Doctoral Degrees (Institute for Wine Biotechnology)

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Browsing Doctoral Degrees (Institute for Wine Biotechnology) by browse.metadata.advisor "Du Toit, Maret"

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    Effect of non-Saccharomyces yeasts and lactic acid bacteria interactions on wine flavour.
    (Stellenbosch : Stellenbosch University, 2018-03) Du Plessis, Heinrich Wilbur; Jolly, Neil; Du Toit, Maret; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: Wine aroma and flavour are important indicators of quality and are primarily determined by the secondary metabolites of the grape, by the yeast that conducts the primary fermentation and also the lactic acid bacteria (LAB) that performs malolactic fermentation (MLF). This is a complex environment and each microorganism affects the other during the wine production process. Therefore, the overall aim of this study was to investigate the interactions between Saccharomyces, non-Saccharomyces yeasts and LAB, and the effect these interactions had on MLF and wine flavour. Contour-clamped homogeneous electric field gel electrophoreses (CHEF) and matrix-assisted laser desorption ionization using time-of flight mass spectrometry (MALDI-TOF MS) were useful tools for identifying and typing of Hanseniaspora uvarum, Lachancea thermotolerans, Candida zemplinina (synonym: Starmerella bacillaris) and Torulaspora delbrueckii strains. Hanseniaspora uvarum strains had β-glucosidase activity and Metschnikowia pulcherrima strains had β-glucosidase and protease activity. Only Schizosaccharomyces pombe and C. zemplinina strains showed mentionable malic acid degradation. Candida stellata, C. zemplinina, H. uvarum, M. pulcherrima and Sc. pombe strains were slow to medium fermenters, whereas L. thermotolerans and T. delbrueckii strains were found to be medium to strong fermenters, comparable to S. cerevisiae. The effect of non-Saccharomyces yeast species on MLF varied and inhibition was found to be strain dependent. In a Shiraz winemaking trial where seven non-Saccharomyces strains were evaluated in combination with S. cerevisiae and three MLF strategies, the C. zemplinina and the one L. thermotolerans isolate slightly inhibited LAB growth in wines where yeast and LAB were inoculated simultaneously. However, the same effect was not observed during sequential inoculation of LAB. Mixed culture fermentations using non-Saccharomyces yeasts contained lower alcohol levels, and were more conducive to MLF than wines produced with S. cerevisiae only. Yeast treatment and MLF strategy resulted in wines with significantly different flavour and sensory profiles. Yeast selection and MLF strategy had a significant effect on berry aroma, but MLF strategy also had a significant effect on acid balance and astringency of wines. In a follow up trial, H. uvarum was used in combination with two S. cerevisiae strains, two LAB (Lactobacillus plantarum and Oenococcus oeni) species and three MLF strategies. One of the S. cerevisiae strains had an inhibitory effect on LAB growth, while H. uvarum in combination with this S. cerevisiae strain had a stimulatory effect on MLF. Simultaneous MLF completed faster than sequential MLF and wines differed with regard to their chemical and sensory characteristics. Isoamyl acetate, ethyl hexanoate, ethyl octanoate, ethyl-3-hydroxybutanoate, ethyl phenylacetate, 2-phenyl acetate, isobutanol, 3-methyl-1-pentanol, hexanoic acid and octanoic acid were important compounds in discriminating between the different wines. Yeast treatment had a significant effect on fresh vegetative and spicy aroma, as well as body and astringency of the wines. The LAB strain and MLF strategy had a significant effect on berry, fruity, sweet associated and spicy aroma, as well as acidity and body of the wines. Mid-infrared (MIR) spectroscopy was used to differentiate between wines produced with the selected Saccharomyces and non-Saccharomyces yeast combinations, LAB species and MLF strategies. This study provides valuable information about the interactions between non-Saccharomyces, Saccharomyces yeast, LAB and MLF strategies, and how important pairing of strains are to ensure successful AF and MLF. Furthermore, the results also showed how these interactions can be applied to diversify wine flavour.
