Abstract

The effect of oxygen transfer rate on the growth rate, the specific ethanol productivity and the volumetric ethanol productivity of the thermotolerant yeast Kluyveromyces marxianus IMB3 in a continous ethanol fermentation was investigated. Under anaerobic conditions the strain grew poorly, with a maximum biomass concentration of 0.4 g L(-1). A small increase in oxygen transfer to the culture led to an increase in both the specific growth rate and the specific ethanol productivity. Further increases in the oxygen transfer rate to the culture caused a linear increase in the growth rate but a rapid decrease in the specific ethanol productivity. The specific ethanol productivity decreased to a constant minimum value of 0.25 (+/-0.05) g ethanol (g cells h)(-1). An initial maximum volumetric ethanol productivity, 0.14 g L(-1) h(-1) was achieved at an oxygen transfer rate to the culture broth of 3 mmol L(-1) h(-1). The volumetric ethanol productivity increased again as the biomass concentration continued to rise although the specific ethanol productivity had levelled, reaching a maximum of 0.44 g L(-1) h(-1). The biomass concentration in a non-aerated chemostat was estimated using the cell yield on oxygen and the oxygen transfer rate to the broth.