EFFECT OF RECYCLED LABORATORY BACKSET ON FERMENTATION OF WHEAT MASHES

Distillers' solubles, soluble components and the liquid obtained after distillation of fermented mash and removal of distillers' grains, are often recycled as ''backset' to reduce effluent treatment costs and water usage. In this study, recycling of laboratory distillers' solubles was carried out over five successive fermentations. When 50% backset was used in a 30-gal mash (30 U.S. gal/56-lb bushel, a term used by corn distillers) using wheat as the substrate, yeast growth and ethanol production remained unaffected over five successive fermentations. The final cell yields approximated 9 X 10(7) yeasts/mL. The alcohol yields ranged from 5.7 to 6.0% w/v. No undesirable substances accumulated in high enough concentrations to inhibit normal yeast metabolism. In fact, calcium, lactic acid, and acetic acid were utilized by the yeasts and removed from the mash. Wheat mashes contained low levels of utilizable free amino nitrogen. Yeast extract added to the mash stimulated the rate (but not the amount) of alcohol produced. Optimal supplementation allowed a 40% reduction in fermentation time compared to that of unsupplemented controls. Although yeast extract contains unassimilable substances, no compounds in the backset were found to impede yeast fermentation through four cycles of backsetting at 50%.