Glycerol production by various strains of Saccharomyces cerevisiae : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Microbiology at Massey University
The influence of yeast strain, fermentation procedure and media on cell growth and the production of glycerol and ethanol was studied. Two fermentation procedures were compared (a) fermentation at a constant temperature of 15°C and (b) fermentation at higher temperatures (15–20°C) maintaining a constant rate of sugar utilization. Three wine-making yeasts and three high glycerol producing hybrid yeasts were fermented on two types of grape juice and a synthetic [control] media. The effect of the fermentation procedure on glycerol, ethanol production and cell growth was variable and appeared to depend on the yeast strain. Comparison of the yeast strains showed glycerol production to vary considerably depending on the yeast this effect was also dependent on the media. The yeast strain is important for maximum fermentation efficiency in a specific grape juice. Selective hybridisation of pure culture wine yeasts was employed to develop yeast strains capable of maximum glycerol yield, without jeopardising ethanol production in Muller Thurgau and in Chenin Blanc grape juices. Improved yields were achieved, but those yeasts selected for fermentation in one type of grape juice did not give outstanding yields when fermented in the other type of grape juice. This suggests that for wine-making it is possible to tailor yeasts for fermentation in specific grape juices. The addition of sulphur dioxide [0–300 ppm] and its influence on glycerol and ethanol production was studied using a wine-making yeast and a high glycerol producing hybrid. The effect was strain dependent and as expected, the addition of sulphur dioxide to the wine-making yeast showed enhanced glycerol production and depressed ethanol production. However, the converse was apparent with the high glycerol producing hybrid. The addition of glycerol to the media prior to fermentation at levels of 0 to 20 g/1 was tested in an attempt to simulate the conditions of grapes attacked by the fungus Botrytis cinerea [noble rot]. No inhibition or stimulation of glycerol or ethanol production was apparent by either the wine-making yeast or the high glycerol producing hybrid yeast tested.