PROGRESS IN BIOPULPING OF NON-WOODY MATERIALS - CHEMICAL, ENZYMATIC AND ULTRASTRUCTURAL ASPECTS OF WHEAT-STRAW DELIGNIFICATION WITH LIGNINOLYTIC FUNGI FROM THE GENUS PLEUROTUS

During screening of basidiomycetes for wheat straw delignification, considerable lignin degradation with a limited attack to cellulose was attained with Pleurotus eryngii. Straw solid-state fermentation (SSF) was optimized, and the enzymatic mechanisms for lignin degradation were investigated. No lignin peroxidase was detected under liquid or SSF conditions, but high laccase and aryl-alcohol oxidase levels were found. The latter enzyme has been fully characterized in Pl. eryngii and it seems to be involved in a cyclic redox system for H2O2 generation from aromatic compounds. Results obtained using homoveratric acid suggest that Pleurotus laccase could be involved in degradation of phenolic and non-phenolic lignin moieties. Histological and ultrastructural studies provided some general morphological characteristics of the fungal attack on wheat straw. Whereas a simultaneous degradation pattern was observed in straw treated with Phanerochaete chrysosporium, Pl. eryngii caused partial degradation of middle lamella and separation of individual sclerenchymatic fibers. When these straw samples were subjected to refining tests, energy saving after biological treatment was the highest in the case of straw treated with Pl. eryngii, which also produced the lowest substrate loss. From these results, a correlation between preferential removal of lignin, separation of sclerenchymatic fibers and pulping properties was provided during fungal treatment of wheat straw.