans. Following enzyme digestion, tri- and tetrasaccharides were the main products (91.1-95.5% of the total oligomers), while the DP3/DP4 molar ratios varied between 2.80 and 3.49. Limiting viscosity values, critical concentration, c**, and coil overlap parameter c** [eta] varied within 0.63-3.01 dl/g, 0.95-1.91 g/dl and 2.12-2.86, respectively. The concentration dependence of apparent viscosity revealed typical pseudoplastic behavior of the polysaccharide and the onset of shear thinning behavior depended on the molecular weight and concentration. All (1 --> 3,1 --> 4)-beta-glucans (10%, w/v) followed well the generalized shear thinning behavior expressed by the equation eta = eta(0)/[1 + (gamma/gamma(1/2))(0.76)] (r(2) = 0.98). Low molecular weight samples tended to aggregate very quickly showing time-dependent rheological behavior, as evidenced by thixotropic loop experiments. Gelation of all barley samples was also feasible under certain storage conditions; the gelation time was shorter for samples of low molecular size. The gelation rate increased with concentration but as a function of gel cure temperature, a maximum was obtained at 31 degreesC. Melting of gels by heating at a certain rate confirmed the thermoreversibility of the network structures. Calorimetric measurements showed broad endothermic transitions at 50-75 degreesC with apparent melting enthalpies being greater for the lower molecular weight samples. The melting temperatures increased with increasing concentration and gel curing temperature in agreement with the dynamic rheometry measurements. Compression tests up to failure were made to examine the effects of molecular weight and concentration on gel strength, firmness and brittleness. Higher molecular weight samples exhibited higher 'yield stress' (sigma(TR)) and lower compression modulus (E) values; instead, the dynamic storage modulus (G'(max)) values decreased with increasing molecular size of the polysaccharide. True stress at failure (sigma(TR)) increased with increasing concentration, whereas true strain at failure (epsilon(TR)) increased reaching a maximum value and then decreased with increasing concentration of the polysaccharide. (C) 2003 Elsevier Ltd. All rights reserved.