13-04: The implications of depressed water activity during high solids saccharification of lignocelluloses

Operation of lignocellulose conversion processes at high dry matter levels (>20%) has been shown necessary for cost effectiveness at industrial scale. To date, a number of studies have reported depressed enzymatic conversions with increasing solids concentrations but only a couple have attempted to uncover underlying reasons for this phenomenon. Previously, we evaluated this “high solids” effect across multiple enzyme activity types to determine if it could be isolated to specific enzyme groups using a custom milliliter-scale high-solids saccharification tumbler. With this system the detrimental effect of high solids loading was observed with commercial enzyme preparations on cellulose, xylan and pretreated lignocellulose yet it was found that hydrolyses with individual/combinations of purified cellulases produced the opposite trend. We hypothesized that concentration of low molecular weight components in commercial preparations under high solids conditions may negatively impact water activity and enzyme effectiveness. Here we address this and report the impact low molecular weight components from commercial enzyme preparations have on the saccharification of lignocelluloses and make observations related to the measurement of water activity under varied hydrolysis conditions. Studies where sub-10,000 Da fractions of commercial enzyme preparations were introduced at industrially relevant quantities proved inhibitory with respect to the enzymatic conversion of both cellulose and xylan. Additionally, experiments where common preservatives were added to base-case hydrolyses proved inhibitory to  conversions. Furthermore, water activity measurements associated with hydrolysis conditions showed these non-enzymatic components depressed water activity and that there is a correlative response in depressed conversion associated with the decreased water activity.