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    Maltotriose transport in yeast
    (Stellenbosch : Stellenbosch University, 2007-12) Smit, Annel; Cordero Otero, Ricardo R.; Pretorius, Isak S.; Du Toit, Maret; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: The conversion of sugar into ethanol and carbon dioxide is a process that has been intertwined with human culture and long as civilized man has existed. This fermentation process has been dominated by the micro-organism Saccharomyces cerevisiae and from providing ancient seafaring explorers of a non perishable beverage to equipping bakers with a raising agent to turn flour into bread; this organism with its fermentative potential, has formed an essential part of most societies. In more recent times, many industries still rely on this basic principle. The complexities and efficiencies of the conversion of sugar into its various fermentative byproducts have been studied and optimised extensively to meet the specific demands of industries. Depending on the raw material used as starting point, the major beneficiaries of the useful characteristics have been alcoholic beverage producers (wine, beer, and whiskey amongst others), bakers (bread leavening) and biofuel producers. One of the obstacles in fermentation optimisation is the sugar consumption preferences displayed by the organism used. S. cerevisiae can consume a wide variety of sugars. Depending on the complexities of its structures, it shows a preference for the simpler saccharides. The fermentation of certain more complex sugars is delayed and runs the risk of being left residually after fermentation. Many of the crops utilised in fermentation-based products contain large amounts of starch. During the starch degradation process many different forms of sugars are made available for fermentation. Improved fermentation of starch and its dextrin products would benefit the brewing, whiskey, and biofuel industries. Most strains of Saccharomyces ferment glucose and maltose, and partially ferment maltotriose, but are unable to utilise the larger dextrin products of starch. This utilisation pattern is partly attributed to the ability of yeast cells to transport the aforementioned mono-, di- and trisaccharides into the cytosol. The inefficiency of maltotriose transport has been identified as the main cause for residual maltotriose. The maltotriose transporting efficiency also varies between different Saccharomyces strains. By advancing the understanding of maltotriose transport in yeast, efforts can be made to minimise incomplete fermentation. This aim can be reached by investigating the existing transporters in the yeast cell membrane that show affinity for maltotriose. This study focuses on optimising maltotriose transport through the comparison of the alpha glucoside transporter obtained from different strains of Saccharomyces. Through specific genetic manipulations the areas important for maltotriose transport could be identified and characterised. This study offers prospects for the development of yeast strains with improved maltose and maltotriose uptake capabilities that, in turn, could increase the overall fermentation efficiencies in the beer, whiskey, and biofuel industries.
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    Metabolomic profiling of non-Saccharomyces yeasts in wine
    (Stellenbosch : Stellenbosch University, 2016-03) Whitener, Margaret Elizabeth Beckner; Du Toit, Maret; Divol, Benoit; Vrhovsek, Urska; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: Recent trends in wine making have led to the commercial production and use of non- Saccharomyces yeasts in wine making. Very little is understood however about how the use of these yeasts affects the final product. The purpose of this study was to evaluate the chemical and sensory characteristics of wine fermented with non-Saccharomyces yeasts using a sequential inoculation strategy. Targeted and untargeted analysis techniques were developed to help identify and quantify the volatile fraction of the wines produced. By combining this and sensory data we were able to build the most comprehensive picture to date of the volatile wine metabolome as it is influenced by various yeast species. The first step was a literature review dedicated to summarizing the current knowledge surrounding the metabolomics of the yeasts used in the subsequent chapters. Specifically, we sought to understand what is currently known about the use of non-Saccharomyces yeasts in wine. Also investigated were the technologies currently being used in the fields of food, wine, and yeast metabolomics. The goal was to provide the background necessary to understand the research in the subsequent chapters, as well as aid in the development and planning of the experiments discussed here within. Two stages of research were conducted. Not only did we want to understand the effects of non-Saccharomyces yeasts on wine aroma but we were interested in whether or not these effects were the same in both red and white wines. As such the first research stage, was a preliminary investigation of the yeast response to two different grape musts. Five different species of non- Saccharomyces yeasts, were chosen and grown in both Shiraz and Sauvignon blanc must and samples were collected for analysis just prior to the point at which Saccharomyces cerevisiae would usually be added to complete the fermentation. The fermentation rates were monitored and the chemical profile of the musts was evaluated. A solid-phase microextraction-Gas Chromatography-Mass spectrometry method that targeted 90 different compounds known to be found in wine was used to evaluate the headspace of the fermented musts. The results obtained helped shape the experimental design for the next phase of the project. The scale was increased to full wine production to evaluate how the yeasts could influence a completed wine product. Again, Sauvignon blanc and Shiraz were chosen and an untargeted chemical analysis method was developed to ensure that the widest possible range of analytes could be evaluated. The finished Sauvignon blanc wine was also subjected to sensory analysis which provided even greater insight into how these inoculation strategies can change the sensory profile of the wine. This research was undertaken in an attempt to answer the questions of ‘What will the wine smell and taste like if I use non-Saccharomyces yeasts during fermentation?’ and ‘Could it be superior to standard wines only inoculated with S. cerevisiae?’ The experiments conducted provided a great deal of insight that can help to begin answering these questions but there is much that remains unknown. In general, we were able to build a detailed volatiles chemical profile for each of the yeast treatments used in both Shiraz and Sauvignon blanc. While some treatments proved to be somewhat detrimental to the aroma and flavor of the wine, others showed promise in possibly enhancing its complexity. We were also able to demonstrate that the yeasts behave very differently in the two different musts. As comprehensive as these studies were, future work should be undertaken to improve the understanding of why and how these yeasts can make an impact on wine production. For example, our work did not include any genetic expression analysis of the yeasts used. Correlating genetic expression to quantitative chemical analysis would provide a much more complete picture of the wine yeast metabolome.