Simultaneous effect of divalent cation in hydrolyzed cassava starch medium used by immobilized yeast for ethanol production

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 (-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.