Doctoral Degrees (Viticulture and Oenology)

Permanent URI for this collection

https://scholar.sun.ac.za/handle/10019.1/575

Browse

Browsing Doctoral Degrees (Viticulture and Oenology) by browse.metadata.advisor "De Beer, D."

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    The colour and phenolic content of Robertson Red grape cultivars : distribution, correlation with wines and analyses
    (Stellenbosch : Stellenbosch University, 2013-03) Van der Merwe, Hanneli; Du Toit, Wessel J.; Nieuwoudt, Helene; De Beer, D.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: South African red wine is often acknowledged world wide as being full bodied and deep in colour. This is often the result of high temperatures that is experienced during the important growth stages of grapes especially post véraison. In the Robertson area in South Africa however, temperatures often exceeds the range for optimal anthocyanin development during these growth stages. The distinction between grapes being technologically ripe and being ripe on a phenolic level is also accepted as an important determining factor for the perfect time to pick grapes. In co-operative wineries such as Robertson Winery (RW) where grapes are delivered from a large area and different producers, it is difficult to individualise grape blocks when it comes to ripeness level in terms of sugar or phenolic ripeness. In most circumstances a generalised set of parameters for deeming grapes ripe or acceptable for delivery is the best substitute. The levels of these parameters are based on research literature that is available for the area as well as data collected through years of maintaining the vineyards of that area. The grape parameters that are currently being used by RW for ripeness and quality are pH, titratable acidity (TA) and sugar level. In recent years RW in conjunction with the Department of Viticulture and Oenology, Stellenbosch University, decided to investigate more parameters to determine the quality of grapes at the time of harvest. Most importantly for the grape growers this quality is connected to a price point and therefore compensation. Two important quality parameters of red wine are the red colour and mouth feel of wine. Anthocyanin and tannins are respectively connected to these two quality attributes and are both widely accepted as quality indicators. Wine with high anthocyanin and tannin content often originates from grapes with a high colour and phenolic profile. The existence of a correlation between grape and wine anthocyanin and tannin content is therefore the basis of attempting to use these parameters in the 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 set of determinations. However, sample preparations, extractions and consumables needed are all factors that need to be reduced to make the measurement and therefore the use of these parameters more viable in a co-operative cellar laboratory, where large volumes of grapes are received during harvest. The first objective of this work was to determine the levels of anthocyanin and tannin in red grapes 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 tannin content for grapes received at the cellar. For this purpose, blocks of the most important red wine cultivars for RW was selected and analysed for these compounds. The ranges and average levels of anthocyanin and tannin content were determined using measurement techniques that could be used by any winery. The average mono flavanol and total colour level of 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 that correlated with those found in other studies. The possible reasons for differences in levels of occurrence 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 phenolic compounds in grapes and their corresponding wines. Such correlations will form the foundation for the use of phenolic content to predict the colour and phenolic potential of the wine and possibly wine quality as well. When the grape and wine colour and phenolic data were correlated for all seasons and cultivars inclusive it was found that grape and wine colour showed better correlations than for instance total phenols and tannins. This was especially true for total colour pigments in red grapes, measured with HPLC, when correlated with certain spectrophotometric analysis of wine colour. Cultivar and season as well as the synergism between the two were further investigated for its role in affecting correlations. When these relationships were further differentiated by season and by cultivar the resulting correlations varied. This work contributed a great deal of information to support the use of grape colour and phenolic 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 to rapidly measured anthocyanins, tannins and total phenolics in red grapes. If proven successfully, this could be employed by a large cellar such as RW. FT-NIR has been used with success on grape extracts and in this instance the focus was to establish a calibration on the grape homogenate itself. Preliminary results showed that FT-NIR could be applied for the use of 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 the reference method that was used. This work brought some interesting, practical information not only of importance for RW, but all wineries that are concerned with improving the basis on which grape quality is determined. The use of aerial data mapping for indicating areas regarding important grape colour and phenolic parameters was used in this study and is a very visual way of showing the distribution of certain ripeness parameters over a large area. Correlations between the grape and wines of such a large amount of red grape blocks for a specific area have not also been reported in South Africa before. The use of FT-NIR to determine anthocyanins and tannin concentrations in grape homogenates is also novel for its use in South African wineries. This work may assist grape and wine producers as well as analysts on the phenolic and colour profile of grapes and wines from RW.