Role of metal ions in fermentative metabolism of yeast

Abstract

This thesis concerns the influence of several metal cations on the growth and fermentative metabolism of industrial strains of yeast. Experiments were conducted in laboratory growth media (both synthetic and complex) and industrial fermentation media (molasses and malt wort) in order to investigate influences of K⁺, Mg²⁺, Ca²⁺ and Zn²⁺ on ethanol production by several strains of the yeast species, Saccharomyces cerevisiae and Kluyveromyces marxianus and Schizosaccharomyces pombe. Mg²⁺ supplementation studies in the media revealed positive effects on cell growth and ethanol production o f all yeasts studied. Although differences existed between yeast species with regard to fermentation, the yeast Saccharomyces cerevisiae, distillers strain DCL'M', was found to be the highest ethanol producer in all media studied.

Defined media experiments were designed which mimicked high, intermediate and low K⁺, Ca²⁺ and Mg²⁺ levels reported in sugarcane molasses and Ca²⁺, Mg²⁺ and Zn²⁺ in malt wort in order to investigate their interactive effects on alcoholic fermentation. Subsequent analysis of fermentations revealed that the yeast (distillers strain of S.cerevisiae) produced higher alcohol levels in the presence of higher levels of K⁺ and Mg²⁺ in synthetic molasses and Mg²⁺ and Zn²⁺ in synthetic malt wort. Analysis of variance showed that all two-factor and three-factor interactions were significant but the main effect of Ca²⁺ was not significant at higher levels of Mg²⁺ in synthetic molasses wort. Although there were significant interactions in synthetic malt wort between Zn²⁺*Ca²⁺ (* = interaction) and Ca²⁺*Mg²⁺*Zn²⁺, neither Ca²⁺ nor Mg²⁺*Zn²⁺ exhibited significant interactions. The estimated quadratic response surface for ethanol production in molasses in the presence of low, intermediate and high levels of magnesium (16, 70 and 270 ppm, respectively) fitted well (r² = 81.9, 83.3 and 83.5, respectively), and the overall regression model was statistically significant (each p<0.01). Maximum predicted ethanol yields were found from the response surface at respective combinations of K⁺/Mg²⁺/Ca²⁺, at 2159/270/1118 ppm, 5102/70/509 ppm and 7231/16/1006ppm. The predicted values were 7.75, 7.49 and 5.50 (%v/v), respectively and confirmatory experimental yields (7.88, 7.08 and 5.63 (%v/v), respectively) were within their 95% prediction interval. Similarly for malt wort fermentations, models showed high coefficients o f determination (r² = 86.3, 81.9 and 81.9) under high, intermediate and low levels of Zn²⁺, respectively. It is therefore suggested that such statistical modelling could prove a useful tool in predicting ethanol yields from fermentation media with known levels of K⁺, Mg²⁺ and Ca²⁺ and Zn²⁺. The studies on the influence of Mg²⁺ on ethanol tolerance in yeast revealed that Mg²⁺ supplemented yeast (Saccharomyces cerevisiae and Kluvyromyces marxianus) maintained higher cell viabilities, produced more ethanol and withstood higher levels of added ethanol than the unsupplemented yeast.

It is concluded that the experimental findings reported in this thesis, particularly with regard to the beneficial effects of Mg²⁺ on yeast fermentation and ethanol tolerance, are directly pertinent to yeast biotechnologies concerned with the commercial production of ethanol.

Date of Award	Sept 1996
Original language	English
Sponsors	Sugarcane Research Institute of the Ministry of Plantation Industries
Supervisor	Graeme Walker (Supervisor)