AbstractThis 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|
|Sponsors||Sugarcane Research Institute of the Ministry of Plantation Industries|
|Supervisor||Graeme Walker (Supervisor)|