High-solids enzymatic hydrolysis of steam-exploded willow without prior water washing
A laboratory reactor equipped with a screw press was used for the hydrolysis of steam-SO2-exploded willow Salix caprea by a composition of Trichoderma reesei and Aspergillus foetidus enzyme preparations at high substrate concentration. Optimal conditions providing the maximal volume of hydrolysis syrup with maximal sugar concentrations were determined. Two different hydrolysis procedures were developed in order to exclude the initial washing of steam-pretreated plant raw material by large volumes of water, which was necessary to eliminate the inhibitory effect of explosion byproducts on enzymatic hydrolysis. The first procedure included enzymatic prehydrolysis of the substrate for 1 h; separation of sugar syrup containing 40-60 g/l glucose, 20-25 g/l xylose, and up to 10 g/l disaccharides, as well as up to 35% of the initial enzymatic activity; then addition of a diluted acetate buffer (pH 4.5); and subsequent hydrolysis of the substrate by the adsorbed enzymes leading to the final accumulation of up to 140 g/l glucose and up to 15 g/l of xylose. In the second scenario, the exploded willow was initially adjusted by alkali to pH 4.5 and then hydrolyzed directly by the added enzymes over 24 h. This procedure resulted in a nearly total polysaccharide hydrolysis and accumulation of up to 170 g/l glucose and 20 g/l xylose. The reasons for inhibition of enzymatic hydrolysis are discussed.