Browsing by Author "Du Toit, Maret"

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    Effect of saccharomyces, non-saccharomyces yeasts and malolactic fermentation strategies on fermentation kinetics and flavor of Shiraz Wines
    (MDPI, 2017) Du Plessis, Heinrich; Du Toit, Maret; Nieuwoudt, Helene; Van der Rijst, Marieta; Kidd, Martin; Jolly, Neil
    The use of non-Saccharomyces yeasts to improve complexity and diversify wine style is increasing; however, the interactions between non-Saccharomyces yeasts and lactic acid bacteria (LAB) have not received much attention. This study investigated the interactions of seven non-Saccharomyces yeast strains of the genera Candida, Hanseniaspora, Lachancea, Metschnikowia and Torulaspora in combination with S. cerevisiae and three malolactic fermentation (MLF) strategies in a Shiraz winemaking trial. Standard oenological parameters, volatile composition and sensory profiles of wines were investigated. Wines produced with non-Saccharomyces yeasts had lower alcohol and glycerol levels than wines produced with S. cerevisiae only. Malolactic fermentation also completed faster in these wines. Wines produced with non-Saccharomyces yeasts differed chemically and sensorially from wines produced with S. cerevisiae only. The Candida zemplinina and the one L. thermotolerans isolate slightly inhibited LAB growth in wines that underwent simultaneous MLF. Malolactic fermentation strategy had a greater impact on sensory profiles than yeast treatment. Both 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. Winemakers should apply the optimal yeast combination and MLF strategy to ensure fast completion of MLF and improve wine complexit
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    Evaluating the influence of maceration practices on biogenic amine formation in wine
    (Elsevier, 2013) Smit, Anita Y.; Du Toit, Wessel J.; Stander, Marietjie; Du Toit, Maret
    Biogenic amines are formed during winemaking from precursor amino acids, mainly by lactic acid bacteria during malolactic fermentation (MLF). Various factors can influence the amino acid content of the grape must and wine; including contact with the grape skins before, during and after alcoholic fermentation. The quantity and composition of amino acids in the must can potentially dictate the subsequent formation of biogenic amines. In this study we investigate the influence of compounds extracted from the grape skins by different maceration practices applied during winemaking on the formation of biogenic amines. Wines were made on small scale with two red grape cultivars. Treatments consisted of free-run juice (no skin contact), skin contact during alcoholic fermentation, cold maceration and extended maceration; followed by MLF in all treatments. Our results show that higher levels of precursor amino acids and biogenic amines were detected in the absence of skin contact, extended maceration and to a lesser extent in conventional maceration. Cold maceration before fermentation initially increased the extraction of amino acids and formation of biogenic amines, but resulted in the lowest concentrations of these harmful compounds in the final wines. Cold maceration therefore appears to have a protective effect against biogenic amine accumulation during MLF.
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    The grapevine and wine microbiome : insights from high-throughput amplicon sequencing
    (Frontiers Media, 2017) Morgan, Horatio H.; Du Toit, Maret; Setati, Mathabatha E.
    From the time when microbial activity in wine fermentation was first demonstrated, the microbial ecology of the vineyard, grape, and wine has been extensively investigated using culture-based methods. However, the last 2 decades have been characterized by an important change in the approaches used for microbial examination, due to the introduction of DNA-based community fingerprinting methods such as DGGE, SSCP, T-RFLP, and ARISA. These approaches allowed for the exploration of microbial community structures without the need to cultivate, and have been extensively applied to decipher the microbial populations associated with the grapevine as well as the microbial dynamics throughout grape berry ripening and wine fermentation. These techniques are well-established for the rapid more sensitive profiling of microbial communities; however, they often do not provide direct taxonomic information and possess limited ability to detect the presence of rare taxa and taxa with low abundance. Consequently, the past 5 years have seen an upsurge in the application of high-throughput sequencing methods for the in-depth assessment of the grapevine and wine microbiome. Although a relatively new approach in wine sciences, these methods reveal a considerably greater diversity than previously reported, and identified several species that had not yet been reported. The aim of the current review is to highlight the contribution of high-throughput next generation sequencing and metagenomics approaches to vineyard microbial ecology especially unraveling the influence of vineyard management practices on microbial diversity.
