Browsing by Author "Snyman, Carla"

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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.
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    Optimised extraction and preliminary characterisation of mannoproteins from non-saccharomyceswine yeasts
    (MDPI, 2021-04-22) Snyman, Carla; Nguela, Julie Mekoue; Sieczkowski, Nathalie; Marangon, Matteo; Divol, Benoit
    The exogenous application of yeast-derived mannoproteins presents many opportunities for the improvement of wine technological and oenological properties. Their isolation from the cell wall of Saccharomycescerevisiae has been well studied. However, investigations into the efficiency of extraction methods from non-Saccharomyces yeasts are necessary to explore the heterogeneity in structure and composition that varies between yeast species, which may influence wine properties such as clarity and mouthfeel. In this study, nine yeast strains were screened for cell wall mannoprotein content using fluorescence microscopy techniques. Four species were subsequently exposed to a combination of mechanical and enzymatic extraction methods to optimize mannoprotein yield. Yeast cells subjected to 4 min of ultrasound treatment applied at 80% of the maximum possible amplitude with a 50% duty cycle, followed by an enzymatic treatment of 4000 U lyticase per g dry cells weight, showed the highest mannoprotein-rich yield from all species. Furthermore, preliminary evaluation of the obtained extracts revealed differences in carbohydrate/protein ratios between species and with increased enzyme incubation time. The results obtained in this study form an important step towards further characterization of extraction treatment impact and yeast species effect on the isolated mannoproteins, and their subsequent influence on wine properties.