Browsing by Author "Viljoen-Bloom, M."
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- ItemThe biochemistry of malic acid metabolism by wine yeasts – a review(South African Society for Enology and Viticulture, 2006) Saayman, M.; Viljoen-Bloom, M.L-Malic acid is an essential intermediate of cell metabolism and the D,L-racemic mixture is used as an acidulant in a variety of foods and beverages. In the wine industry, it plays an important role during grape must fermentation, contributing to the “fixed acidity” that is important. The latter is important in defining the quality of wine. Genetic and biochemical characterisation of the L-malate utilising pathways in several yeast species has indicated that the physiological role and regulation of L-malate metabolism differ significantly between the K(-) and K(+) yeasts. A variety of factors influence the ability of a yeast species to effectively degrade L-malate, including the conditions associated with wine fermentation and the yeast’s intrinsic ability to transport and effectively metabolise L-malate inside the cell. This paper reviews the ability of different yeast species associated with grapes and wine to degrade extracellular L-malate, and the underlying mechanisms in the differential utilisation of L-malate by different yeast species.
- ItemMalic 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.
- ItemMalic 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.
- ItemUtilising Grape Pomace for Ethanol Production(South African Society for Enology and Viticulture, 2002) Korkie, L. J.; Janse, B. J. H.; Viljoen-Bloom, M.Chemical analyses of grape pomace revealed the presence of significant amounts of fermentable sugars that are retained in the pomace after pressing of the grapes. Furthermore, treatment of the pomace with purified hydrolases indicated that the enzymatic biodegradation of the pomace could release additional fermentable sugars. We isolated and evaluated yeast strains associated with grape pomace for their ability to hydrolyse the complex polysaccharides found in grape pomace and to utilise the fermentable sugars for the production of ethanol. Two Pichia rhodanensis isolates were able to partially hydrolyse the pomace polysaccharides, but fermentation of the pomace resulted only in a small increase in the amount of ethanol produced. The study revealed that significant amounts of ethanol could be obtained from the residual sugars associated with grape pomace. However, the complex structure of the pomace polysaccharides apparently renders it unsusceptible to efficient hydrolysis under fermentative conditions.