Influence of sodium chloride on wine yeast physiology and fermentation performance
This thesis concerns research into the influence of salt on physiology of the yeast, Saccharomyces cerevisiae. Specifically, the work focused on how sodium chloride affected the growth, viability and fermentation performance in industrial (winemaking) strains of this yeast in both laboratory-scale and industrial-scale experiments. Comparative fermentations were also conducted with selected non-Saccharomyces yeasts that are of relevance in enology. One of the main findings of the research presented involved the influence of salt "preconditioning” of yeasts which represents a method of pre-culturing cells in the presence of salt in an attempt to improve subsequent fermentation performance. Such an approach resulted in preconditioned yeasts having an improved capability to ferment high-sugar containing media with increased cell viability and with elevated levels of produced ethanol. Salt-preconditioning was most likely influencing the stress-tolerance of yeasts by inducing the synthesis of key metabolites such as trehalose and glycerol which act to improve cells’ ability to withstand osmostress and ethanol toxicity. The industrial-scale trials using salt-preconditioned yeasts verified the benefit of the physiological engineering approach to practical winemaking fermentations. Benefits were also observed in a specialized fermentation system (WITY produced by the first letters of the words Wine, Immobilization, Tower, and Yeast) that utilized immobilized yeast. Overall, this research has demonstrated that a relatively simple method designed to physiologically adapt yeast cells - by salt-preconditioning - can have distinct advantages for alcohol fermentation processes.
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