Glucuronoarabinoxylans from sorghum grain

Water-unextractable cell wall materials (WUS) were prepared from raw, polished, and malted sorghum ( Sorghum vulgare cv. Fara Fara). Except for the amounts, hardly any difference could be observed between the WUS of these three raw materials. This means that cell wall materials of the endosperm cell walls are basically the same as those of the outer endosperm and pericarp layers, and that the cell walls largely persist, during malting. These preparations were further fractionated by a sequential extraction procedure using aqueous solutions of saturated Ba(OH) ₂ , 1M KOH and 4M KOH. The WUS preparations were composed of glucuronoarabinoxylans (GAX), (1→3),(1→4)-β-D-glucans, cellulose, and some protein. GAX was primarily extracted by Ba(OH) ₂ solutions. All GAX fractions were composed of a highly substituted (1→4)-β-D-xylan backbone, substituted by arabinose and uronic acid. It was concluded that sorghum GAX populations were characterized by a reasonable homogeneity, since they could not be separated further by several chromatographic and precipitation techniques.

Degradation studies using purified xylanases, arabinofuranosidases and a glucuronidase alone or in combination, showed that the GAX populations were hardly broken down. Some oligomers were formed by digesting Ba(OH) ₂ extracted GAX with a combination of endoxylanase I and (1→4)-β-D-arabinoxylan arabinofuranohydrolase, both purified from Aspergillus awamori . These oligomers were found to have a main chain of three or four xylose units, and to contain α-glucuronic acid linked to O -2 of the non-reducing terminal xylose unit. Two oligomers were found to have a dimeric (1→2)-linked arabinose side, chain linked at O -3 of an internal xylose unit. Also single arabinose substitution occured at O -3 of an internal xylose unit. There are strong indications that these side groups can also be linked at O -2 of an internal xylose residue. The reducing xylose units were unsubstituted. A model for the GAX populations from sorghum was proposed combining the results of the degradation studies, the identification of the oligomers, and knowledge about the mode of action of the enzymes used.

Finally, the developed techniques to investigate GAX in particular, were used to study the behaviour of GAX in the brewing process. Worts and spent grains of mashes, supplemented with commercial enzyme preparations containing xylanases among others were studied. Except for the amount of solubilized GAX, the GAX hardly changed with respect to the sugar composition and molecular weight distribution. A direct relationship between GAX, xylanases, and filtration behaviour of worts prepared from malted sorghum, could therefore not be established.