Browsing by Author "Prior, B. A."

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    Effect of alternative NAD +-regenerating pathways on the formation of primary and secondary aroma compounds in a Saccharomyces cerevisiae glycerol-defective mutant
    (2012) Jain, V. K.; Divol, Benoit; Prior, B. A.; Bauer, Florian
    Saccharomyces cerevisiae maintains a redox balance under fermentative growth conditions by re-oxidizing NADH formed during glycolysis through ethanol formation. Excess NADH stimulates the synthesis of mainly glycerol, but also of other compounds. Here, we investigated the production of primary and secondary metabolites in S. cerevisiae strains where the glycerol production pathway was inactivated through deletion of the two glycerol-3-phosphate dehydrogenases genes (GPD1/GPD2) and replaced with alternative NAD +-generating pathways. While these modifications decreased fermentative ability compared to the wild-type strain, all improved growth and/or fermentative ability of the gpd1Δgpd2Δ strain in self-generated anaerobic high sugar medium. The partial NAD + regeneration ability of the mutants resulted in significant amounts of alternative products, but at lower yields than glycerol. Compared to the wild-type strain, pyruvate production increased in most genetically manipulated strains, whereas acetate and succinate production decreased in all strains. Malate production was similar in all strains. Isobutanol production increased substantially in all genetically manipulated strains compared to the wild-type strain, whereas only mutant strains expressing the sorbitol producing SOR1 and srlD genes showed increases in isoamyl alcohol and 2-phenyl alcohol. A marked reduction in ethyl acetate concentration was observed in the genetically manipulated strains, while isobutyric acid increased. The synthesis of some primary and secondary metabolites appears more readily influenced by the NAD +/NADH availability. The data provide an initial assessment of the impact of redox balance on the production of primary and secondary metabolites which play an essential role in the flavour and aroma character of beverages. © 2011 Springer-Verlag.
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    Elimination of glycerol and replacement with alternative products in ethanol fermentation by Saccharomyces cerevisiae
    (2011) Jain, V. K.; Divol, Benoit; Prior, B. A.; Bauer, Florian
    Glycerol is a major by-product of ethanol fermentation by Saccharomyces cerevisiae and typically 2-3% of the sugar fermented is converted to glycerol. Replacing the NAD+-regenerating glycerol pathway in S. cerevisiae with alternative NADH reoxidation pathways may be useful to produce metabolites of biotechnological relevance. Under fermentative conditions yeast reoxidizes excess NADH through glycerol production which involves NADH-dependent glycerol-3-phosphate dehydrogenases (Gpd1p and Gpd2p). Deletion of these two genes limits fermentative activity under anaerobic conditions due to accumulation of NADH. We investigated the possibility of converting this excess NADH to NAD+ by transforming a double mutant (gpd1δgpd2δ) with alternative oxidoreductase genes that might restore the redox balance and produce either sorbitol or propane-1,2-diol. All of the modifications improved fermentative ability and/or growth of the double mutant strain in a self-generated anaerobic high sugar medium. However, these strain properties were not restored to the level of the parental wild-type strain. The results indicate an apparent partial NAD+ regeneration ability and formation of significant amounts of the commodity chemicals like sorbitol or propane-1,2-diol. The ethanol yields were maintained between 46 and 48% of the sugar mixture. Other factors apart from the maintenance of the redox balance appeared to influence the growth and production of the alternative products by the genetically manipulated strains. © 2010 Society for Industrial Microbiology.
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    Genetic analysis of the metabolic pathways responsible for aroma metabolite production by Saccharomyces cerevisiae.
    (SPRINGER, 233 SPRING STREET, NEW YORK, USA, NY, 10013, 2013) Styger, G.; Jacobson, D.; Prior, B. A.; Bauer, Florian
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    Glycerol in South African table wines : an assessment of its relationship to wine quality
    (South African Society for Enology and Viticulture, 2002) Nieuwoudt, Helene; Prior, B. A.; Pretorius, I. S.; Bauer, Florian
    Glycerol is an important by-product of glycolysis and is quantitatively one of the major components of wine. While the physicochemical and sensory characteristics of pure glycerol are well established, the impact of varying levels of glycerol on general wine quality remains a topic of debate. Previous reports have relied on limited numbers of either commercial or experimental wines to assess the role of glycerol, leading to contradictory conclusions. Here we report on a large-scale assessment of the relationship between glycerol concentration and wine quality, based on the analysis of a significant number of commercial South African table wines of adjudged quality. The mean glycerol concentrations of 237 dry red (10.49 g/L), 158 dry white (6.82 g/L), 22 off-dry white (6.55 g/L), 16 special late harvest (8.26 g/L) and 14 noble late harvest wines (15.55 g/L) were found to be associated with considerable variation within each respective style. The final glycerol concentrations were significantly associated with the wine style (P<0.05). Shiraz wines had a mean glycerol concentration (10.22 g/L) which was significantly lower than that of Cabernet Sauvignon (10.81 g/L), Pinotage (10.46 g/L) and Merlot (10.62 g/L) wines (P<0.05). In both the dry white and off-dry white styles, the mean glycerol concentrations of Sauvignon blanc wines (6.31and5.42 g/L, respectively) were significantly lower (P<0.05) than those of the Chardonnay wines (7.08 and 7.03 g/L, respectively) and the Chenin blanc wines (6.81 and 6.85 g/L, respectively). No significant association between the final glycerol concentrations in commercial wines and the vintage, geographic origin or yeast strain used in inoculated fermentations could be established (P>0.05). The mean glycerol concentrations for South African dry red wines were significantly higher than those of dry white and off-dry white wines. Wine quality could not be significantly associated with glycerol concentrations in the dry red wines (P>0.05). For the dry white, off-dry white and late harvest wines this association was significant (P<0.05), although the exact nature of the association was somewhat different for the respective styles. Despite this positive statistical association, the observed differences between the mean glycerol concentrations of dry white and off-dry white wines of different quality ratings were too small to be of major practical value. The relationship between glycerol concentration and wine quality is reassessed on the basis of results obtained in this study as well as on recent reports in the literature.
