Browsing by Author "De Koker, Simone"

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    Nitrogen utilisation of selected non-Saccharomyces yeasts and the impact on volatile compound production
    (Stellenbosch : Stellenbosch University, 2015-12) De Koker, Simone; Divol, Benoit; Bauer, Florian; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: During fermentation, nitrogenous compounds serve as nutrients for the yeasts, which enable their growth, functioning and maintenance of the yeasts cells. From a winemaking perspective, a certain amount of nitrogen is required for the yeasts in order to avoid sluggish or stuck fermentation. Moreover, nitrogen metabolism leads to the production of aroma compounds such as higher alcohols, fatty acids and esters which contribute positively to overall sensory characteristics of wine. Nitrogen metabolism (uptake of ammonium and amino acids) have been extensively studied in Saccharomyces cerevisiae. Nonetheless, the fairly great variances observed between strains in terms of preference for certain nitrogen sources and metabolism thereof are not so well understood. Additionally, these mechanisms nitrogen metabolism of non- Saccharomyces yeasts are even vaguer and simply assumed to be globally similar to those of S. cerevisiae. This study aimed to investigate the uptake of nitrogen compounds (ammonium and individual amino acids) by selected non-Saccharomyces yeasts (Lachancea thermotolerans IWBT Y1240, Torulaspora delbrueckii Biodiva TD291, Pichia kluyveri FrootZen, Metschnikowia pulcherrima IWBT Y1123 and Metschnikowia pulcherrima Flavia) to assess the impact of fermentation kinetics and the production of aroma compounds during sequential fermentations with S. cerevisiae under different initial YAN concentrations, with 300 mg/L, 150 mg/L and 75 mg/L, respectively). Fermentations were performed in a synthetic grape juice medium with pure and sequential fermentations. The data showed that the assimilation of nitrogen compounds were species specific. For example, L. thermotolerans preferred alpha amino nitrogen above ammonia, where the opposite hold true for T. delbrueckii. Notable differences could also be identified for the uptake of certain single amino acids. Irrespective of the initial YAN concentrations during sequential fermentations, the yeasts only assimilated about half of the initial YAN. The non-Saccharomyces yeasts did not influence fermentation performance during sequential fermentations. However, a low initial YAN (75 mg/L) had a strong influence on the fermentation kinetics and aroma compound production. The higher uptake (compare to S. cerevisiae) of specific single amino acids by non-Saccharomyces yeasts (especially L. thermotolerans), can be tentatively correlated with certain aroma compounds produced at the end of fermentation. The results also revealed that agitation could impact overall fermentation performance and aroma compound production. This study contributes to an improved understanding of how different initial nitrogen concentrations affect growth, fermentation performances and aroma compound production of wine-related yeasts under fermentative conditions. Moreover, the uptake of single amino acids by selected non-Saccharomyces yeasts had also been identified, which is a good starting point to better understand non- Saccharomyces yeasts nitrogen requirements which may be used for the optimization of nitrogen source addition, during alcoholic fermentation, when used in mixed fermentations in order to ensure a complete alcoholic fermentation. To the best of our knowledge, the uptake of single amino acids and YAN consumption by selected non-Saccharomyces yeasts under fermentation conditions tested, have never been studied before.