Zinc is an essential trace element in biological systems. For example, it activates many enzymes, acts as a cellular membrane stabiliser and is a constituent of the zinc finger proteins that bind specific DNA sequences, playing a critical role in gene expression and genome modification. The present study has focused on the influence of zinc on cell physiology of the yeast Saccharomyces cerevisiae. Zinc uptake by industrial strains of S. cerevisiae, including brewing strains, and the subsequent utilisation of this key metal during fermentation was studied. Yeast strains take up most of the available zinc very quickly following pitching, with some strains increasing their cellular zinc ten-fold at the onset of fermentation. Zinc content of yeast cell walls was found to remain constant during fermentation and zinc was localised in the vacuole. These and other findings indicated that most of the initial zinc uptake was metabolism-dependent, rather than via a cell surface biosorption phenomenon. After initial periods of cellular zinc accumulation, and during the course of the subsequent fermentation, zinc became virtually undetectable in wort. As yeast cells grew during this period, zinc was distributed to daughter cells at cell division and this effectively lowered their individual cellular zinc concentration. Depending on the extent of yeast growth during fermentation, this may result in the generation of zinc-depleted biomass at the time of yeast harvesting. A brewing yeast strain was also investigated following exposure to stresses typically encountered during the brewing process and a relationship existed between zinc loss from cells and decrease in cell viability during various environmental insults. In order to extend initial laboratory studies to industrial scale, yeast fermentative performance was investigated under various zinc concentrations, firstly in 1 L conical vessels and subsequently in 200 L brewing fermenters. A cell physiological study was also made of zinc-limitation in the haploid reference strain S. cerevisiae CEN.PK113-7D in chemostat continuous culture cultivations, in order to pave the way for more in-depth studies aimed at identifying zinc-responsive molecular biomarkers in yeasts. The ultimate aim of such a study was to provide a valuable and rapid way to determine the status of cellular zinc in yeast during fermentation. The consequences of this for efficient industrial processes are discussed including implications for brewing fermentation optimisation based on control of wort zinc bioavailability. Overall, this research has provided new insights into the influence of zinc on yeast cell physiology and the fundamental information gained has practical implications for yeast-based biotechnologies.
|Date of Award||Mar 2006|
|Sponsors||Heineken Supply Chain, European Social Fund (ESF) & American Society of Brewing Chemists (ABSC)|
|Supervisor||Graeme Walker (Supervisor)|