Kinetics of Saccharomyces cerevisiae Fermentation under Metal Ions Stress during Ethanol Production

Abstract

This research investigated the effects of inorganic compounds or metal ions (calcium ion, Ca²⁺; potassium ion, K⁺; magnesium ion, Mg²⁺) on ethanol production efficiency invertase, an enzyme produced by Saccharomyces cerevisiae, in sucrose solution, which was the substrate for yeast fermentation. The results showed that all metal ions (concentration 0.20 and 0.60 % (w/v)) acted as inhibitors on invertase activity in the order Ca²⁺ > K⁺ > Mg²⁺. Subsequently, these ions inhibited sugar conversion, reducing sucrose utilization and less glucose and fructose consumption based on the high content of remaining sugars in the culture medium. The reduction of the substrate was due to the consumption and an increased growth rate of S. cerevisiae, which all resulted in low efficiency of ethanol production and an increase in glycerol content. The glycerol content was increased due to yeast cells' developed mechanism or adaptation to enhance cell survival following metal ion contamination, especially from Ca²⁺ and K⁺; furthermore, the glycerol content significantly increased during the changed conditions, such as when the sugars were nearly all consumed. The kinetic parameters such as specific growth rate (µ^-1), substrate consumption rate (Qs), and ethanol production of the research work were also undertaken. In conclusion, metal ion contamination in the sucrose substrate of yeast fermentation resulted in low efficiency of ethanol production, specific growth rate, and substrate consumption rate decrease with the Ca²⁺ ion (concentration 0.20 - 0.60 % (w/v)) acting more harshly as an inhibitor of ethanol production than the other ions, particularly where there was a high concentration of contamination.