Browsing by Author "Volschenk, H."

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    Malic acid distribution and degradation in grape must during skin contact : the influence of recombinant malo-ethanolic wine yeast strains
    (South African Society for Enology and Viticulture, 2005) Van Staden, J.; Volschenk, H.; Van Vuuren, H. J. J.; Viljoen-Bloom, M.
    Wine acidity plays an important role in determining wine quality and ensuring physiochemical and microbiological stability. In high-acid wines, the L-malic acid concentration is usually reduced through bacterial malolactic fermentation, while acidulation in low-acidity wines is usually done during final blending of the wine before bottling. This study showed that skin contact did not influence the relative concentration of L-malic acid in the pulp and juice fractions from Colombard, Ruby Cabernet and Cabernet Sauvignon grape musts, with 32%-44% of the L-malic acid present in the pulp fraction. Four recombinant malo-ethanolic (ME) Saccharomyces wine yeast strains containing the malic enzyme (mae2) and malate transporter (mael) genes of Schizasaccharomyces pombe, effectively degraded the L-malic acid in both the juice and pulp fractions of all three cultivars, with a complete degradation of malic acid in the juice fraction within 2 days.
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    Malic acid in wine : origin, function and metabolism during vinification
    (South African Society for Enology and Viticulture, 2006) Volschenk, H.; Van Vuuren, H. J. J.; Viljoen-Bloom, M.
    The production of quality wines requires a judicious balance between the sugar, acid and flavour components of wine. L-Malic and tartaric acids are the most prominent organic acids in wine and play a crucial role in the winemaking process, including the organoleptic quality and the physical, biochemical and microbial stability of wine. Deacidification of grape must and wine is often required for the production of well-balanced wines. Malolactic fermentation induced by the addition of malolactic starter cultures, regarded as the preferred method for naturally reducing wine acidity, efficiently decreases the acidic taste of wine, improves the microbial stability and modifies to some extent the organoleptic character of wine. However, the recurrent phenomenon of delayed or sluggish malolactic fermentation often causes interruption of cellar operations, while the malolactic fermentation is not always compatible with certain styles of wine. Commercial wine yeast strains of Saccharomyces are generally unable to degrade L-malic acid effectively in grape must during alcoholic fermentation, with relatively minor modifications in total acidity during vinification. Functional expression of the malolactic pathway genes, i.e. the malate transporter (mae1) of Schizosaccharomyces pombe and the malolactic enzyme (mleA) from Oenococcus oeni in wine yeasts, has paved the way for the construction of malate-degrading strains of Saccharomyces for commercial winemaking.