Ethanol fermentation of sugar beet thick juice diluted with crude cheese whey by the flex yeast Kluyveromyces marxianus KD-15

Kluyveromyces marxianus KD-15, called flex yeast, is a strain that is insensitive to catabolite repression and has the capacity to produce ethanol efficiently from a mixture of beet molasses and whey powder. When a fermentation test was conducted in 50 mL of a medium containing 200 mg mL−1 of sugar as sugar beet thick juice diluted with an arbitrary amount of crude whey, strain KD-15 produced over 99 mg mL−1 ethanol in all the media tested, and ethanol formation decreased in proportion to the volume of whey by K. marxianus NBRC 1963, the parental strain of KD-15, and Saccharomyces cerevisiae NBRC 0224, the reference strain for conventional ethanol production. Fermentation of thick juice diluted with whey alone by strain KD-15 at 30 °C initially proceeded slower than that at 33 °C–37 °C but finally bore the highest level of ethanol. The maximum ethanol concentration obtained in 1.5 L of a medium using a 2-L fermentor was elevated by aeration of 15–50 mL min−1and reduced by that in excess of 100 mL min−1. Under optimized conditions in 72 h, strain KD-15 converted all of the sugars derived from thick juice and whey to ethanol at 102 mg mL−1, corresponding to 92.9% of the theoretical yield.