Fermentative capacity in high-cell-density fed-batch cultures of baker's yeast

High-cell-density fed-batch processes for bakers' yeast production will inv olve a low-average-specific growth rate due to the limited oxygen-transfer capacity of industrial bioreactors. The relationship between specific growt h rate and fermentative capacity was investigated in aerobic, sucrose-limit ed fed-batch cultures of an industrial bakers' yeast strain. Using a define d mineral medium, biomass concentrations of 130 g dry weight/L were reprodu cibly attained. After an initial exponential-feed phase (mu = 0.18 h(-1)), oxygen-transfer limitation necessitated a gradual decrease of the specific growth rate to ca. 0.01 h(-1). Throughout fed-batch cultivation, sugar meta bolism was fully respiratory, with a biomass yield of 0.5 g biomass/g sucro se(-1). Fermentative capacity (assayed off-line as ethanol production rate under anaerobic conditions with excess glucose) showed a strong positive co rrelation with specific growth rate. The fermentative capacity observed at the end of the process (mu = 0.01 h(-1)) was only half that observed during the exponential-feed phase (mu = 0.18 h(-1)). During fed-batch cultivation , activities of glycolytic enzymes, pyruvate decarboxylase and alcohol dehy drogenase in cell extracts did not exhibit marked changes. This suggests th at changes of fermentative capacity during fed-batch cultivation were not p rimarily caused by regulation of the synthesis of glycolytic enzymes. (C) 2 000 John Wiley & Sons, Inc.