THE EFFECT OF SULFUR-DIOXIDE APPLICATION LEVEL ON THE BIODEGRADATION OF WHEAT STRAW CARBOHYDRATES BY RUMEN MICROORGANISMS AND BY TRICHODERMA-VIRIDE CELLULASE

Wheat straw was treated with SO2 at different levels of application under mild temperature conditions, and the effect of treatment on cell wall (CW) composition and biodegradation was tested. The SO2 treatments gradually solubilized CW non-glucose polysaccharide (NGP = hemicellulose), decreasing the content of this fraction in residual CW from 31.5 g 100 g-1 CW in the untreated straw, to 17.3 g 100 g-1 CW in the 5% SO2-treated material Lignin and cellulose were only slightly solubilized by SO2 treatments. Concomitantly, the in vitro digestibility by rumen bacteria of total straw monosaccharides increased gradually from 60.5% in the untreated straw to a plateau of 87.7% in the 4% SO2-treated straw. The digestibility of residual CW carbohydrates followed a similar pattern, increasing from 55.4% in the untreated straw to 81.1% in the 4% SO2-treated material The 4% and 5% SO2 application levels were also the most effective as pretreatments for subsequent hydrolysis of straw by Trichoderma viride cellulases, and this was expressed by an increase in the degree of saccharification of total straw carbohydrates from 7.76% in the untreated material to 62.3% in the straw treated by the 4% SO2, plus cellulase. Data from this study support the theory that hemicellulose is the major biodegradation obstacle in the CW of wheat straw.