Effect of yeast inoculation rate on the metabolism of contaminating lactobacilli during fermentation of corn mash.

Two separate 4 (bacterial concentrations)x6 (yeast concentrations) full factorial experiments were conducted in an attempt to identify a novel approach to minimize the effects caused by bacterial contamination during industrial production of ethanol from corn. Lactobacillus plantarum and Lactobacillus paracasei, commonly occurring bacterial contaminants in ethanol plants, were used in separate fermentation experiments conducted in duplicate using an industrial strain of Saccharomyces cerevisiae, Allyeast Superstart. Bacterial concentrations were 0, 1x10(6), 1x10(7) and 1x10(8) cells/ml mash. Yeast concentrations were 0, 1x10(6), 1x10(7), 2x10(7), 3x10(7), and 4x10(7) cells/ml mash. An increased yeast inoculation rate of 3x10(7) cells/ml resulted in a greater than 80% decrease (P < 0.001) and a greater than 55% decrease (P < 0.001) in lactic acid production by L. plantarum and L. paracasei, respectively, when mash was infected with 1x10(8) lactobacilli/ml. No differences (P > 0.25) were observed in the final ethanol concentration produced by yeast at any of the inoculation rates studied, in the absence of lactobacilli. However, when the mash was infected with 1x10(7) or 1x10(8) lactobacilli/ml, a reduction of 0.7-0.9% v/v (P < 0.005) and a reduction of 0.4-0.6% v/v (P < 0.005) in the final ethanol produced was observed in mashes inoculated with 1x10(6) and 1x10(7) yeast cells/ml, respectively. At higher yeast inoculation rates of 3x10(7) or 4x10(7) cells/ml, no differences (P > 0.35) were observed in the final ethanol produced even when the mash was infected with 1x10(8) lactobacilli/ml. The increase in ethanol corresponded to the reduction in lactic acid production by lactobacilli. This suggests that using an inoculation rate of 3x10(7) yeast cells/ml reduces the growth and metabolism of contaminating lactic bacteria significantly, which results in reduced lactic acid production and a concomitant increase in ethanol production by yeast.