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    Managing your wine fermentation to reduce the risk of biogenic amine formation
    (Frontiers Research Foundation, 2012-03) Smit, Anita Yolandi; Engelbrecht, Lynn; Du Toit, Maret
    Biogenic amines are nitrogenous organic compounds produced in wine from amino acid precursors mainly by microbial decarboxylation. The concentration of biogenic amines that can potentially be produced is dependent on the amount of amino acid precursors in the medium, the presence of decarboxylase positive microorganisms and conditions that enable microbial or biochemical activity such as the addition of nutrients to support the inoculated starter cultures for alcoholic and malolactic fermentation(MLF). MLF can be conducted using co-inoculation or an inoculation after the completion of alcoholic fermentation that may also affect the level of biogenic amines in wine. This study focused on the impact of the addition of complex commercial yeast and bacterial nutrients and the use of different MLF inoculation scenarions on the production of biogenic amines in wine. Results showed that the addition of complex nutrients to real grape must could potentially increase histamine concentrations in wine. The same experiment in synthetic grape must showed a similar trend for putrescine and cadaverine. The effect of different MLF inoculation scenarios was examined in two cultivars, Pinotage and Shiraz. Conflicting results was obtained. In the Shiraz, coinoculation resulted in lower biogenic amine concentrations after MLF compared to before MLF, while the concentration was higher in the Pinotage. However,the production of biogenic amines was affected more by the presence of decarboxylase positive lactic acid bacteria than by the addition of complex nutrients or the inoculation scenario.
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    Modulation of wine flavor using hanseniaspora uvarum in combination with different saccharomyces cerevisiae, lactic acid bacteria strains and malolactic fermentation strategies
    (MDPI, 2019-07-2019) Du Plessis, Heinrich; Du Toit, Maret; Nieuwoudt, Helene; Van Der Rijst, Marieta; Hoff, Justin; Jolly, Neil
    Hanseniaspora uvarum is one of the predominant non-Saccharomyces yeast species found on grapes and in juice, but its effect on lactic acid bacteria (LAB) growth and wine flavor has not been extensively studied. Therefore, the interaction between H. uvarum, two Saccharomyces cerevisiae yeast strains, two LAB species (Lactobacillus plantarum and Oenococcus oeni) in combination with two malolactic fermentation (MLF) strategies was investigated in Shiraz wine production trials. The evolution of the different microorganisms was monitored, non-volatile and volatile compounds were measured, and the wines were subjected to sensory evaluation. Wines produced with H. uvarum in combination with S. cerevisiae completed MLF in a shorter period than wines produced with only S. cerevisiae. Sequential MLF wines scored higher for fresh vegetative and spicy aroma than wines where MLF was induced as a simultaneous inoculation. Wines produced with H. uvarum had more body than wines produced with only S. cerevisiae. The induction of MLF using L. plantarum also resulted in wines with higher scores for body. H. uvarum can be used to reduce the duration of MLF, enhance fresh vegetative aroma and improve the body of a wine.
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    Transcriptomics unravels the adaptive molecular mechanisms of Brettanomyces bruxellensis under SO2 stress in wine condition
    (Elsevier, 2020-03-10) Valdetara, Federica; Skalic, Miha; Fracassetti, Daniela; Louw, Marli; Compagno, Concetta; Du Toit, Maret; Foschino, Roberto; Petrovic, Uros; Divol, Benoit; Vigentini, Ileana
    Sulfur dioxide is generally used as an antimicrobial in wine to counteract the activity of spoilage yeasts, including Brettanomyces bruxellensis. However, this chemical does not exert the same effectiveness on different B. bruxellensis yeasts since some strains can proliferate in the final product leading to a negative sensory profile due to 4-ethylguaiacol and 4-ethylphenol. Thus, the capability of deciphering the general molecular mechanisms characterizing this yeast species’ response in presence of SO2 stress could be considered strategic for a better management of SO2 in winemaking. A RNA-Seq approach was used to investigate the gene expression of two strains of B. bruxellensis, AWRI 1499 and CBS 2499 having different genetic backgrounds, when exposed to a SO2 pulse. Results revealed that sulphites affected yeast culturability and metabolism, but not volatile phenol production suggesting that a phenotypical heterogeneity could be involved for the SO2 cell adaptation. The transcriptomics variation in response to SO2 stress confirmed the strain-related response in B. bruxellensis and the GO analysis of common differentially expressed genes showed that the detoxification process carried out by SSU1 gene can be considered as the principal specific adaptive response to counteract the SO2 presence. However, nonspecific mechanisms can be exploited by cells to assist the SO2 tolerance; namely, the metabolisms related to sugar alcohol (polyols) and oxidative stress, and structural compounds.