KINETICS OF GROWTH AND SUGAR CONSUMPTION IN YEASTS

An overview is presented of the steady- and transient state kinetics o f growth and formation of metabolic byproducts in yeasts. Saccharomyce s cerevisiae is strongly inclined to perform alcoholic fermentation. E ven under fully aerobic conditions, ethanol is produced by this yeast when sugars are present in excess. This so-called 'Crabtree effect' pr obably results from a multiplicity of factors, including the mode of s ugar transport and the regulation of enzyme activities involved in res piration and alcoholic fermentation. The Crabtree effect in S. cerevis iae is not caused by an intrinsic inability to adjust its respiratory activity to high glycolytic fluxes. Under certain cultivation conditio ns, for example during growth in the presence of weak organic acids, v ery high respiration rates can be achieved by this yeast. S. cerevisia e is an exceptional yeast since, in contrast to most other species tha t are able to perform alcoholic fermentation, it can grow under strict ly anaerobic conditions. 'Non-Saccharomyces' yeasts require a growth-l imiting supply of oxygen (i.e. oxygen-limited growth conditions) to tr igger alcoholic fermentation. However, complete absence of oxygen resu lts in cessation of growth and therefore, ultimately, of alcoholic fer mentation. Since it is very difficult to reproducibly achieve the righ t oxygen dosage in large-scale fermentations, non-Saccharomyces yeasts are therefore not suitable for large-scale alcoholic fermentation of sugar-containing waste streams. In these yeasts, alcoholic fermentatio n is also dependent on the type of sugar. For example, the facultative ly fermentative yeast Candida utilis does not ferment maltose, not eve n under oxygen-limited growth conditions, although this disaccharide s upports rapid oxidative growth.