The colour and phenolic content of Robertson Red grape cultivars :distribution, correlation with wines and analyses

[摘要] ENGLISH ABSTRACT: South African red wine is often acknowledged world wide as being full bodied and deep incolour. This is often the result of high temperatures that is experienced during the importantgrowth stages of grapes especially post véraison. In the Robertson area in South Africahowever, temperatures often exceeds the range for optimal anthocyanin development duringthese growth stages. The distinction between grapes being technologically ripe and being ripeon a phenolic level is also accepted as an important determining factor for the perfect time topick grapes. In co-operative wineries such as Robertson Winery (RW) where grapes aredelivered from a large area and different producers, it is difficult to individualise grape blockswhen it comes to ripeness level in terms of sugar or phenolic ripeness. In most circumstances ageneralised set of parameters for deeming grapes ripe or acceptable for delivery is the bestsubstitute. The levels of these parameters are based on research literature that is available forthe area as well as data collected through years of maintaining the vineyards of that area. Thegrape parameters that are currently being used by RW for ripeness and quality are pH, titratableacidity (TA) and sugar level. In recent years RW in conjunction with the Department ofViticulture and Oenology, Stellenbosch University, decided to investigate more parameters todetermine the quality of grapes at the time of harvest. Most importantly for the grape growersthis quality is connected to a price point and therefore compensation. Two important qualityparameters of red wine are the red colour and mouth feel of wine. Anthocyanin and tannins arerespectively connected to these two quality attributes and are both widely accepted as qualityindicators. Wine with high anthocyanin and tannin content often originates from grapes with ahigh colour and phenolic profile. The existence of a correlation between grape and wineanthocyanin and tannin content is therefore the basis of attempting to use these parameters inthe grape to predict end wine's colour and phenolic quantity. Determination of anthocyanin and tannin content of grapes has already become part of some private owned wineries' standard setof determinations. However, sample preparations, extractions and consumables needed are allfactors that need to be reduced to make the measurement and therefore the use of theseparameters more viable in a co-operative cellar laboratory, where large volumes of grapes arereceived during harvest.The first objective of this work was to determine the levels of anthocyanin and tannin in redgrapes from different vineyard blocks from the producers of RW from three successive vintages.This would give insight as to what can be seen as a low and high anthocyanin and tannincontent for grapes received at the cellar. For this purpose, blocks of the most important redwine cultivars for RW was selected and analysed for these compounds. The ranges andaverage levels of anthocyanin and tannin content were determined using measurementtechniques that could be used by any winery. The average mono flavanol and total colour levelof the grapes were found to be lower than those often reported in literature, with total grape flavanols being higher. However, a wide range of values for these compounds were found thatcorrelated with those found in other studies. The possible reasons for differences in levels ofoccurrence of these compounds were discussed and mostly pertain to differences in cultivar,micro climatic and season.The second objective was to determine the correlation between levels of colour and phenoliccompounds in grapes and their corresponding wines. Such correlations will form the foundationfor the use of phenolic content to predict the colour and phenolic potential of the wine andpossibly wine quality as well. When the grape and wine colour and phenolic data werecorrelated for all seasons and cultivars inclusive it was found that grape and wine colourshowed better correlations than for instance total phenols and tannins. This was especially truefor total colour pigments in red grapes, measured with HPLC, when correlated with certainspectrophotometric analysis of wine colour. Cultivar and season as well as the synergismbetween the two were further investigated for its role in affecting correlations. When theserelationships were further differentiated by season and by cultivar the resulting correlationsvaried. This work contributed a great deal of information to support the use of grape colour andphenolic compounds for the prediction of end wine colour and phenolic composition.The third objective was to investigate near infrared spectroscopy (FT-NIR) as a viable option torapidly measured anthocyanins, tannins and total phenolics in red grapes. If provensuccessfully, this could be employed by a large cellar such as RW. FT-NIR has been used withsuccess on grape extracts and in this instance the focus was to establish a calibration on thegrape homogenate itself. Preliminary results showed that FT-NIR could be applied for the useof determination of anthocyanin and tannin levels in red grapes originating from RW. The prediction of total phenols was not found to be as accurate, but this could also be due to thereference method that was used.This work brought some interesting, practical information not only of importance for RW, but allwineries that are concerned with improving the basis on which grape quality is determined. Theuse of aerial data mapping for indicating areas regarding important grape colour and phenolicparameters was used in this study and is a very visual way of showing the distribution of certainripeness parameters over a large area. Correlations between the grape and wines of such alarge amount of red grape blocks for a specific area have not also been reported in South Africabefore. The use of FT-NIR to determine anthocyanins and tannin concentrations in grapehomogenates is also novel for its use in South African wineries. This work may assist grapeand wine producers as well as analysts on the phenolic and colour profile of grapes and winesfrom RW.

[发布机构] Stellenbosch University