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
Acknowledgements
The authors thank Sara All and Kara Podkaminer (Dartmouth),
and Miguel Rodriguez (ORNL) for technical assistance, and Genencor/
Danisco for providing samples of SpezymeCP and Accellerase
1000 cellulases. This work was funded in part by the National
Institute of Standards and Technology (NIST) through Grant Number
#60NANB1D0064 awarded to Dartmouth College, where much
of the early work was performed. Additional support was obtained
from the Oak Ridge National Laboratory BioEnergy Science Center,
a US Department of Energy Bioenergy Research Center supported
by the Office of Biological and Environmental Research in the US
Department of Energy Office of Science.