Effect of specific growth rate on fermentative capacity of baker's yeast.

The specific growth rate is a key control parameter in the industrial production of baker's yeast. Nevertheless, quantitative data describing its effect on fermentative capacity are not available from the literature. In this study, the effect of the specific growth rate on the physiology and fermentative capacity of an industrial Saccharomyces cerevisiae strain in aerobic, glucose-limited chemostat cultures was investigated. At specific growth rates (dilution rates, D) below 0.28 h-1, glucose metabolism was fully respiratory. Above this dilution rate, respirofermentative metabolism set in, with ethanol production rates of up to 14 mmol of ethanol . g of biomass-1 . h-1 at D = 0.40 h-1. A substantial fermentative capacity (assayed offline as ethanol production rate under anaerobic conditions) was found in cultures in which no ethanol was detectable (D < 0.28 h-1). This fermentative capacity increased with increasing dilution rates, from 10.0 mmol of ethanol . g of dry yeast biomass-1 . h-1 at D = 0.025 h-1 to 20.5 mmol of ethanol . g of dry yeast biomass-1 . h-1 at D = 0.28 h-1. At even higher dilution rates, the fermentative capacity showed only a small further increase, up to 22.0 mmol of ethanol . g of dry yeast biomass-1 . h-1 at D = 0.40 h-1. The activities of all glycolytic enzymes, pyruvate decarboxylase, and alcohol dehydrogenase were determined in cell extracts. Only the in vitro activities of pyruvate decarboxylase and phosphofructokinase showed a clear positive correlation with fermentative capacity. These enzymes are interesting targets for overexpression in attempts to improve the fermentative capacity of aerobic cultures grown at low specific growth rates.