Fermentation of wheat straw xylan by the white-rot fungus Dichomitus squalens

Abstract

The pattern of degradation of purified wheat straw xylan by submerged cultures of the ligninolytic white-rot fungus Dichomitus squalens has been determined. After a 22-h lag period, extracellular xylanase and protease activities developed during the first phase of secretion, the exponential growth phase. A rapid drop in the degree of polymerization of the soluble xylan was observed, corresponding to the release of xylobiose, followed by the release of xylotriose and xylose. After 70 h cultivation, soluble sugars were almost exhausted; laminarinase activities were secreted, followed by endoxylanases, β-xylosidases, CM-cellulases, and an α-L-arabinofuranosidase. All the endoxylanases exhibited β-xylosidase activity. No α-D-glucuronidase could be determined. Accumulation of an undegradable soluble polymer, highly substituted by glucuronic acid residues, was observed. Since no free arabinose or free glucuronic acid could be detected in the culture medium, it is suggested that a progressive increase of unsubstituted xylose units arises from preferential attack on the more highly substituted regions of the xylan backbone by endoxylanases.