Browsing by Author "Van Rensburg, P."
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- ItemEnzymes 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.
- ItemThe 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.
- Itemn Ondersoek na die vlugtige samestelling van geselekteerde Suid-Afrikaanse jong wyne met behulp van GC-FID tegnologie (Deel 1): Inleiding tot vlugtige verbindings in wyn.(Wynboer, 2010) Louw, L.; Tredoux, A. G. J.; Nieuwoudt, Helene; Van Rensburg, P.
- Itemn Ondersoek na die vlugtige samestelling van geselekteerde Suid-Afrikaanse jong wyne met behulp van GC-FID tegnologie (Deel 2): Die veranderinge in die vlugtige samestelling van jong wyne oor drie oesjare.(Wynboer, 2010) Louw, L.; Tredoux, A. G. J.; Nieuwoudt, Helene; Van Rensburg, P.
- Itemn Ondersoek na die vlugtige samestelling van geselekteerde Suid-Afrikaanse jong wyne met behulp van GC-FID tegnologie (Deel 3): Die verskille in die vlugtige samestelling van ses belangrike kultivarwyne.(Wynboer, 2010) Louw, L.; Tredoux, A. G. J.; Nieuwoudt, Helene; Van Rensburg, P.
- Itemn Ondersoek na die vlugtige samestelling van geselekteerde Suid-Afrikaanse jong wyne met behulp van GC-FID tegnologie (Deel 4): Die klassifisering van wynkultivars op grond van hul vlugtige samestelling.(Wynboer, 2010) Louw, L.; Tredoux, A. G. J.; Nieuwoudt, Helene; Van Rensburg, P.
- ItemRegulation of endo-polygalacturonase activity in Saccharomyces cerevisiae(2010) Louw, C.; Young, P. R.; Van Rensburg, P.; Divol, BenoitPectolytic 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.
- ItemThe 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.
- ItemValidation 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