STRUCTURE AND PHYSICOCHEMICAL PROPERTIES OF BARLEY NONSTARCH POLYSACCHARIDES - II - ALKALI-EXTRACTABLE BETA-GLUCANS AND ARABINOXYLANS

Three fractions containing hemicellulosic material were obtained by se quential extraction of barley residue (left after removal of water-ext ractable polysaccharides) with saturated barium hydroxide [Ba(OH)(2) f raction], distilled water [Ba(OH)(2)/H2O fraction], and 1M sodium hydr oxide [NaOH fraction]. The yields of the fractions were 1.6, 1.7, and 2.6% (w/w), respectively, of the dry barley grist. The Ba(OH)2 fractio n contained mainly arabinose and xylose, 35.8% and 60.9%, respectively . The Ba(OH)(2)/H2O fraction in addition to 26.7% Ara and 36.6% Xyl co ntained also 34.8% Glc. The NaOH fraction was composed of 14.2% Ara, 4 4.0% Xyl, and 40.9% Glc. The Ba(OH)(2)/H2O and NaOH extracts were furt her fractionated by stepwise (NH4)(2)SO4 precipitation into several su bfractions with varying amounts of beta-glucans and arabinoxylans. bet a-Glucans in Ba(OH)(2)/H2O and NaOH fractions were characterized by hi gh ratios of beta-(1-->4)/beta-(1-->3) linkages, large amounts of cont iguously linked beta-(1-->4) segments, and high ratios of cellotriosyl /cellotetraosyl units. The alkali-extractable arabinoxylans, especiall y those NaOH-extractable, were characterized by a very low degree of s ubstitution, high xylose/arabinose ratio, and a small content of doubl y substituted xylose residues. Some populations of arabinoxylans displ ayed structural features that would enable them to self-associate or t o interact with beta-glucans. (C) 1998 Elsevier Science Ltd. All right s reserved.