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A Technical Guide for Wine Producers
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DETERMINING OPTIMUM RIPENESS IN WINE GRAPES 1 Viticultural consultant, VinPro. Project submitted in
partial compliance with the requirements for Hons BSc in Viticulture at the University of
Stellenbosch. 2 Department of Viticulture and Oenology, University of
Stellenbosch. Introduction The road travelled by a bottle of wine before it ends up on a
consumer's table is complex and consists of a series of wide-ranging processes in the
vineyard as well as the cellar. In the production of a bottle of wine, the roles played by
the vineyard and by the cellar cannot be separated from each other and one is justified in
saying that the winemaking process starts in the vineyard. One of the most critical and
difficult decisions that have a direct bearing on wine quality is determining when the
grapes have reached optimum ripeness to satisfy the specific wine goal. Overripe as well
as green grapes have a detrimental influence on the eventual quality of the wine. The
decision is a difficult one, but there are a few aids to help determine optimum ripeness. Chemical Analyses The measurement of sugar and acid concentrations, pH and combinations
thereof is generally used to determine whether the grapes have reached optimum ripeness.
The importance of sugar, acid and pH in the winemaking process is well-known and these
components can be measured with ease and accuracy. The danger of using sugar
concentration by itself as a yardstick of ripeness was pointed out a long time
ago. Du Plessis (1977) obtained a fluctuation in wine quality of between 8 and 27 per cent
in Pinotage wines by pressing the grape four years in a row at 22°B. During the same
period, using a sugar:acid ratio of 2,5 as a yardstick, the best wine quality in Chenin
blanc occurred when the grapes ranged from 17°B to 24°B. This clearly indicates that
from one year to the next, optimum ripeness is obtained at varying sugar concentrations
and that used by itself as a yardstick, may cause great variation in wine quality. The same danger exists when total titrateable acid concentration is
used as the only yardstick. Du Plessis (1977) obtained optimum wine quality in Chenin
blanc grapes with acid concentrations ranging from 7 to 10 g/l from one year to the next.
This does not mean that organic acids are not important in the determination of optimum
ripeness; on the contrary, they determine the nature and content of other components such
as flavourants and colourants. Table 1: Grape composition components to determine
optimum ripeness for various wine types.
The ratio between sugar and acid in the juice of the berry is a lot more significant in the determination of optimum ripeness than any of the components taken in isolation. Du Plessis (1984) found optimum ripeness of Chenin blanc in sugar:acid ratios of between 2,4 and 2,6. In the case of Pinotage, optimum ripeness was encountered at a ratio of approximately 4,0. From this and other research it is clear, however, that the optimum ratio differs not only from cultivar to cultivar, but also from year to year and therefore this cannot be used as a single yardstick either. The pH of the juice is probably the most important factor to determine microbic and chemical stability. It also has an important effect on the colour and taste of the wine. Usually the pH increases with an increase in sugar concentration and it may be used as a means of determining the date of picking. All researchers are in agreement, however, that pH by itself is not a reliable yardstick for the determination of optimum ripeness and that it should rather be used with other measurements in an index. Butzke & Boulton (1997) listed the three pH titrateable acid combinations which may be expected under various conditions, namely:
This shows that a ripening index with pH and acid as components should also take the ripening conditions into account and that this will probably differ from one area to the next. In an attempt to overcome the above-mentioned shortcomings, Bouton et al (1997) suggested the following combinations of sugar, acid and pH for various wine types: Combined with this should be a subjective evaluation of colour and flavour to determine a more accurate date for picking. Van Rooyen et al (1984) found an index of sugar x pH to be a better yardstick for the measurement of optimum ripeness in Cabernet Sauvignon and Pinotage than sugar by itself or sugar:acid ratio. In this instance the best wine quality was obtained at index values of 85 to 95. By refining this index even further, Coombe et al (1980) found sugar x pH2 to be an even better indicator of optimum ripeness. The motivation for attaching a bigger value to pH lies in the significant role it plays in fermentation as well as in wine stability. According to this measurement, the best wines are made at index values ranging from 200 to 270. Flavourants usually occur as fruity and grassy combinations in grapes. The amount of these flavourants is extremely small and difficult to determine. Marais, Hunter & Haasbroek (1999) found shadow levels in the foliage to determine the type of flavourants (fruity or grassy). Furthermore, it