Use of response surfaces to investigate metal ion interactions in yeast fermentations.
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The metal cations K(^+), Mg(^2+), Ca(^2+), and Zn(^2+) are known to directly influence fermentative metabolism in yeast, and therefore knowledge of their interactions is essential to manipulate their availability in industrial fermentations to optimal levels. Defined media experimental fermentations were designed to mimic high, intermediate, and low levels of K(^+), Mg(^2+), and Ca(^2+) previously reported in sugarcane molasses and Mg(^2+), Ca(^2+), and Zn(^2+) previously reported in malt wort. Subsequent analysis of fermentations revealed that the yeast (distillers strain of Saccharomyces cerevisiae) produced higher levels of ethanol in the presence of higher levels of Mg(^2+) in synthetic molasses and malt wort. Analysis of variance showed that yeast fermentation performance depended on complex interactions among the metal cations studied. For simulated molasses fermentations with fixed levels of Mg(^2+), ethanol production varied with changing levels of Ca(^2+) and K(^+) in a predictable way that was well fitted by the quadratic response surface model. Maximum predicted ethanol yields found from the quadratic response surface model were generally confirmed by authentic molasses fermentations. In simulated malt wort fermentations with fixed levels of Zn(^2+), ethanol production varied in a predictable way with changing levels of Ca(^2+) and Mg(^2+). However, quadratic response surface model predictions of ethanol yield failed to match results obtained from authentic malt wort fermentations, indicating significant effects of extraneous factors in wort. Although the results from defined media experiments suggest that statistical modeling could prove a useful tool in predicting yeast fermentation performance, further analysis is required of the influence of other components in industrial fermentation media, such as brewers' wort.