Barley endosperm cell walls: a review of cell wall polysaccharides and cell-wall degrading enzymes.

Thompson, R.G. and LaBerge, D.E.

Abstract
Over some 50 years it was gradually established that the endosperm cell walls of barley were disrupted during germination and at least two nonstarchy polysaccharides (NSP) and the enzymes that degrade them were involved. Intensive studies in the 1950's showed that the water soluble and alkali soluble NSP from barley and malt were essentially similar in composition, the major constituents being glucose, arabinose and xylose. Beta glucan and arabinoxylan were identified in wort and beer. Analysis of isolated endosperm cell walls indicates that they are composed predominantly of beta glucan and arabinoxylan with perhaps 2 to 4% of glucomannan and 5 to 6% of protein. Cellulose and pectic substances are of negligible importance. Beta glucan is linear polysaccharide composed of glucose units linked by beta 1,3 and beta 1,4 bonds in roughly equal proportions. Studies of the arrangement of the linkages indicate that there are primarily two types of repeating unit: a tetrameric unit linked 1,3 1,4 1,4 and a pentameric unit linked 1,3 1,4 1,4 1,4. Segments of the molecule consisting of these repeating units are interrupted by less orderly regions containing longer sequences of 1,3 linkages. Water soluble arabinoxylan has a main chain of beta 1,4 linked xylopyranose units to which L arabino furanose units are attached at intervals via the C2 or C3 position, or at both positions to give a double branch. The alkali soluble arabinoxylan appears to have more of the doubly branched residues than the water soluble material. The distribution of the arabinose branches has not been studied in barley but in some other cereals the branches occur most frequently on isolated xylose residues, less often on two or three consecutive residues and never on four or more in sequence. The arrangement of the side chains may determine the ability of arabinoxylans to bind other cell wall components. Enzymes in barley and malt capable of attacking beta glucan include both endo and exo beta glucanases. The endo enzymes, important in degrading high viscosity beta glucan solubilized during mashing, appear to include at least three types of activity, specific for beta 1,3, beta 1,4 and for the mixed beta 1,3; beta 1,4a linkages of barley beta glucan respectively. The exo beta glucanase activity of barley and malt has not been clearly defined. Cellobiase and laminaribiase activities have been demonstrated, together with evidence for a true exo beta 1,4 glucanase. Enzymes capable of attacking arabinoxylan (arabinosidase, xylobiase, exo and endo xylanase) are present in barley and are enhanced during malting. Kilning decreases exo enzyme activity to or below that of the original grain. Arabinose is the first detectable hydrolysis product and the action of arabinosidase may be a prerequisite for hydrolysis of the xylan main chain. The endosperm cell walls serve as a barrier to migration of enzymes during malting and slow malting barleys may lack sufficient enzymes to disrupt this structure quickly.
Keywords: barley cell wall endosperm enzyme glucanase gums hydrolysis polysaccharide survey