Effects of fermentation substrate conditions on corn–soy co-fermentation for fuel ethanol production

► A successful integration of soy and corn biorefineries is shown. ► Soy skim from soybean aqueous extraction processing was shown to have rate promotion effect on corn-ethanol fermentation without affecting the ethanol yield. ► The soy skim provided all nutrients needed for the yeast. ► Such corn–soy co-fermentation benefits soybean aqueous extraction processing and ethanol fermentation. ► Soy-enhanced whole stillage had high protein and lysine contents and can provide feed with improved quality. Soy skim, a protein-rich liquid co-product from the aqueous extraction of soybeans, was co-fermented with corn to produce ethanol. Effects of soy skim addition level, type of skim, corn particle size, water-to-solids ratio, and urea on co-fermentation were determined. The addition of 20–100% skim increased the fermentation rate by 18–27% and shortened the fermentation time by 5–7h without affecting ethanol yield. Finely ground corn or high water-to-solids ratio (⩾3.0) in the mash gave higher fermentation rates, but did not increase the ethanol yield. When the water was completely replaced with soy skim, the addition of urea became unnecessary. Soy skim retentate that was concentrated by nanofiltration increased fermentation rate by 25%. The highest level of skim addition resulted in a finished beer with 16% solids, 47% protein (dwb) containing 3.6% lysine, and an ethanol yield of 39g/100g dry corn.