Browsing by Author "Van Rensburg, P."

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    Characterization of selected South African young cultivar wines using FTMIR Spectroscopy, Gas chromatography, and multivariate data analysis
    (2009) Louw, L.; Roux, K.; Tredoux, A.; Tomic, O.; Naes, T.; Nieuwoudt, Helene; Van Rensburg, P.
    The powerful combination of analytical chemistry and chemometrics and its application to wine analysis provide a way to gain knowledge and insight into the inherent chemical composition of wine and to objectively distinguish between wines. Extensive research programs are focused on the chemical characterization of wine to establish industry benchmarks and authentication systems. The aim of this study was to investigate the volatile composition and mid-infrared spectroscopic profiles of South African young cultivar wines with chemometrics to identify compositional trends and to distinguish between the different cultivars. Data were generated by gas chromatography and FTMIR spectroscopy and investigated by using analysis of variance (ANOVA), principal component analysis (PCA), and linear discriminant analysis (LDA). Significant differences were found in the volatile composition of the cultivar wines, with marked similarities in the composition of Pinotage wines and white wines, specifically for 2-phenylethanol, butyric acid, ethyl acetate, isoamyl acetate, isoamyl alcohol, and isobutyric acid. Of the 26 compounds that were analyzed, 14 had odor activity values of > 1. The volatile composition and FTMIR spectra both contributed to the differentiation between the cultivar wines. The best discrimination model between the white wines was based on FTMIR spectra (98.3% correct classification), whereas a combination of spectra and volatile compounds (86.8% correct classification) was best to discriminate between the red wine cultivars. © 2009 American Chemical Society.
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    Enzymes in winemaking : harnessing natural catalysts for efficient biotransformations
    (South African Society for Enology and Viticulture, 2000) Van Rensburg, P.; Pretorius, I. S.
    Enzymes play a definitive role in the ancient and complex process of winemaking. From a scientific and technical point of view, wine can be seen as the product of enzymatic transformation of grape juice. From the pre-fermentation stage, through fermentation, post-fermentation and aging, enzymes are the major driving forces catalysing various biotransformation reactions. These biocatalysts originate not only from the grape itself but also from yeasts and other microbes (fungi and bacteria) associated with vineyards and wine cellars. Through better understanding of these enzymatic activities, winemakers have come to learn how to control the unwanted enzymes while optimising the desired activities. Today, winemakers reinforce and extend the action of these endogenous enzymes by the judicious application of an ever-increasing spectrum of commercial enzyme preparations. These enzyme preparations are applied to winemaking with the aims of improving the clarification and processing of wine, releasing varietal aromas from precursor compounds, reducing ethyl carbamate formation and lowering alcohol levels. This review article summarises the most important enzymes applied to winemaking, the nature and structure of their substrates, and the reactions catalysed by these enzymes. This paper also reviews the limitations of the endogenous enzymes derived from grapes and microbes present in must and wine, along with the effects of commercial enzyme preparations on process technology and the quality of the final product. Prospects of developing wine yeast strains expressing tailored enzymes are also highlighted.
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    Epigenetic regulation of PGU1 transcription in Saccharomyces cerevisiae
    (2010) Louw, C.; Young, P. R.; Van Rensburg, P.; Divol, Benoit
    The PGU1 gene of Saccharomyces cerevisiae has been shown to encode a polygalacturonase. The polygalacturonase activity in S. cerevisiae is strain specific. There are no significant differences in the PGU1 promoter regions of strains with and without polygalacturonase activity. The PGU1 gene is subtelomeric because it is located within 25 kb of the right telomere of chromosome X. Expressions of genes located in subtelomeric regions in the yeast S. cerevisiae are inhibited compared with the rest of the genome. In this study, we showed that the deletion of genes involved in telomere silencing enhances polygalacturonase activity. PGU1 transcription and polygalacturonase activity are increased when PGU1 is shifted to a different location in the genome, away from the telomere located close to this gene, and the depletion of the histone H4 leads to an increase in PGU1 transcription. We concluded that PGU1 is silenced in strains without polygalacturonase activity due to an epigenetic effect. The results of this study suggest that PGU1 is silenced by being folded into a heterochromatin-like structure at its subtelomeric position on chromosome X. Formation of this silent structure is dependent on the Isw2p chromatin remodeling complex, its histone fold motif containing subunit Dls1p and the N-terminal tail of the H4 histone. © 2010 Federation of European Microbiological Societies. Published by Blackwell Publishing Ltd.
