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

Abstract

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 β-glucan contents, as well as in grain hardness. The CW were initially extracted with water at 45 °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 (CW ND) was further fractionated by sequential extraction with water at 95 °C (CW ND-WE95), saturated barium hydroxide (CW ND-BaE), and 1 N sodium hydroxide (CW ND-NaE) at 25 °C. Composition and molecular structure analyses were carried out for all fractions including the remaining cell wall residue (CW RES). Extraction of CW with water followed by digestion with malt crude enzyme extract solubilized the majority of β-glucans (∼55–70%) and glucomannans (∼60–80%) but only a small portion of arabinoxylans (∼20–30%) present in the intact CW. The CW-WE45 and CW ND-WE95 fractions consisted mostly of β-glucans exhibiting high average molecular weights ( M w) (2–3 × 10 6), whereas the CW ND-BaE consisted mainly of arabinoxylans with M w about 1–1.5 × 10 6. The CW ND-NaE contained almost equal amounts of β-glucans and arabinoxylans and a small amount of glucomannans, whereas the CW RES contained approximately equal proportions of β-glucans, arabinoxylans and glucomannans. β-Glucans in CW ND-WE95, CW ND-NaE, and CW RES exhibited a higher ratio of 3- O-β- d-cellobiosyl- d-glucose to 3- O-β- d-cellotriosyl- d-glucose (DP3/DP4) compared to β-glucans in CW-WE45 and CW-MD. β-Glucans in CW ND-NaE showed the highest level of long cellulosic oligosaccharides with DP ≥ 5, whereas those in the CW RES had the highest DP3/DP4 ratio. The CW-MD was fractionated by ultrafiltration into high (CW-MD HMW) and low-molecular weight (CW-MD LMW) sub-fractions, with weight-average M w of ∼150–350 × 10 3 and <10 × 10 3, respectively, as confirmed by size-exclusion chromatography. The monosaccharide composition of the sub-fractions indicated a more extended enzymic degradation of β-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.