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

High‐cell‐density fed‐batch processes for bakers' yeast production will involve a low‐average‐specific growth rate due to the limited oxygen‐transfer capacity of industrial bioreactors. The relationship between specific growth rate and fermentative capacity was investigated in aerobic, sucrose‐limited fed‐batch cultures of an industrial bakers' yeast strain. Using a defined mineral medium, biomass concentrations of 130 g dry weight/L were reproducibly attained. After an initial exponential‐feed phase (μ = 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 metabolism was fully respiratory, with a biomass yield of 0.5 g biomass/g sucrose−1. Fermentative capacity (assayed off‐line as ethanol production rate under anaerobic conditions with excess glucose) showed a strong positive correlation with specific growth rate. The fermentative capacity observed at the end of the process (μ = 0.01 h−1) was only half that observed during the exponential‐feed phase (μ = 0.18 h−1). During fed‐batch cultivation, activities of glycolytic enzymes, pyruvate decarboxylase and alcohol dehydrogenase in cell extracts did not exhibit marked changes. This suggests that changes of fermentative capacity during fed‐batch cultivation were not primarily caused by regulation of the synthesis of glycolytic enzymes. © 2000 John Wiley & Sons, Inc. Biotechnol Bioeng 68: 517–523, 2000.