Repeated batch ethanolic fermentation of very high gravity medium by immobilized Saccharomyces cerevisiae

The main objective of this study was to evaluate the effect of yeast immobilization on ethanolic fermentation of very high gravity (VHG) medium and to determine the concentrations of yeast storage carbohydrates like trehalose and glycogen during the process. Repeated batch ethanolic fermentation of VHG medium was carried out using Saccharomyces cerevisiae immobilized separately within Ca-alginate and k-carrageenan polymers. Immobilization yields (Y-I) were between 80 and 90% and ethanol yields (Y-P/S) were more than 0.41 with both carriers. An average fermentation efficiency of nearly 70% was observed in 48 h fermentation batches. Compared to free cells, a reduction of more than 50% in the accumulated trehalose, and a twofold increase in intracellular glycogen levels were observed in immobilized yeast cells at 24 and 48 h of fermentation, respectively, with both carriers. The increased viability (up to four-fold higher) upon 18% ethanol treatment for 2 h, and the sustained viability over four successive batches of immobilized cells showed the protective nature of the polymer carriers. The chemical nature of the carriers was not found to have any adverse effect on ethanol yields. Application of immobilized yeast in porous matrices may serve as a feasible and better technique for ethanol production, at both pilot and industrial scale.