A fermentation process for producing both ethanol and lysine-enriched yeast

Abstract

In 18 batch-fermentation experiments, baker's yeast was grown in an enriched mineral medium, containing 10% by weight glucose, at various pH and temperature levels. The pH and temperature are just two representative engineering variables which and be easily varied at negligible cost. The commercial yeast inoculum, .20% by weight or about .16% viable cells, was selected to represent industrial (nonsterile) conditions. Free L-lysine, ethanol, and cell growth were followed in time for each batch run held at a fixed pH and temperature. The maximum free lysine level reached at either 10½ or 24 hr occurred at a pH of 5 and 32°C. At 24 hr, the peak free lysine level, 120 mg/liter, is three times as great as the uncontrolled pH counterpart. In terms of total L-lysine, (free plus protein-bound) the peak represents a 25% improvement over the uncontrolled case, based on an average 3.5% lysine level per cell weight. The greatest measured cell level, .9% by weight in the fermentation broth, or a 5½-fold increase over the inoculum, was reached during the 36°C and pH 3 run, while the largest measured ethanol value (3%, or 30% conversion by weight from glucose) was achieved during the 28°C and pH 6 experiment. The optimal lysine run produced, however, no less than 15% of the maximum cell and 30% of the maximum ethanol levels.