Water extractable (1 leads to 3, 1 leads to 4)-beta-D-glucans from barley and oats: an intervarietal study on their structural features and rheological behaviour

The fine structures and rheological behaviours of aqueous flour dispersions and of beta-glucan, (1 leads to 3,1 leads to 4)-beta-D-glucan isolates obtained from 18 registered varieties of normal covered barley seeds and four registered oat varieties, grown in the same location in Greece, were investigated. The beta-glucan content of the barleys and oats varied between 2.5-5.4 and 2.1-3.9%, respectively (dry matter basis). Heat treatment of the barley and oat flour dispersions with 80% (v/v) ethanol, to inactivate endogenous beta-glucanases, had a stabilizing effect on the viscosity profile of the flour slurries. The relationship between total beta-glucan content and aqueous slurry viscosity (at 247 s(-1)) of the heat-treated barley flours was weak (r2=0.45, p<0.05, n=18). beta-Glucans were isolated by water extraction at temperatures slightly below the gelatinization temperature of starch, enzymatic removal of starch and partial removal of contaminating proteins by adjustment to pH 4.0-4.5, and subsequent precipitation of the water-soluble beta-glucans with 80% (v/v) ethanol. The cellulosic oligomers released by the action of a (1 leads to 3,1 leads to 4)-beta-D-glucan hydrolase showed cellotriosyl and cellotetraosyl units, accounted for 91.1-92.1% for barley and between 92.4 and 94.1% for the oat preparations; the respective molar ratios of tri- to tetra-saccharides (DP3/DP4) ranged between 2.73-3.05 (barley) and 2.16-2.42 (oat). Steady shear measurements confirmed the random coil type behaviour of freshly prepared beta-glucan solutions (5 and 7%, w/v). The rate at which shear thinning began was dependent on both concentration and molecular size of the polysaccharide. Most of the beta-glucan dispersions followed the Cox-Merz rule, except Mucio, a variety with high M(w) beta-glucan (2.39X10(5)). Viscoelastic characterization, at 8% (w/v), of three barley beta-glucan aqueous dispersions differing in molecular size, indicated that the low molecular weight sample exhibited shorter gelation time and higher gelation rate (I(E)=[dlog G'/dt]max) than its higher molecular weight counterparts. Small deformation oscillatory measurements on gels of all barley beta-glucan isolates (10% (w/v), 7 d storage, 25 degrees C) revealed a strong negative relationship (r2=0.88, p<0.01) between G' (1 Hz, strain 0.1%) and apparent M(w) of the polysaccharide.