Ethanol production from d-lactic acid by lactic acid-assimilating Saccharomyces cerevisiae NAM34-4C

The lactic acid-assimilating yeast Saccharomyces cerevisiae NAM34-4C grew rapidly in minimal d -lactate medium (pH 3.5) at 35°C, compared with minimal l -lactate medium. A laboratory strain, S. cerevisiae S288C, did not grow in either medium at pH 3.5. Strain NAM34-4C produced remarkably high levels of ethanol in YPDL medium at pH 3.5, but not at pH 5.5, when d -lactate was provided as the carbon source. Optimal cultivation conditions for ethanol production from d -lactate by strain NAM34-4C were as follows: shaking speed, 60 rpm; initial pH, 3.0; cultivation temperature, 35°C; yeast extract, 5 g/L; peptone, 10 g/L; and d -lactate, 30 g/L. Under these conditions, strain NAM34-4C produced 2.7 g/L ethanol, which is 18% of the theoretical maximal yield (0.51 × initial d -lactate concentration).