Abstract

Response surface methodology was adopted in a central composite design to optimize ethanol production from cassava starch hydrolysate medium. Starch hydrolyzate was prepared from TMS 98/0581, a genetically developed cassava mosaic disease-resistant variety. The yeast whole cell, Saccharomyces pastorianus, a lager brewing strain (726 x 10⁶ cells/ml, 98.78% viability) and fungamyl and termamyl (a-amylase enzymes), used for the 120 h fermentation, were immobilized by entrapment in calcium alginate gel. Effects of three divalent cationic concentrations Mg²⁺, Zn²⁺ and Ca²⁺ on ethanol yield were investigated at five variable combinations in 20 experimental runs in accordance with the experimental design. Maximum ethanol concentration of 12.53 %v/v was produced in the 96 h of fermentation when the divalent cationic combination was 64, 0.48 and 30 mg/l (Mg²⁺, Zn²⁺ and Ca²⁺), respectively.  The study showed that effect of Zn²⁺ on ethanol yield was significantly (P≤0.05) quadratic.

Keywords: Ethanol, immobilization, Saccharomyces pastorianus, divalent cations, optimization, response surface methodology, cassava mosaic disease.