The coloration of the Shiraz grape berries during their ripening. Relations between the color descriptors, <em>in situ</em> dynamic and influence of some environmental factors

Abstract

The coloration of Shiraz grape berries was studied all along the ripening period using the CR 200 Minolta chromameter. The analysis of the relations between the three chromatic descriptors of the color, luminance (L*), saturation (C) and hue (Ho), shows three distinct phases in the averaged course followed by the color of a standard berry from its unripe and green stade up to its fully ripe and black stade. These three phases are considered as reflecting different steps of the biosynthesis of the berry pigments, particularly that of the polyphenols. The first phase concerns the green berries with a constant Ho value near 115 degree , but with a noticeable decrease of the two quasi-linearilly linked C and L* descriptors. The beginning of the C or L* changes could be only attributed to modifications or destruction of green and yellow-green pigments (chlorophylles and carotenoids). In this phase, green unsoftened and softened berries coexist. The second phase is characterized by to the great change of Ho from 115 degree to near 25 degree (red evolution of the color), the two others descriptors being continuously decreasing. The color change of this fast and transitory phase is not easily observed by the viticulturist and could be explained by biosynthesis or migration of tanins or proanthocyanidins compounds. The third phase, where a new important change of Ho values, from 25 degree to near -100 degree (green-blue evolution of the color), with a slower decrease of C and L*, is explained by the biosynthesis of skin anthocyanidins or by glycosylation of the previously appeared tanins or proanthocyanidins. This phase is really the color inception of berries (classical veraison). The in situ evolution of the color descriptors with time allows the determination of the averaged rate of the color change of Shiraz berries during their maturation: decreases of 1.6 and 6 units by day for L* (or C) and Ho, respectively. High levels of heterogeneities due to berry asynchronism and to individual coloration rates were also observed. Results indicate that these heterogeneities were maximum at the veraison and that the onset of the color change (not seen by the viticulturist) appears fiveteen or twenty days before. Compared with previous results, the first color modification of this cultivar is exactly concomitant with the onset of the softening process and with the thermic constraint also appearing at this period in berries and clusters. The analysis of such coloration dynamics effected in situ on two different cultural situations, modelizing important changes in the vine microclimate parameters, shows that the environmental factors of the vinestock modify the color descriptors values and consecutively, the polyphenol composition of the berries during their development and at the harvest. Conclusively, the in situ use of a chromameter appears as a pertinent way for studying the dynamics and biosynthesis of pigments under the various development or ripening conditions especially for the black grape cultivars.

Keywords

• Vitis vinifera L.
• Shiraz
• grape ripening
• véraison
• berry coloration
• chromatic descriptors
• polyphenols
• anthocyanins
• environmental factors