EFFECT OF SPECIFIC GROWTH-RATE ON FERMENTATIVE CAPACITY OF BAKERS-YEAST

The specific growth rate is a key control parameter in the industrial production of baker's yeast. Nevertheless, quantitative data describin g its effect on fermentative capacity are not available from the liter ature, In this study, the effect of the specific growth rate on the ph ysiology and fermentative capacity of an industrial Saccharomyces cere visiae strain in aerobic, glucose-limited chemostat cultures was inves tigated. 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 o f 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 pro duction rate under anaerobic conditions) was found in cultures in whic h no ethanol was detectable (D < 0.28 h(-1)). This fermentative capaci ty 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 e thanol g of dry yeast biomass(-1) h(-1) at D = 0.28 h(-1). At even hig her dilution rates, the fermentative capacity showed only a small furt her 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, pyruv ate decarboxylase, and alcohol dehydrogenase were determined in cell e xtracts. Only the in vitro activities of pyruvate decarboxylase and ph osphofructokinase showed a clear positive correlation with fermentativ e capacity. These enzymes are interesting targets for overexpression i n attempts to improve the fermentative capacity of aerobic cultures gr own at low specific growth rates.