Department of Viticulture and Oenology

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Browsing Department of Viticulture and Oenology by browse.metadata.advisor "Benoit, Divol"

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    Impact of the protease-secreting yeast Metschnikowia pulcherrima IWBT Y1123 on wine properties and response of protease production to nitrogen sources
    (Stellenbosch : Stellenbosch University, 2019-04) Snyman, Carla; Benoit, Divol; Theron, Louwrens; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: Yeast of oenological origin belong to various genera and harbour unique metabolic properties that may significantly impact aspects of wine processing and composition. Extracellular aspartic proteases, and their secretion by wine yeasts, have received much attention due to their protein degradative ability as a possible solution to the quality problem of wine turbidity. This fault is generally addressed by wine-makers with the use of bentonite, but this fining agent is associated with various technological and organoleptic issues. A key contributing factor to haze formation in wine is the presence of heat unstable grape proteins, and their removal by proteases therefore presents an attractive alternative to the use of bentonite. The yeast Metschnikowia pulcherrima IWBT Y1123 has been isolated from grape juice and secretes an extracellular aspartic protease named MpAPr1. This enzyme demonstrated activity against grape proteins and reduced wine haze-forming potential under winemaking conditions after 48 h incubation with a purified exogenous MpAPr1 preparation. However, inoculation of M. pulcherrima IWBT Y1123 as a co-starter culture to wine fermentation for the secretion of MpAPr1 directly into the matrix presents a possible time- and cost-effective alternative that would eliminate the need for enzyme purification steps. Nevertheless, understanding protease regulation by environmental conditions and relating protease secretion and activity to its impact on wine properties could prove useful when considering inoculation strategies for this yeast. This study sought to establish the potential for developing co-inoculation strategies of M. pulcherrima IWBT Y1123 with the efficient fermenter Saccharomyces cerevisiae, by assessing the impact of nitrogen sources and protein availability on protease production and activity by M. pulcherrima IWBT Y1123, as well as the impact of protease production and activity in grape juice inoculated with M. pulcherrima IWBT Y1123 on grape protein content, haze formation potential and wine volatile aroma profile. Protease production was shown to be subject to nitrogen catabolite repression in the presence of preferred sources of nitrogen, as well as induction by proteins. Upon inoculation into grape juice, an up-regulation of MpAPr1 gene expression could be observed, as well as protease production and activity. With regard to the impact of M. pulcherrima IWBT Y1123 co-inoculation with S. cerevisiae on wine properties at the end of fermentation, total protein content and haze forming potential were lower compared to controls and the volatile profile was altered. Future work should focus on enhancing protease production by M. pulcherrima IWBT Y1123 to improve its viability as a commercial protease-producing yeast strain. Nevertheless, the results generated in this study contribute to knowledge in both fundamental and biotechnological aspects of protease secretion by M. pulcherrima IWBT Y1123.