ABSTRACT

Divalent cations (Mg2 , Zn2Ca2 ) are known to directly influence fermentative metabolism in yeast and consequently increase ethanol yield. The bioavailability of these cations and the correct concentration combination directly influence sugar metabolism by yeast in indtitrial fermentation. Therefore, the optimum combination is vital for maximum production of ethanol. Saccharonyces pastorianus-based fermentations were conducted in batch culiures 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 was by entrapment in calcium alginate gel. Maximum ethanol production (11.12%v/v was obtained with variable divalent cation combination of 64, 0.48 and 30mg/I in the flee 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 fermentation. 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 aloe 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 (I 0.07%v/v) devoid of divalent cation. The cationic combinations with high concentration 2+ 2+ . 2~ of Mg and Zn and low concentration Ca favoured 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 2-'- 2+ . 2+ of Mg and Zn (64 and 0.48mg/I), increase in Ca from 30 to 76mg/I brought the ethanol production to a minimum values.