seems that fruity flavours are obtained at complete maturity, while grassy flavours peak at a slightly lower sugar concentration. Another factor which makes it more difficult to determine the stage of ripeness at which optimum flavourant content is achieved in grapes, is the occurrence of so-called flavourant combinations. These are the glycosidally bound precursor flavourant combinations which cannot be tasted (Reynolds & Wardle, 1988). In general the levels of glycosides in the berry increase progressively with ripening. These glycosides may be hydrolised by enzymes, which means that the bound flavours are released, thus increasing the flavour perception of grapes. A method has been developed to determine these bound flavours (the so-called glycosyl-glucose test) simply by determining the bound glucose content (Williams, et al, 1995). The method has its drawbacks, however, since it does not work well with red grapes and also because it is relatively difficult to apply in practice on a large scale. Tasting the grapes during ripening to determine flavourant ripeness remains the second best alternative. Colourants, especially in red grapes, are also a good indicator of optimum ripeness. It is difficult to determine, however, and at this stage too expensive to be practical on a large, commercialised scale. Vineyard and Grape Judging As indicated, chemical analyses of grapes are handy yardsticks to help decide when grapes have reached optimum ripeness. There are other yardsticks, however, which may be used to supplement the information obtained from chemical analyses. Particularly with regard to flavourant ripeness, the tasting of grapes by experienced judges is of great value. Although the bound flavours are not easily recognised when the grapes are tasted, the analysis of unbound flavourants are a good indication of when grape flavours reach optimum peaks. As with wine tasting, it is important that grape tasters should be trained to note the various grape flavours. So for example the flavourants in Cabernet Sauvignon grapes have been described as green, herbaceous, nutty, red berry-like, black berry-like and jammy as the ripening progresses (Long, 1997). The tannins change from being tart and dusty to soft. By tasting the grapes, additional indications of optimum ripeness may be obtained. The ease with which the grape skins burst, the amount of browning of the pips and the change in colour of the brush (red cultivars) are all observations which play a role in deciding when grapes reach optimum ripeness. It is important to remember that while physical and especially chemical changes are still taking place in the berries as a result of photosynthesis, the grapes have not yet reached maturity. As soon as chemical changes take place due to moisture loss, the grapes are either overripe or moisture stress has occurred to such an extent that further natural ripening is impossible. Conclusion In contrast with other winemaking countries such as Chili, South Australia and the biggest parts of France, grapes in South Africa ripen while temperatures are increasing. This often results in sugar ripeness being reached before other components such as flavours and tannins have reached optimum ripeness. This makes it very difficult to determine optimum ripeness in South Africa. Although there is no easy recipe, the use of indices such as sugar concentration (°B) x pH2, backed by the tasting of grapes and accompanying observations (browning of the pip, colour of the brush, etc), are important considerations in the decision about the date of picking. Additional literature ARCHER E, 1981, Rypwording en oesmetodes, In Wingerdbou in Suid-Afrika, 468-471, Reds, J Burger & J Deist, Stellenbosch: NIWW BOULTON RB, SINGLETON VL, BISSON LF & KUNKEE RE, 1997, Principles and practices of winemaking , California ; Chapman & Hill. BUTZKE CE & BOULTON RB, 1997. Acidity, pH and potassium for grapegrowers. Practical Winery & Vineyard 18, 10-16. COOMBE BG, DUNDON RJ & SHORT AWS, 1980. Indices of sugar-acidity as ripeness criteria for winegrapes. J Sci Food Agric. 3, 495-502. DU PLESSIS CS, 1977, GRape components in relation to white wine quality, Proc, Int , Symp , on the quality of the vintage, 14-21 Febr., 1977, Cape Town 117-128 DU PLESSIS CS, 1984. Optimum maturity and quality parameters in grapes: A review, S, Afr. J, Enol, Vitic, 5, 35-42 LONG Z, 1977. Developing wine favour in the vineyard. Practiacal Winery and Vineyard 18, 6-9. MARAIS J, HUNTER JJ & HAASBROEK PD, 1999. Effect of canopy microclimate, season and region on Sauvignon blanc grape composition and wine quality. S , Afr. J, Enol, Vitic. 20, 19-30. REYNOLDS AG, WARDLE DA, 1988. Canopy microclimate of Gewürztraminer and monoterpene levels. Proc. Int. Symp. Cool Climate Viticulture and Oenology, 11-15 Jan 1988, Auckland, 116-122. VAN ROOYEN PC, ELLIS LP & DU PLESSIS CS, 1984. Interactions between grape maturity and quality for Pinotage and Cabernet Sauvignon wines from four locations. S. Afr. Enol, Vitic. 5, 29-34. WILLIAMS PJ, CYNKAR W, FRANCIS IL, GRAY JD, ILANO PG & COOMBE BG, 1995. Qualification of glycosides ingrapes, juices and wines through a determination og glycosyl glucose.
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