Browsing by Author "Nel, Anton Pieter"

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    The influence of different winemaking techniques on the extraction of grape tannins
    (Stellenbosch : University of Stellenbosch, 2011-03) Nel, Anton Pieter; Lambrechts, Marius; Van Rensburg, Pierre; University of Stellenbosch. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: Grape and wine phenols consist of flavanols which is the building blocks for tannins. These building blocks are called monomers which consist of catechins, epicatechins, epigallocatechins and epicatechin-gallate. Tannin is important in wine as it contributes to bitterness, mouth feel (astringency) and maturation potential of the wine. Futhermore it has a health benefit as an antioxidant. Anthocyanins are responsible for the colour of red wine. The anthocyanins combine with tannins to form stable polymeric pigments. Due to the importance of tannins and anthocyanins in wine, it is imperitative that different winemaking techniques are used to extract as much of these components as possible and that the analysis is done quickly and accurately. The aim of this study was to evaluate different winemaking techniques and their extraction of tannins and anthocyanins into the wine. Too much tannin extraction can have a negative effect on the sensory quality of the wine. Therefore a second aim was to evaluate the mouth feel properties of a Shiraz wine. A third aim was to compare the two tannin precipitation methods in terms of time efficiency, repeatability and the ease of practice. To investigate the amount of tannin concentration extracted by different winemaking techniques, two cultivars (Cabernet Sauvignon and Shiraz) were used. These treatments included the addition of an enzyme during fermentation [E], cold maceration [CM], post maceration [PM] and the combination of cold and post maceration [CM+PM]. The grapes were harvested in two different climatic areas during the 2008 and 2009 vintages. The two climatic areas were classified according to the Winkler scale as a III (Morgenster) and a IV (Plaisir de Merle). The grapes were harvested at two different ripeness levels in order to evaluate the effect of the different winemaking processes on the extraction of tannins and anthocyanins. One harvest was before (LB) and the other after (HB) the commercial harvest. The results of this study showed significant differences in the phenolic composition of the wines. It was found that the warmer area showed higher tannin concentrations than the cooler area for both cultivars. In the 2008 Cabernet Sauvignon the CM extracted higher concentrations of tannin from the cooler area at both ripeness levels. In the warmer area, CM extracted the highest tannin concentration HB, but the CM+PM extracted the highest tannin concentration from Cabernet Sauvignon at the LB and CM at the HB of the warmer area. In 2009 the PM extracted the highest concentration of tannin at the lower ripeness level, while the E treatment extracted the highest concentration from the warmer area. In the cooler area the CM+PM extracted the highest concentration of tannin at a lower ripeness level, while there were no siginicant differences between the different treatments at the higher ripeness level. The highest anthocyanin concentration was found in the cooler area. The CM treatment was found to have no effect on anthocyanin extraction. Different methods are available to quantify the tannin concentration in wine. Two of the most popular tannin analytical methods are the bovine serum albumin (BSA) and the methyl cellulose precipitable tannin (MCP) methods. The BSA method is a very complex method which uses at least 3 times more reagents than the MCP method. The MCP method only analyzes tannins, while the BSA method analyzes tannins, monomeric pigments (MP), small polymeric pigments (SPP) and large polymeric pigments (LPP). In this study a good correlation was found between the two tannin precipitation methods (R2 – 0.88). There is controversy regarding the variability of these methods. Some scientists found that the two methods show a good correlation with HPLC, while others found that there was no such correlation between the precipitation methods and the HPLC. The MCP method had a practical advantage as it could be performed in half the time required for the BSA method. This has a significant impact in scenarios where a high sample throughput is required although it only measures total tannin. The phenolic composition and mouth feel of the wine was strongly influenced by the climatic area. In the warmer area the effect of tannin concentration on mouth feel was much less than in the cooler area. The wine made of riper grapes, was more grippy, bitter and numbing than the wines made from greener grapes. The E treatment was especially associated with a dry, grippy sensation.
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    Pinking of wine – influence of different winemaking processes, causative agents and pinking treatments
    (Stellenbosch : Stellenbosch University, 2021-03) Nel, Anton Pieter; Van Jaarsveld, Francois; Du Toit, Wessel J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: Pinking is a slight salmon-red discolouration in certain white wines after some oxygen ingress. This discolouration can lead to some economical loss to the winemaker. This phenomenon was first described in the late 1960s to the early 1970s. In the 50 odd years that this phenomenon is known, only about 4 peer-reviewed scientific articles were ever written with a handful of popular articles. In the first article on pinking, the researcher looked at how to test for pinking susceptibility (PS) as well as the influence of SO2 and pH on a pinked wine. This article established the method that is now used all over wine-producing countries. The influence of SO2 was established and the outcome was that the wine needs at least 40 mg/L of SO2 to protect the wine against PS. It was also established that the change in pH does not influence the pink colour showing that pinking is not associated with anthocyanins. In another article, preventative agents were researched. It was found that PVPP can bind certain compounds, presumably phenolic compounds, and settle them out, decreasing the PS. This research led to a phenomenal increase in the sale of PVPP. Literature indicated the possibility of anthocyanins in white wine and found malvidin-3-O-glucoside to play a role in PS of Siria grapes. In this study, the influence of temperature, skin contact time and pressing methods were researched on PS. Whole bunch press was tested against crushed & destemmed grapes. Dissolved oxygen (DO) was also measured to deduce the influence of oxygen on PS. The DO of the whole bunch pressed grapes was significantly higher than the crushed & destemmed grapes. This was due to more air space between the berries. Micro oxygenation in the initial stages of the winemaking practices led to a higher PS in the whole bunch pressed wines. The temperature had the highest influence on PS. Cooler (4ºC) grapes also had a significantly higher DO than grapes at ambient (20ºC) temperature. This finding was confirmed by different winemakers which had problems with pinking. All of them picked their Sauvignon blanc grapes at the coolest temperature, keeping them as cool as possible before processing. This and the higher DO in the juice help with the oxidation of the pinking precursors leading to a wine with a high PS. Longer skin contact time leads to a higher potential to pink, but this PS was not significant. When long skin contact time was combined with a cool temperature, the PS increases significantly. In two articles by Portuguese researchers, the presence of malvidin-3-O-glucoside was established. The theory that all cultivars have the genes to produce anthocyanins, but that the genes are repressed in white wines grapes, could hold some truth. In UV-visible spectrophotometric analysis, the spectra of pinked wines and control wines were analysed. The ranges where the differences in spectra were visible was not associated with any phenolic or anthocyanin compounds. During LC-MS analysis of the peaks showed that the compounds causing PS does not fit any of the known wine phenols. During the LC-MS analysis, the mass and retention times were given. The masses were compared to the library of the LC-MS and no fits to the mass data were established, thus making the identification of the compounds near impossible. A sensory analysis was done on pinked wines which are novel work and was never done before. The anecdotal evidence shows that pinked wine does not influence the aroma and taste. This was tested against a trained panel. It was found that the trained panel could not pick up the difference between a pinked wine and a control wine, although some of the tasters could pick up oxidation characters on the aroma. The sensory study showed that the panelists could pick up differences by smell but not on the taste of the wines. This study proved the anecdotal evidence and could lead to further sensory studies in the sensory science of pinked white wines. This study paved the way for further sensory research on pinked wines. The data also showed that panels can now be trained in identifying pink wine as a wine fault or to contribute a new style of wine to the public.