Grapes phenolic maturity

A similar effect was observed in other fruits and vegetables, where UV-C treated strawberries showed a higher increment of phenols and PAL activity 12 hours after treatment than unirradiated (control)(Pan and others 2004), which could be the reason for the increment in total phenol constituents (Lancaster and others 2000). UV-C and UV-B caused a two- and threefold increase in content of resveratrol (a grape phenol constituent). Thus, mature Napoleon grapes that had been irradiated with UV-C light can provide up to 3 mg of resveratrol per serving (Cantos and others 2001). Therefore, UV-C treatments clearly cause a benefit effect, increasing total phenol content, which can be mainly attributed to the increment of PAL activity. 

Although a high concentration of anthocyanins in the skins is necessary to obtain a deep-colored wine, it is not the only condition. The cells must also be sufficiently decayed to make these molecules easily extractable by non-aggressive technology. At phenolic maturity, grapes have both... 

Table 6.16. Evolution of the phenolic maturity of Cabernet Sauvignon grapes from different vineyards in 1993 (Glories, 1994, unpnbhshed)...

The organoleptical quality of tannins is directly related to maturation conditions. Enologists have defined this quality in terms of good tannins and bad tannins. The chemical understanding of these phenolic compounds has made it possible to make a better choice of winemaking techniques that optimize the quality of various kinds of grapes. A perfect state of phenolic maturity not only supposes a maximum tannin concentration it also corresponds to soft, non-aggressive, non-bitter tannins. 

Table VIII. Phenolic Compounds in Grape Skins at Crop Maturity in Vineyard SC...

This notion of anthocyanin extractability depends on the state of maturity that controls the breakdown of skin cells (Table 6.15). All other conditions being equal, when grapes are perfectly ripe or slightly overripe, the anthocyanin content in the wine is higher than it would have been prior to maturity, although these pigments tend to decrease in grapes. Both color and total phenol content are at a maximum. 

The following phenolic compounds are present in the grape skin at maturity benzoic and cinnamic acid, flavonols and tannins. They are distribnted in the cells of the epidermis and the first snb-epidermal layers in both white and red grapes. In addition, the red grape skin contains anthocyanins, essentially located in the hypodermal cell layers. Exceptionally, in certain years, the cells adjacent to the pulp can be colored. The pulp itself is colored in the case of Tenturier varieties and some American vines or direct producer hybrids. Antho-cyanin composition varies from cultivar to cultivar, depending on the anthocyanidin substitution and heterosidic nature of the cultivar (see Volume 2, Section 6.2.3). 

The biosynthetic pathways are active as early as the start of grape development. Consequently, the total phenolic compound concentration continues to increase during this period. The rapid increase in tannin concentration at the beginning of development, however, is followed by a slower accumulation during maturation. The biosynthesis may therefore be less active than the increase in berry volume. 

In white grapes, the concentrations of phenolic acids esterified by tartaric add, flavan-3-ols and oligomeric procyanidins are high at the beginning of development They then diminish to minimal concentrations at maturity. 

Consequently, the definition of maturity varies, depending on the objective. For example, the production of dry white wines requires grapes whose aromatic substances are at a maximal concentration and whose acidity is still sufficient. In certain situations, an early harvest can be interesting. Conversely, when the elaboration of a quality red wine is desired, grape development must be left to continue to obtain the most easily extractable phenolic compounds. 

In general, grape maturation results from several biochemical transformations that are not necessarily related to each other. To simplify matters, the increase in sugar concentration and the decrease in acidity are monitored. The accumulation and refinement of white grape aromas and phenolic compounds in red grapes should also be taken into account. The essential property of a quality wine-producing area is to permit a favorable maturation. This corresponds with a harmonious evolution of the various transformations to reach the optimum point simultaneously at the time of the harvest. 

---