Fermentation of soybean hulls to ethanol while preserving protein value
Soybean hulls were evaluated as a resource for production of ethanol by the simultaneous saccharification
and fermentation (SSF) process, and no pretreatment of the hulls was found to be needed to realize
high ethanol yields with Saccharomyces cerevisiae D5A. The impact of cellulase, b-glucosidase and pectinase
dosages were determined at a 15% biomass loading, and ethanol concentrations of 25–30 g/L were
routinely obtained, while under these conditions corn stover, wheat straw, and switchgrass produced
3–4 times lower ethanol yields. Removal of carbohydrates also concentrated the hull protein to over
25% w/w from the original roughly 10%. Analysis of the soybean hulls before and after fermentation
showed similar amino acid profiles including an increase in the essential amino acids lysine and threonine
in the residues. Thus, eliminating pretreatment should assure that the protein in the hulls is preserved,
and conversion of the carbohydrates to ethanol with high yields produces a more concentrated
and valuable co-product in addition to ethanol. The resulting upgraded feed product from soybean hulls
would likely to be acceptable to monogastric as well as bovine livestock