Browsing by Author "Lombard, Judith"

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    Characterisation of Wickerhamomyces anomalus and Kazachstania aerobia : investigating fermentation kinetics and aroma production
    (Stellenbosch : Stellenbosch University, 2016-12) Lombard, Judith; Bauer, Florian; Musarurwa, Hannibal; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: Non-Saccharomyces yeasts have been studied extensively in the past two decades to use as catalysts for adjusting the aroma and chemical properties of wine. Many non-Saccharomyces yeasts dominate in grape must, but Wickerhamomyces anomalus and Kazachstania aerobia have recently been found to be more dominant in several musts in South Africa than what has been reported from other wine growing areas. It has been hypothesised that regional microflora can lead to a terroir specific wine. To further establish these claims, the impact of these non-Saccharomyces yeasts on the chemical profile and sensory perception of wine, in particular when present in high numbers, has yet to be fully elucidated. This study was designed to better characterise isolated strains of non-Saccharomyces species, determining its phenotypic space, as well as to assess their fermentation potential and volatile aroma compound production in synthetic and real grape must. Eight K. aerobia and thirteen W. anomalus isolates were used for characterisation. DNA based taxonomic differences between isolates were investigated using the Random Amplification of Polymorphic DNA (RAPD) method and phenotypic heterogeneity was established using stress assays to determine heat, saline, osmotic and oxidative stress tolerance. Phenotypically diverse K. aerobia and W. anomalus strains were then selected for co- and sequential fermentations with two S. cerevisiae strains, VIN13 and EC1118, in synthetic grape must. In addition, sequential culture fermentations were conducted in Sauvignon blanc grape must by individually pairing two strains of K. aerobia and two strains of W. anomalus with S. cerevisiae EC1118. Wine aroma compounds were quantified using GC-FID. RAPD analysis classified W. anomalus isolates into five distinct groups according to place of origin. Phenotypic variations were evident within and between the proposed strains as was exhibited by heterogeneous resistance to oxidative, saline and osmotic stresses compared to S. cerevisiae, VIN13. The K. aerobia isolates showed no marked genetic differences, although exhibiting slight variations in stress responses. During fermentation the non-Saccharomyces yeasts persisted for longer when S. cerevisiae was only inoculated after 48 hours, or at a lower density. The longer the non-Saccharomyces yeasts proliferated in the must the more pronounced was the effect on aroma production. Kazachstania aerobia yeasts did not achieve a high biomass compared to W. anomalus, but survived for longer in fermentation, especially in Sauvignon blanc grape must. Although W. anomalus displayed strong growth, it was inhibited by the growth of S. cerevisiae. Kazachstania aerobia and W. anomalus gave a unique aroma profile to the wines. The latter yeast produced high concentrations of ethyl acetate, while K. aerobia was characterised by increased acetic acid concentration. Most aroma compounds were increased in mixed culture fermentations, especially higher alcohols, with a significant increase in the esters 2-phenylethyl acetate by K. aerobia, and ethyl caproate and caprylate by W. anomalus. Although, as single cultures these yeasts did not ferment wines to dryness in synthetic grape must and only completed fermentation after 28 days in Sauvignon blanc grape must, they are capable of conferring favourable wine aroma when in association with S. cerevisiae strains with no risk of sluggish fermentation. This study provides a basis for future work on wine quality improvement through exploitation of non-Saccharomyces yeasts and gives insight to the possible impact of K. aerobia and W. anomalus present in grape must in a South African context.