Effect of enzymatic hydrolysis on the morphology and fine structure of pretreated cellulosic residues

Abstract

The action of Trichoderma cellulases on the various characteristics of cellulosic fractions obtained from pretreated eucalyptus chips was investigated. A screened fraction derived from a fully bleached eucalyptus kraft pulp (FBEP-48 fraction) was shown to consist of fibers with an average fiber length and an average degree of polymerization (DP) considerably higher than that obtained for the screened fraction derived from SO 2-impregnated steam-exploded eucalyptus chips which had been posttreated with alkaline hydrogen peroxide (SEE-H 2O 2, F-150 fraction). Both fractions were shown to be readily and effectively hydrolyzed by the cellulases present in the Trichoderma Celluclast preparation (Novo Nordisk). The process of enzymatic hydrolysis of both FBEP-48 and F-150 fractions resulted in a rapid decrease in the length distribution of the fibers, while small particles rapidly accumulated in the hydrolysis mixture. These particles were subsequently degraded to equivalent amounts of soluble sugars. Glucose was the main hydrolysis product, due to the high amount of β-glucosidase activity which had been added. However, at the molecular level, it was apparent that the two cellulosic substrates were hydrolyzed in distinct ways and that the mechanism appeared to be influenced by the fine structure of the substrate. As hydrolysis resulted in a gradual decrease in both the degree of polymerization and the degree of crystallinity of the F-150 fraction, it was probable that the depolymerization of this substrate was predominantly due to exoglucanase activity. By contrast, the enzymatic hydrolysis of the FBEP-48 fraction resulted in little change in either the cellulose DP or the degree of crystallinity of the substrate. This suggested a “peeling off” type of mechanism. The susceptibility of the pretreated substrates to enzymatic hydrolysis could not be easily predicted from the differences in their cellulose DP or crystallinity.