Ethanol fermentation of cheese whey powder solution by repeated fed-batch operation

Ethanol fermentation of cheese whey powder (CWP) solution was realized in a fed-batch operated fermenter using the pure culture of Kluyveromyces marxianus (DSMZ 7239) at different feed CWP concentrations varying between 51 and 408 g l−1. Total sugar content of the feed varied between 25 and 200 g l−1 with a constant flow rate of 0.084 l h−1. Sugar utilization, ethanol formation and biomass growth were investigated as function of the feed sugar concentration in a five-cycle repeated fed-batch operation where the system reached the quasi steady-state. Variations of the growth and the product yield coefficients with the feed sugar concentration were quantified. The growth yield coefficient (YX/S) decreased with increasing feed sugar concentrations. The product yield coefficient (YP/S) was almost constant (0.54 ± 0.02 g EtOH g−1 S) for sugar concentrations between 25 and 150 g l−1, but decreased at the feed sugar concentration of 200 g l−1 due to high osmotic pressure at high sugar concentrations. The highest ethanol concentration (63 g l−1) and the productivity (5.3 g EtOH h−1) were obtained with a 125 g l−1 feed sugar concentration and sugar loading rate of 10.5 g S h−1.