Institute for Wine Biotechnology

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Browsing Institute for Wine Biotechnology by Author "Beukes, Louisa"

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    Impact of complex yeast nutrient products on selected non Saccharomyces yeasts
    (Stellenbosch : Stellenbosch University, 2020-03-31) Beukes, Louisa; Jolly, Neil; Divol, Benoit; Bauer, Florian; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: In recent years there has been a growing interest in non-Saccharomyces yeasts for winemaking due to their ability to produce more complex wines. These yeasts, considered weak fermenters, are used in combination with Saccharomyces cerevisiae and compete for nutrients such as nitrogen. Therefore, it is important for the winemaker to know what nutrients may be insufficient so that corrective action can be taken. Yeast assimilable nitrogen (YAN), a growth limiting resource naturally occurring in grape must, is important for yeast metabolism as well as for production of desirable aromatic compounds. When YAN is deficient it can lead to slow or stuck fermentations and production of undesirable compounds. Thus, to ensure a complete alcoholic fermentation and desirable aroma profile, nitrogen supplementation is required. Traditionally, ammonium salts are added as a nitrogen supplement, however, recently several complex yeast nutrients have also become commercially available. These yeast nutrients are yet to be investigated for fermentation with non-Saccharomyces yeasts. This study investigated the impact of eight complex commercial yeast nutrients on three commercial non-Saccharomyces yeasts (Torulaspora delbrueckii Biodiva™ TD291, Pichia kluyveri Viniflora® Frootzen™ and Metschnikowia pulcherrima Flavia® MP346). Fermentations were carried out with single yeasts or combined with S. cerevisiae in sequential fermentations in synthetic grape must. The M. pulcherrima sequential fermentation was repeated in Chenin blanc grape must. For the single yeast fermentations, it appeared that the nutrients had a greater effect on the onset of fermentation than on the growth of the yeasts and that one nutrient (nutrient treatment Y2) was preferred by all the yeasts. This is the first time that nitrogen supplementation at the same level but with different content was investigated for non-Saccharomyces wine yeast sequential fermentations. The ability of non-Saccharomyces yeasts to persist in sequential fermentations could be improved with nutrient selection. Further investigations with M. pulcherrima sequential fermentations in Chenin blanc must found clear differences for the two different matrices. Although synthetic must is a defined medium that reduces the risk of unknown variables, it is not a true representation of how these nutrients can influence non-Saccharomyces yeasts in real grape must. Nutrient selection can also increase desirable esters and influence the sensory properties of wine; however, this should be further investigated and confirmed through sensory evaluation. This study improved the current knowledge of non-Saccharomyces yeasts and their utilisation of complex yeast nutrients. It demonstrated that nutrient selection can improve non-Saccharomyces yeast implantation as well as improve production of desirable volatiles.