Composition and molecular structure of polysaccharides released from barley endosperm cell walls by sequential extraction with water, malt enzymes, and alkali

Isolated and purified endosperm cell walls (CW), used in this study, were derived from a Canadian malting barley variety, AC Metcalfe, grown in three different environments in Canada in 2003, and varying in grain protein and beta-glucan contents, as well as in grain hardness. The CW were initially extracted with water at 45 degrees C and subsequently digested with barley malt crude enzyme extract resulting in two fractions designated CW-WE45 and CW-MD, respectively. The remaining non-digested cell wall material (CWND) was further fractionated by sequential extraction with water at 95 degrees C (CWND-WE95), saturated barium hydroxide (CWND-BaE), and 1 N sodium hydroxide (CWND-NaE) at 25 degrees C. Composition and molecular structure analyses were carried out for all fractions including the remaining cell wall residue (CWRES). Extraction of CW with water followed by digestion with malt crude enzyme extract solubilized the majority of beta-glucans (similar to 55-70%) and glucomannans (similar to 60-80%) but only a small portion of arabinoxylans (similar to 20-30%) present in the intact CW. The CW-WE45 and CWND-WE95 fractions consisted mostly of beta-glucans exhibiting high average molecular weights (M-w) (2-3 x 10(6)), whereas the CWND-BaE consisted mainly of arabinoxylans with Mw about 1-1.5 x 10(6). The CWND-NaE contained almost equal amounts of beta-glucans and arabinoxylans and a small amount of glucomannans, whereas the CWRES contained approximately equal proportions of beta-glucans, arabinoxylans and glucomannans. beta-Glucans in CWND-WE95, CWND-NaE, and CWRES exhibited a higher ratio of 3-O-beta-D-cellobiosyl-D-glucose to 3-O-beta-D-cellotriosyl-D-glucose (DP3/DP4) compared to beta-glucans in CW-WE45 and CW-MD. beta-Glucans in CWND-NaE showed the highest level of long cellulosic oligosaccharides with DP >= 5, whereas those in the CWRES had the highest DP3/DP4 ratio. The CW-MD was fractionated by ultrafiltration into high (CW-MDHMW) and low-molecular weight (CW-MDLMW) sub-fractions, with weight-average M-w of similar to 150-350 x 10(3) and <10 x 10(3), respectively, as confirmed by size-exclusion chromatography. The monosaccharide composition of the sub-fractions indicated a more extended enzymic degradation of beta-glucans and glucomannans than arabinoxylans. Some differences in composition and molecular structure of the cell wall constituents among the three barley samples were related to their solubility and enzymic digestibility. (C) 2007 Elsevier Ltd. All rights reserved.