Evaluation of dilute-acid pretreated bagasse, corn cob and rice straw for ethanol fermentation by Saccharomyces cerevisiae

Bagasse, corn cob, and rice straw agricultural wastes were found to consist of 37, 39 and 34% cellulose and 24, 41 and 22% hemicellulose, respectively, on a dry solid (w/w) basis and thus have the potential to serve as a low cost feedstock for ethanol production. Hydrolysates produced by dilute-acid pretreatment followed by cellulase digestion were evaluated as substrates for ethanol fermentation by Saccharomyces cerevisiae. After pretreatment by 141 mM sulphuric acid, bagasse waste released glucose (134 mg/g) at a higher level than that from corn cob (75 mg/g) and rice straw (8 mg/g). Hydroxymethylfurfural (HMF) levels derived from acid pretreatment of bagasse (1.5 g/l), but not corn cob (0.8 g/l) or rice straw (0.1 g/l) attained levels likely to be toxic (1.5 g/l) for S. cerevisiae growth and ethanol fermentation rates. All three agricultural wastes released likely non-toxic levels of furfural (< 0.5 g/l) and lactic acid (negligible for bagasse and rice straw and 0.7 g/l for corn cob). After cellulase saccharification of the dilute-acid pretreated agricultural wastes, the glucose content of corn cob hydrolysates (13 +/- 0.17 g/l) was marginally higher than that of bagasse (12 +/- 0.27 g/l) or rice straw (11 +/- 0.07 g/l), yet the ethanol conversion yield by S. cerevisiae on corn cob hydrolysate (0.45 +/- 0.006 g/g) was lower than that attained with bagasse hydrolysate (0.49 +/- 0.007 g/g). Synergistic adverse effects between furfural and HMF with weak acids, or other lignin derived products in the corn cob hydrolysate are proposed as the effective inhibitor(s) for ethanol fermentation by S. cerevisiae.