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    Fermentation-derived aroma compounds in varietal young wines from South Africa
    (SASEV, 2010-09) Louw, L.; Tredoux, A. G. J.; Van Rensburg, P.; Kidd, M.; Naes, T.; Nieuwoudt, Helene
    The volatile composition of 925 single cultivar young Sauvignon blanc, Chardonnay, Pinotage, Merlot, Shiraz and Cabernet Sauvignon wines of vintages 2005 to 2007, was determined using gas chromatography-flame ionisation detection. Compositional data were compared to published data on young wines from South Africa and other countries. South African young wines analysed in this study had a largely similar volatile composition to that reported in the literature. Significant between-vintage and between-cultivar differences were observed in the volatile composition of the wines investigated in this study. The concentration ranges of four compounds in red wines, hexanol, propanol, diethyl succinate and ethyl lactate, and four compounds in white wines, 2-phenylethanol, hexanoic acid, isoamyl acetate and propanol, were not influenced by vintage effects. This finding was interpreted as the first indication that typical concentration ranges for some aroma compounds can be established for South African young cultivar wines. A trend was observed in the white wines that the alcohols and their respective acetate esters, as well as fatty acids and their ethyl esters, were responsible for the vintage-related effects. Differences in volatile composition between Chardonnay and Sauvignon blanc wines could also largely be explained on the same basis. Classification models were established to discriminate between individual red wine cultivars and between the two white wine cultivars and correct classification rates of respectively, 79 % and 85 % were achieved.
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    Genetic screening of wine-related enzymes in Lactobacillus species isolated from South African wines
    (2010) Mtshali, P. S.; Divol, Benoit; Van Rensburg, P.; Du Toit, M.
    Aims: The objective of this study was to investigate the presence of genes coding for enzymes of oenological relevance in wine Lactobacillus strains isolated from South African grape and wine samples during the 2001 and 2002 harvest seasons. Methods and Results: A total of 120 wine lactobacilli isolates belonging to Lactobacillus plantarum, Lactobacillus hilgardii, Lactobacillus brevis, Lactobacillus pentosus, Lactobacillus paracasei, Lactobacillus sakei and Lactobacillus paraplantarum were genetically screened for enzyme-encoding genes using PCR with primers specific for β-glucosidase, protease, esterase, citrate lyase and phenolic acid decarboxylase. The results of PCR screening showed that the Lactobacillus strains possessed different combinations of enzymes and that some strains did not possess any of the enzymes tested. Confirmation analysis with gene sequencing also showed high similarity of genes with those available in GenBank database. Conclusion: In this study, we have demonstrated the existence of genes coding for wine-related enzymes in wine lactobacilli that could potentially hydrolyse wine precursors to positively influence wine aroma. Significance and Impact of the Study: An expansion of knowledge on the genetic diversity of wine-associated lactic acid bacteria will enable the selection of novel malolactic fermentation starter cultures with desired oenological traits for the improvement of the organoleptic quality of the wine, and hence wine aroma. © 2009 The Authors.
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    The influences of different winemaking techniques on the mouthfeel of Shiraz grapes
    (South African Society for Enology and Viticulture, 2015) Nel, A. P.; Louw, L.; Lambrechts, M. G.; Van Rensburg, P.
    The objective of this study was to determine the effect of ripeness and of different tannin extraction methods on the sensory properties of wine, with a specific focus on mouthfeel properties. Quantitative descriptive analysis (QDA) was performed to evaluate the sensory properties of 20 young Shiraz wines in two phases. In Phase 1, wines from a cool area were evaluated and, in Phase 2, wines from a warm area were evaluated. Clear differences were found between the wines from the two regions. Wines from the cooler region were generally associated with higher levels of total non-flavonoids and total anthocyanins, and more intense numbing and puckering sensations. In contrast, the wines from the warmer region as a group were associated with a more drying and grippy mouthfeel, as well as less total anthocyanins and total non-flavonoids. In the set of wines from the cooler region, the effect of ripeness was more pronounced than in the set of wines from the warmer region. In both cases, riper grapes resulted in a coarser surface smoothness, a more numbing sensation, a bitter aftertaste and less adhesive mouthfeel. The wines from the cooler region that were harvested at a riper stage were associated with many of the anthocyanins/ anthocyanin derivatives and were negatively associated with hydroxycinnamate, procyanidin B1 and delphinidin-3-glucoside-p-coumaric acid. In the warmer area, the riper grapes were again associated with anthocyanins/anthocyanin derivatives, but this time were strongly associated with procyanidin B2, caffeic acid, p-coumaric acid, catechin, coutaric acid and total non-flavonoids. The effect of tannin extraction method on the sensory properties of the wines from the warmer region was more pronounced than in the wines from the cooler region. Unfortunately, the differentiation between treatments was not consistent from one ripeness level to the next. However, it appeared that the cold soak treatment differed the least from the control, regardless of region or ripeness, whereas the post-maceration treatment differed the most based on mouthfeel and phenolic composition. Although some mouthfeel attributes and phenolic compounds were consistently associated with region and ripeness, it is not clear if mouthfeel can be manipulated consistently by tannin extraction methods.
