Rapid lactic acid production at high cell concentrations in whey ultrafiltrate by Lactobacillus helveticus

The effects of inoculum levels, temperature, pH, and the supplementation of whey ultrafiltrate with yeast extract on cell growth and lactic acid production by Lactobacillus helveticus were investigated. The objective was to accelerate fermentation batch cycle times of 1-2 h. This was a first step to evaluate the feasibility of a tubular continuous process. Addition of 10 g l^-1 yeast extract was required to optimize the volumetric productivity. Higher yeast extract levels further improved cell growth, but lactic acid production remained unchanged. A temperature of 42°C and pH 5.8 were found to be optimal. pH values above 5.8 resulted in a decrease in productivity and yield, mainly because galactose was only partially utilized. Volumetric productivities were increased up to 35 g l^-1 h^-1 at cell densities of 27 g l^-1. The time for complete conversion of 55-60 g l^-1 lactose was reduced to 90 min. Cell growth and lactic acid formation followed the model of Luedeking and Piret. Copyright (C) 1999 Elsevier Science Inc.