Comparison of Alcoholic Fermentation Performance of the Free and Immobilized Yeast on Water Hyacinth Stem Pieces in Medium with Different Glucose Contents

Ethanol fermentation with Saccharomyces cerevisiae cells was performed in medium with different glucose concentrations. As the glucose content augmented from 200 to 250 g/L, the growth of the immobilized cells did not change while that of the free cells was reduced. At higher glucose concentration (300, 350, and 400 g/L), the cell proliferation significantly decreased and the residual sugar level sharply augmented for both the immobilized and free yeast. The specific growth rate of the immobilized cells was 27–65 % higher than that of the free cells, and the final ethanol concentration in the immobilized yeast cultures was 9.7–18.5 % higher than that in the free yeast cultures. However, the immobilized yeast demonstrated similar or slightly lower ethanol yield in comparison with the free yeast. High fermentation rate of the immobilized yeast was associated with low unsaturation degree of fatty acids in cellular membrane. Adsorption of S. cerevisiae cells on water hyacinth stem pieces in the nutritional medium decreased the unsaturation degree of membrane lipid and the immobilized yeast always exhibited lower unsaturation degree of membrane lipid than the free yeast in ethanol fermentation.