The role of selected metal ions in the growth and physiology of wine yeasts
Wine production is a complex biochemical process carried out by the main protagonist, the yeast Saccharomyces cerevisiae and its related species. The industrial processes leading to the production of alcoholic beverages, e.g. wine, are diverse and complex and yeast, during these processes, are subjected to a variety of conditions, not least a range of environmental stresses. This study investigated various aspects of yeast cell physiology in relation to ionic influences. The roles of magnesium and calcium in cell growth and fermentation, commercial biomass production and environmental stress were considered. Elevation of magnesium levels in the growth medium improved yeast cell viability, growth and fermentation parameters of Sacch. cerevisiae and Torulaspora delbrueckii (wine strains) in complex, semi-synthetic and minimal media. Magnesium was also found to alleviate the problems associated with agglomeration in active dried yeast (ADY) production. The phenomenon of agglomeration was identified to be a consequence of a combination of protein (a,β & y proteins) and ionic (Ca2+ induced) influences and elevation of magnesium levels (5/10-fold) of the cultivation medium resulted in reduction of grit levels by 40-60%. Nutrient limitation (organic and inorganic) resulted in induction of a stress response in yeast. Metal ion related stress, either by removal or limitation of essential ions provoked stress protein production, particularly of 46 and 26 kDa proteins. Heat (42°C) and ethanol (10%) stress induced a wide range of stress proteins which were subsequently repressed on elevation of magnesium in the growth medium above 10mM. These Mg2+ levels improved cell viability and reduced gross cell surface damage of yeast on exposure to stress. A protective role of magnesium in the yeast stress response is thus proposed. Magnesium is believed to counteract the stress response with regard to heat and ethanol, by offering physiological protection to yeast cells rather than having a role as a repair mechanism, as is the case with heat shock proteins. It is hypothesised that the mode of action is stabilisation of cell membranes by Mg2+.
Networked Digital Library of Theses and Dissertations