USE OF RESPONSE SURFACES TO INVESTIGATE METAL-ION INTERACTIONS IN YEAST FERMENTATIONS

The metal cations K+, Mg2+, Ca2+, and Zn2+ are known to directly influ ence fermentative metabolism in yeast, and therefore knowledge of thei r interactions is essential to manipulate their availability in indust rial fermentations to optimal levels. Defined media experimental ferme ntations were designed to mimic high intermediate, and low levels of K +, Mg2+, and Ca2+ previously reported in sugarcane molasses and Mg2+, Ca and Zn2+ previously reported in malt wort. Subsequent analysis of f ermentations revealed that the yeast (distillers strain of Saccharomyc es cerevisiae) produced higher levels of ethanol in the. presence of h igher levels of Mg2+ in synthetic molasses and malt wort. Analysis of variance showed that yeast fermentation performance depended on comple x interactions among the metal cations studied. For simulated molasses fermentations with fixed levels of Mg2+, ethanol production varied wi th changing levels of Ca2+ and K+ in a predictable way that was well f itted by the quadratic response surface model. Maximum predicted ethan ol yields found from the quadratic response surface model were general ly confirmed by authentic molasses fermentations. in simulated malt wo rt fermentations with fixed levels of Zn2+, ethanol production varied in a predictable way with changing levels of Ca2+ and Mg2+. However, q uadratic response surface model predictions of ethanol yield failed to match results obtained from authentic malt wort fermentations, indica ting significant effects of extraneous factors in wort. Although the r esults from defined media experiments suggest that statistical modelin g could prove a useful tool in predicting yeast fermentation performan ce, further analysis is required of the influence of other components in industrial fermentation media, such as brewers' wort.