Characterization of a unique ethanologenic yeast capable of fermenting galactose

The hexose and pentose sugars common to substrate mixtures used in yeast-catalyzed industrial processes are often affected by carbon catabolite repression, in which the presence of glucose significantly delays the consumption of other sugars. Screening experiments performed with wild-type Saccharomyces cerevisiae strains have identified a strain that exhibits exceptional fermentative performance on galactose, completely exhausting the sugar in significantly less time (6 h) than that typically required by other S. cerevisiae strains tested (10–24 h). This strain was also capable of initiating fermentation at very high concentrations of galactose (≥225 g L−1). Further experiments on mixed hexose sugar substrates (galactose, glucose, and mannose) indicated the absence of conventional catabolite repression in this strain, including a preference for galactose as carbon source and notable delays in the utilization of glucose and mannose. Additionally, the endogenous formation of extracellular glucose was observed during double sugar fermentations of galactose and mannose. Subsequent experiments illustrated differences between the selected strain and a reference strain in UDP-glucose-4-epimerase activity, and suggested other unique molecular properties.