Ethanol and butanol production by fermentation of enzymatically saccharified SO 2-prehydrolysed lignocellulosics

Abstract

The present paper reports the results of SO 2-catalysed prehydrolysis of lignocellulosics, namely pine, aspen, and corn stover, and their direct saccharification, without washing, by cellulolytic enzymes to produce a mixture of hemicellulose and cellulose sugars. Total sugar yield upon enzymatic hydrolysis of these prehydrolysed residues was over 65% of the weight of raw material, or above 86% of theoretical yields. Fermentation by a strain of Pichia stipitis adapted to wood hydrolysates by recycling, designated the R strain, resulted in efficient utilization of xylose, high ethanol yields, and good tolerance of inhibitors. Fermentation of hydrolysates containing both the hemicellulose and cellulose sugars by P. stipitis R gave yields of 372, 346, and 388 l ethanol tonne −1 (dry weight of prehydrolysed feedstock) from pine, aspen, and corn stover, respectively. These yields represent over 74% of theoretical. Extractive fermentation of these hydrolysates to acetone-butanol-ethanol by Clostridium acetobutylicum P262 using dibutylphthalate as in situ extractant in a semicontinuous mode resulted in rapid fermentation, when using a high cell concentration and recycling the cells for each 24-h fermentation to the succeeding batch. By this means, 255 l of acetone-butanol solvents were obtained per tonne of aspen, 298 l per tonne of pine, and 283 l per tonne of corn stover. The technical aspects of prehydrolysis using SO 2 catalyst, enzymatic saccharification, the advantages of using P. stipitis R for the fermentation of pentose-rich hydrolysates, and the potential of dibutylphthalate for extractive fermentation to obtain high yields of solvents are discussed.