Materials and methods
Four red cultivars, Pinotage, Merlot, Shiraz, and Cabernet Sauvignon were chosen from the 2010 vintage. Vineyard lo cations varied from Stellenbosch, Robertson, Rawsonville, Durbanville, Somerset-West, Franschhoek and Hermanus. Grapes were picked at commercial harvest date after which they were processed. The must was inoculated with NT116 (Anchor Yeast Biotechnologies) one hour after crushing and destemming and punch downs were performed every day to facilitate phenolic and colour extraction. Fermentation was performed at 23°C until dry. The skins were pressed after alcoholic
fermentation in an open basket to a pressure of 0.5 bar after which it was inoculated with Viniflora Oenos (CHR-Hansen) to start the malolactic fermentation (MLF).
Analyses of the berry samples using the Glories, Iland and BSA methods were done and full descriptions of the methods were reported in published scientific articles. BSA analyses were also done on the wines as well as different spectrophotometer analyses to measure colour density, modified wine colour density, total red pigments, total phenolics, estimate of SO2 resistant pigments and copigmented anthocyanins[4, 7]. Other wine analyses included total anthocyanins and tannin concentrations[3, 5].
Results and Discussion
Colour and phenolic composition of grapes There were no significant differences between the cultivars for most of the phenolic and anthocyanins analyses of the grapes (Table 1). The extractability index of the anthocyanins seemed to be higher for the Cabernet Sauvignon grapes, this means the colour was easier to extract for this cultivar. Merlot exhibited a higher seed tannin content, which could be undesirable as it increased the astringency and could associate with proteins and polysaccharides instead of stabilising anthocyanins. There was no clear grouping of the cultivars in the
constructed biplot (Figure 1) pointing to other factors such as the origin of the grapes, terroir, viticultural practices and ripeness levels being the predominant influences on the phenolic composition of the grapes. A strong, positive correlation between the total anthocyanins, extractable anthocyanins and skin tannins, all measures using the Glories method, as well as anthocyanins measured using the Iland method could be seen. In a positive correlation, as the values of one of the variables increases, the values of the other variable will also increase. These groups of compounds were extracted under similar conditions and would thus make sense to be correlated.
Colour and phenolic composition of the wine
The principle component analyses (PCA) (Figure 2a) on the wine data just after fermentation did not show clear trends, or groupings in terms of cultivar. Loadings (Figure 2b) did, however, show positive correlations between total red pigments, anthocyanins, colour density, modified colour density and total phenols which were negatively correlated with tannins. This grouping was not surprising as the former contained all the colour associated compounds. After MLF, some grouping occurred with Merlot positioning more on the left of PC1 (Figure 3a). No other grouping tendencies were seen for the other cultivars. Loadings (Figure 3b) showed that the total phenol, total red pigments, anthocyanin and colour density were significantly lower for the Merlot wines when compared to the other cultivars. In general, the colour and phenolic concentration was lower after MLF. This confirmed the general belief that MLF could affect colour and phenolic characteristics of a wine[10, 11].
Correlations from grape to wine
The BSA method proved to be well equipped to assess grape tannin levels and how they reflected in the wine. Significant positive correlations (data not shown) between grape and wine phenolic characteristics were observed when analysed with both the Glories and Iland methods. Strong correlations were observed between anthocyanins (Iland), colour density (after alcoholic fermentation), modified colour density (after alcoholic fermentation) and modified colour density (after MLF). The total anthocyanin content (Glories) and the extractable anthocyanins were also positively and significantly correlated to the above mentioned wine parameters. It would seem as if the correlations delivered by the Iland method might be slightly better
to predict wine colour. The use of ethanol during the Iland procedure might explain this observation. The importance of assessing correlations not only after alcoholic fermentation but also after the completion of MLF was evident. In the future, studies should also investigate the effect of barrel ageing
on the phenolic composition. Using the extractability index (EA) The extractability index is an indicator of the extractability of anthocyanins,
by taking into account the total anthocyanins and anthocyanins extracted under winemaking conditions (both measured using the Glories method). It would thus be expected that the EA would correlate with the anthocyanins and colour levels in wine. This, however, was not the case in this study. There was no significant correlation between EA and most colour characteristics of the wine. This raises the question of the efficiency of the EA to predict total
extractability of anthocyanins from ripe grapes. However, it could still provide information on how quickly the anthocyanins can be extracted during winemaking operations particularly with regard to skin contact and fermentation.
Which method to use: Glories or Iland?
The question arises, which of these methods to use when attempting to predict the phenolics and colour indicative compounds in grapes that would be convertedto wine? This is especially important for the commercial cellars as equipment such as the HPLC is normally not available. According to the findings in this study, the Iland method ight be slightly more suitable, as it yielded slightly better correlations with most of the wine data. The Glories method, however, had an advantage by delivering additional information such as skin tannins and the contribution of seed tannins to the total phenolic profile of the wine. Of course other factors such as equipment available, chemicals used, waste management and time should be borne in mind
when making this decision. This work might give wine producers, as well as wine analyses laboratories, with valuable information regarding the suitability of these methods to characterise the phenolic composition of South African red grapes and their resulting wines.
The author would like to thank Winetech, Thrip and the NRF for financial support, the vineyards and cellars which donated grapes for this project, Lorraine Geldenhuys, Hanneli van der Merwe and Andy Roediger for technical support and Professor Martin Kidd for the statistical analyses.
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