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    PGU1 gene natural deletion is responsible for the absence of endo-polygalacturonase activity in some wine strains of Saccharomyces cerevisiae
    (2007) Divol, Benoit; Van Rensburg, P.
    The PGU1 gene encodes an endo-polygalacturonase enzyme in Saccharomyces cerevisiae. The literature reports that most S. cerevisiae strains possess this gene, despite a wide range of enzyme activity levels. Nevertheless, a few wine strains lack the PGU1 gene. We investigated the PGU1 locus sequence in these strains. The results indicated that the gene had been replaced by a partial Ty mobile element, whereas the gene promoter was still at the expected location. As all the strains lacking the PGU1 gene experienced the same phenomenon, it was tempting to hypothesize a common phylogenetic origin. However, fingerprints only allowed grouping of a few of them within one cluster. © 2007 Federation of European Microbiological Societies.
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    The production of reduced-alcohol wines using Gluzyme Mono® 10.000 BG-treated grape juice
    (SASEV, 2009) Biyela, B. N. E.; Du Toit, W. J.; Divol, Benoit; Malherbe, D. F.; Van Rensburg, P.
    High alcohol wines have become a major challenge in the international wine trade. Several physical processes are used to produce wines with reduced-alcohol content, all of which involve the selective extraction of ethanol based on volatility or diffusion. In this study, the possibility of Gluzyme Mono® 10.000 BG (Gluzyme) (Novozymes, South Africa) to reduce the glucose content of synthetic grape juice before fermentation was investigated in order to produce wine with reduced-alcohol content. Gluzyme is a glucose oxidase preparation from Aspergillus oryzae, currently used in the baking industry. Glucose oxidase catalyses the oxidation of glucose to gluconic acid and hydrogen peroxide(H2O2) in the presence of molecular oxygen. Gluzyme was initially used in synthetic grape juice, where different enzyme concentrations and factors influencing its efficiency were investigated under winemaking conditions. The results showed up to 0.5% v/v less alcohol at an enzyme concentration of 20 kU compared to the control samples. This reduction in alcohol was increased to 1 and 1.3% v/v alcohol at pH 3.5 and pH 5.5 respectively in aerated (8 mg/L O2) synthetic grape juice using 30 kU enzyme. Secondly, Gluzyme was used to treat Pinotage grape must before fermentation. Gluzyme-treated wines at 30 kU enzyme concentration after fermentation contained 0.68% v/v less alcohol than the control wines. A decrease in acetic acid concentration of the treated compared to control wines was also observed.
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    Regulation of endo-polygalacturonase activity in Saccharomyces cerevisiae
    (2010) Louw, C.; Young, P. R.; Van Rensburg, P.; Divol, Benoit
    Pectolytic activity in Saccharomyces cerevisiae is due to the secretion of an endo-polygalacturonase encoded by the PGU1 gene. The ability to degrade polygalacturonic acid has been shown to vary between different strains. In this study, we attempted to elucidate how pectolytic activity is regulated in S. cerevisiae and to determine whether the means of regulation differ between strains. Saccharomyces cerevisiae strains from different genetic backgrounds, with varying ability to degrade pectin, were compared. Activity was found not to be regulated by sequence differences in the PGU1 gene, but by the transcription level of the gene. Expression of PGU1 was found to be determined by the transcription level of its two transcription factors TEC1 and STE12. The activation of PGU1 transcription by galactose was found to be strain specific, independent of the strain being an industrial or a domesticated one. The EUROSCARF yeast deletion library was screened for genes encoding inhibitors and activators of polygalacturonase activity. Fourteen strains were identified, in which deletion of a specific gene resulted in a recovery of polygalacturonase activity; these genes were identified as encoding inhibitors of polygalacturonase activity, and two activators were identified. © 2009 Federation of European Microbiological Societies.
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    The tailoring of designer grapevines and microbial starter strains for a market-directed and quality-focussed wine industry. Chapter 174
    (2006) Pretorius, I. S.; Bartowsky, E. J.; Bauer, Florian; De Barros Lopes, M.; Du Toit, M.; Van Rensburg, P.; Vivier, M. A.
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    Validation of two Napping techniques as rapid sensory screening tools for high alcohol products.
    (ELSEVIER SCI LTD, THE BOULEVARD, LANGFORD LANE, KIDLINGTON, OXFORD, ENGLAND,OXON, OX5 1GB, 2013) Louw, L.; Malherbe, S.; Naes, T.; Lambrechts, M. G.; Van Rensburg, P.; Nieuwoudt, Helene