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    Glycerol production by four common grape moulds, Aspergillus, Botrytis, Penicillium and Rhizopus : a thesis presented in fulfilment of the requirements for Master of Science in Microbiology at Massey University
    (Massey University, 1989) Ravji, Rajesh Govind
    The production of glycerol by the grape moulds Aspergillus niger, Penicillium italicum, Rhizopus nigricans and Botrytis cinerea growing in juice from Chasselas and Black Hamburg grapes was examined. Juice from both free-run and homogenized whole grapes was filter sterilized and inoculated with single pure cultures of the moulds above. The four juice types were incubated at 25° C for 26 to 29 days. The inoculated juices were incubated in different air relations and during the 26 to 29 day incubation period, samples were taken periodically for the analysis of glycerol, glucose and fructose by HPLC. After 26 to 29 days, the moulds were harvested by filtration so that dry mycelial weights could be obtained. Large differences in glycerol production were noted among the grape moulds. Under similar conditions of cultivation in Chasselas juice, R. nigricans and B. cinerea produced significantly more glycerol than A. niger and P. italicum. The levels of glycerol never exceeded 0.5g/100mL, whereas all cultures of B. nigricans and li. cinerea exceeded this level after 15 to 18 days of incubation. In Black Hamburg juice glycerol was not detected in cultures of A. niger and P. italicum. The levels of glycerol produced by all the four moulds were lower in Black Hamburg than in Chasselas juice. Overall more sugar was utilized in Black Hamburg juice than in Chasselas juice under similar conditions. B. cinerea utilized the most total sugar in Chasselas juice than all the other moulds, while R- nigricans utilized the most total sugar in Black Hamburg juice than all the other moulds. In Chasselas juice B. cinerea and R- nigricans displayed a preference for glucose over fructose, while in Black Hamburg juice no preference was evident. The pattern of sugar utilization over the incubation period between Chasselas and Black Hamburg juice was markedly different. In Chasselas juice under most cultivation conditions the four moulds utilized glucose and fructose throughout the incubation period, while in Black Hamburg juice there was rapid utilization during the first three days followed by a reduced rate of sugar utilization in the latter stages of incubation. The four moulds differed in their production of mycelial dry weight. These differences were most marked in Chasselas juice where ­B. cinerea, depending on air relations, produced five to seven times more mycelial dry weight than R. nigricans and more than twice the mycelial dry weight produced by A. niger and P. italicum. In Black Hamburg juice B. cinerea produced two to three times more mycelial mass than the other three moulds. At present in the Californian wine industry an HPLC method is currently under investigation, where the level of glycerol in the grape juice is used as an indicator of fungal rot of the grapes. This study has demonstrated that certain grape moulds do not produce the same amount of glycerol and that the level of glycerol is not related to the mycelial growth. Thus this investigation has established that glycerol may not be used as a suitable indicator of fungal rot.
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    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
    (Massey University, 1987) Munster, Rosalind Evelyn Gordon
    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.