Cheese whey fermentation by Kluyveromyces lactis: ethanol production and beta-galactosidase activity

Cheese whey (CW) is a by-product of dairy industry formed during the coagulation of milk casein that retains about 55% of milk nutrients (4.5-5.0% lactose) and could be used as a potential source of added value compounds. Lactose of CW may be used as a carbon source in different bioprocesses. Among the microorganisms able to metabolize lactose, Kluyveromyces lactis stands out for its ability of metabolizing this disaccharide through the activities of a lactose permease and a β-galactosidase (β-gal). This Generally Recognized as Safe (GRAS) yeast has become an alternative to the traditional S. cerevisiae owing to its industrial potential. In this study batch fermentations of CW, either as such or about 1.5-fold or twice concentrated, by K. lactis were performed in flasks with (180 rpm) or without agitation at 33 ºC to find the most suitable conditions to simultaneously produce ethanol and biomass with high β-galactosidase activity. With agitation, the highest ethanol production parameters, namely concentration of 15.0 g L-1, yields on consumed lactose and produced biomass of 0.47 and 8.60 g g-1 and volumetric productivity of 0.31 g L-1 h-1 were obtained in CW as such, corresponding to 87.4% fermentation efficiency, but β-galactosidase activity was disappointing (449.3-680.0 U g-1). In the attempt to maximize, at the same time, β-galactosidase activity and ethanol production, the best compromise was obtained in static cultures on twice-concentrated CW, where, in spite of a decrease in fermentation efficiency to 63.2%, ethanol concentration, yield on consumed lactose and volumetric productivity achieved 22.2 g L-1, 0.38 g g-1 and 0.31 g L-1 h-1, respectively, while β-galactosidase activity was no less than 2,100 U g-1 throughout the whole fermentation. These results are promising for possible exploitation of CW as an industrial feedstock.