Evaluation of best parameters in simultaneous saccharification and fermentation process for maximum ethanol yield by K. marxianus IMB3

Abstract

Scope and Method of Study:

Statistical design and analysis of media and immobilization parameters for yeast K. marxianus IMB3.

Findings and Conclusions:

A defined medium was constructed for K. marxianus IMB3 with a very similar cost to the original media and a better yield. This media was then screened and optimized to find an minimal media, but significant blocking dictated which parameters could be used in the optimization. The optimized media produced a higher level of ethanol, but only after 72 hours. The increased yield was offset by a lower rate of conversion of glucose to ethanol in the fermentation media. In an SSF the optimized media did not prove to work as well, most probably due to the lower pH of the starting batch fermentation, and the effect of low pH on the cellulase enzyme being used in the reaction. Optimized media with an initial pH adjustment through the addition of a small amount of base, allowed the same overall yield, but still had a lower initial ethanol production. Media composition did not matter for ethanol yield when the yeast cells were immobilized in a calcium alginate matrix. Immobilization decreased the time to completion of a glucose fermentation over that of free cells, but proved to be inadequate in a simultaneous saccharification and fermentation reaction. The best method for increasing ethanol production in an SSF in this study, was the use of an acetate buffer instead of a citrate buffer for pH control. The use of an acetate buffer (50 mM, pH = 5) served to decrease cell mass (g/L) and increase ethanol production in glucose fermentations. SSF using an acetate buffer also showed higher ethanol yields. No acetate production was observed in any of the flasks using an acetate buffer.