Masters Degrees (Institute for Wine Biotechnology)

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Browsing Masters Degrees (Institute for Wine Biotechnology) by Subject "Cell-cell interaction"

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    Interaction of multiple yeast species during fermentation
    (Stellenbosch : Stellenbosch University, 2015-04) Luyt, Natasha Alethea; Bauer, Florian; Divol, Benoit; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: The use of non-Saccharomyces yeasts together with the yeast S. cerevisiae in multistarter wine fermentations has emerged as a useful tool to modulate wine aroma and/or to decrease the concentration of undesirable compounds. However, upon inoculation, these yeast species do not co-exist passively, but interact in various ways. While competition for nutrients and the excretion of killer toxins in an antagonistic relationship are obvious and well established types of interactions, some studies have suggested the existence of other forms of cellular or molecular interactions. One of these includes physical cell-cell contact and to our knowledge, only one previous study has confirmed its existence in wine yeasts. Yeast interactions are also influenced by other factors, such as ethanol concentration, however some studies have highlighted the role that dissolved oxygen plays on the survival of non-Saccharomyces yeasts and their ability to compete for space with S. cerevisiae and little research has focused on this. This study aimed to investigate the occurrence of a physical cell-cell and/or metabolic interaction between S. cerevisiae and L. thermotolerans in mixed culture fermentations of synthetic grape must. For this purpose, fermentations in a Double Compartment Bioreactor (DCB) which separates yeast population through the use of a membrane were compared to mixed fermentations in the absence of the membrane, using the same reactor. Furthermore, the impact of oxygen supply on yeast behaviour was also assessed. Following mixed culture fermentations in a DCB, it was observed that the presence of S. cerevisiae led to a significant decline in viability in L. thermotolerans. This decline was significantly less prominent in mixed cultures where the cells were in indirect contact. Together, the data provided evidence for both cell-cell and metabolic interactions whereby S. cerevisiae had a strong negative influence on the growth of L. thermotolerans. However, it was also observed that L. thermotolerans had some negative impact on the growth of S. cerevisiae, leading to a reduction in biomass (when in indirect contact) and a reduced maximum CFU/mL compared to pure cultures. The data also suggest that direct physical contact may increase the production of glycerol and propanol, but this needs further investigation. By decreasing the frequency at which oxygen pulses were provided, a reduction in biomass and increase in fermentation duration was observed for all fermentations. However, this effect was somewhat reduced in mixed cultures. Here, no impact on fermentation duration was observed and the decrease in biomass was less compared to pure cultures. The impact of these oxygen pulses was also greater on L. thermotolerans. In the latter yeast’s pure culture a slight increase in glycerol was observed when less oxygen was provided and in general there appeared to be no impact on acetic acid production. Furthermore, there was little or no impact on volatile production, however, more repeats might reveal different results and therefore more research is needed to confirm these results. To our knowledge, this is the first study of its kind to confirm a physical cell-cell interaction between the yeast pair S. cerevisiae and L. thermotolerans.