ABSTRACT

Divalent cations (Mg2 , Zn2Ca2 ) are known to directly influence fermentative metabolism in yeast and consequently increase ethanol yield. The bioavailability of these catiotis and the correct concentration combination directly influence sugar metabolism by yeast in industrial fermentation. Therefore, the optimum combination is vital for maximum production of ethanol. Saccharomyces pastorianus-based fermentations were conducted in batch cult ures to optimize the effect of these divalent cations on ethanol production using the central composite rotatable response surface design. Fermentations for ethanol production ere carried out for a period of 0-120h with free and immobilized yeast cells. Immobilization as by entrapment in calcium alginate gel. Maximum ethanol production (11.12%v/v 'as obtained with variable divalent cation combination of 64, 0.48 and 30mg/I in the free \ east fermentation medium. This maximum value increased to 12.53%v/v when the yeast cells were immobilized in the same medium and at the same period (96h) of ferrnentition. Minimum ethanol production of9.21%v/v was obtained with the immobilized yeast cells in variable cationic combination of 64, 0.48, and 76mg/i fermentation medium. This value further reduced (7.53%v/v) in the free yeast fermentation medium. Ethanol production, whether by free or immobilized yeast cells, was higher at maximum value than the control (10.07%v/v) devoid of divalent cation. The cationic combinations with high concentration of Mg2and Zn2and iowconcentration Ca2favoured ethanol production. This research showed 96h as critical fermentation period at which maximum ethanol is produced irrespective of the freedom of the yeast cells. It also showed that at constant concentrations of Mg2and Zn2(64 and 0.48mg/I), increase in Ca2from 30 to 76mg/i brought the ethanol production to a minimum values.