Cassava starch fermentations were conducted in batch cultures to optimize the effect of divalent cations on ethanol production with Saccharomyces pastorianususing the central composite rotatable response surface design. Divalent cations used were magnesium (Mg2+), zinc (Zn2+) and calcium (Ca2+). Maximum ethanol concentration of 11.12% v/v was obtained with cationic concentration combination of 64, 0.48 and 30 mg/l for Mg2+, Zn2+ and Ca2+, respectively, after 96 h of the fermentations. Minimum ethanol concentration of 7.53% (v/v) was obtained at a variable combination of 64, 0.48 and 76 mg/l for Mg2+, Zn2+ and Ca2+, respectively. Thus response surface methodology was used in a central composite design to optimize the process variables of Mg2+, Zn2+, Ca2+ in the fermentation medium, thereby increasing the ethanol production from 10.5% in the control to 11.12%. There were significant linear and quadratic effects of Zn2+ as well as a significant (P ≤ 0.05) negative quadratic effect of Ca2+ on ethanol production, which are confirmed in the response surface plots.
Key words: Ethanol, Saccharomyces pastorianus, cassava starch, hydrolyzates, central composite design, response surface methodology.
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