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    Glycerol production by the yeast saccharomyces cerevisiae and its relevance to wine : a review
    (South African Society for Enology and Viticulture, 1998) Scanes, K. T.; Hohmann, S.; Prior, B. A.
    Glycerol is a sugar alcohol produced as a by-product of the ethanol fermentation process by Saccharomyces cerevisiae. In wines, levels between 1 and 15 gll are frequently encountered and the higher levels are thought to contribute to the smoothness and viscosity of wine. Glycerol and ethanol levels are inversely related, which may add an additional favourable attribute to wine. The metabolic pathways involved in glycerol synthesis, accumulation and utilisation by yeast are now better understood since a number of the genes involved in glycerol metabolism have been cloned, sequenced and their functions established. These fundamental studies now permit the glycerol levels produced by yeast to be raised by either the specific control of the culture conditions or by the manipulation of the genetic and molecular properties of the yeast. In some instances, the level of glycerol produced under laboratory conditions has been significantly raised. However, a number of undesirable by-products also accumulate during the fermentation and an improved understanding of the glycerol metabolic flux is required before wines with a consistently elevated glycerol concentration can be produced.
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    Impact of yeast breeding for elevated glycerol production on fermentative activity and metabolite formation in chardonnay wine
    (South African Society for Enology and Viticulture, 2000) Prior, B. A.; Toh, T. H.; Jolly, N.; Baccari, C.; Mortimer, R. K.
    Glycerol in wine originates mainly as a by-product during fermentation by yeast and is thought to add to the body and smooth mouth-feel. We evaluated the properties of Chardonnay wine produced using various wine yeast strains of Saccharomyces cerevisiae and hybrid strains that were bred to produce elevated glycerol concentrations in laboratory trial experiments. The wine yeast strains (commercial strains or strains from culture collections) produced a mean glycerol and ethanol concentration of 4.38 and 101.2 g/L (12.8% v/v; n=26) respectively, whereas the glycerol and ethanol concentrations in wine made using the hybrid strains was 7.18 g/L and 96.0 g/L (12.2% v/v; n=15). Considerable variability in the glycerol-producing ability of the wine yeast and hybrid strains was apparent. Coupled to the higher glycerol levels formed by the hybrid strains, acetic acid, volatile acidity, acetoin, acetaldehyde and 2,3-butanediol levels were higher than the levels produced by the wine yeast strains. The levels of some of these metabolites were strongly linked to elevated glycerol production. The hybrid strains fermented the Chardonnay grape juice more slowly than the wine yeast strains, but in most instances dryness was achieved. The concentrations of miscellaneous metabolites (alcohols, acids and esters) were in most instances similar in the wine made with the wine yeast strains and hybrid strains, indicating that the breeding of yeast to produce higher glycerol levels has a minor influence on the production of these compounds. In a wine production experiment one hybrid yeast strain producing elevated glycerol levels yielded a Chardonnay wine with a better or equivalent body than wine made with commercial wine yeast strains, although the aroma and general quality were worse. These results suggest that further breeding and selection might yield yeast strains for fermentation that improves the body of wine without impacting on the overall balance of wine.
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    Principal component analysis applied to Fourier transform infrared spectroscopy for the design of calibration sets for glycerol prediction models in wine and for the detection and classification of outlier samples
    (2004) Nieuwoudt, Helene; Prior, B. A.; Pretorius, I. S.; Manley, M.; Bauer, Florian
    Principal component analysis (PCA) was used to identify the main sources of variation in the Fourier transform infrared (FT-IR) spectra of 329 wines of various styles. The FT-IR spectra were gathered using a specialized WineScan instrument. The main sources of variation included the reducing sugar and alcohol content of the samples, as well as the stage of fermentation and the maturation period of the wines. The implications of the variation between the different wine styles for the design of calibration models with accurate predictive abilities were investigated using glycerol calibration in wine as a model system. PCA enabled the identification and interpretation of samples that were poorly predicted by the calibration models, as well as the detection of individual samples in the sample set that had atypical spectra (i.e., outlier samples). The Soft Independent Modeling of Class Analogy (SIMCA) approach was used to establish a model for the classification of the outlier samples. A glycerol calibration for wine was developed (reducing sugar content < 30 g/L, alcohol > 8% v/v) with satisfactory predictive ability (SEP = 0.40 g/L). The RPD value (ratio of the standard deviation of the data to the standard error of prediction) was 5.6, indicating that the calibration is suitable for quantification purposes. A calibration for glycerol in special late harvest and noble late harvest wines (RS 31-147 g/L, alcohol > 11.6% v/v) with a prediction error SECV = 0.65 g/L, was also established. This study yielded an analytical strategy that combined the careful design of calibration sets with measures that facilitated the early detection and interpretation of poorly predicted samples and outlier samples in a sample set. The strategy provided a powerful means of quality control, which is necessary for the generation of accurate prediction data and therefore for the successful implementation of FT-IR in the routine analytical laboratory.
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    Principal Component Analysis Applied to Fourier Transform Infrared Spectroscopy for the Design of Calibration Sets for Glycerol Prediction Models in Wine and for the Detection and Classification of Outlier Samples.
    (AMER CHEMICAL SOC, 1155 16TH ST, NW, WASHINGTON, USA, DC, 20036, 2004) Nieuwoudt, Helene; Prior, B. A.; Pretorius, I. S.; Manley, M.; Bauer, Florian