Submitted to: American Journal of Enology and Viticulture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/8/2008
Publication Date: 9/1/2008
Citation: Tarara, J.M., Lee, J., Spayd, S.E., Scagel, C.F. 2008. Berry temperature and Solar Radiation Alter Acylation, Proportion, and Concentration of Anthocyanin in 'Merlot' Grapes. American Journal of Enology and Viticulture. 59:235-247. Interpretive Summary: Anthocyanins (red pigments) are important for quality in grapes and red wine because they impart color to these products. Typically grape growers strive to maximize color, among other things, to increase the quality of their product and therefore farm gate returns. A complex interaction in the vineyard between sunlight and temperature appears to determine the final concentration of anthocyanins in the grape skins and their distribution among the fifteen different forms of anthocyanins that are synthesized in grapes. This experiment was undertaken to better understand how temperature and sunlight determine the final balance of anthocyanins among these 15 forms at the time the grapes are harvested. Using a forced air heating and cooling system in a vineyard, we controlled the temperatures of a number of clusters without altering the temperature of the vines to which they were attached. Temperatures were controlled for up to nine weeks during ripening. We found that whether the grapes were exposed to direct sunlight or shaded by the vine's leaves, as the temperature of the grapes increased, so did the proportion of anthocyanins in the malvidin group. High temperatures also changed the anthocyanins by increasing the tendency for small acid groups to be attached to the base anthocyanin molecule. Sunlight appeared to cause more of the anthocyanins to be shunted toward the cyanidin group. High temperature extremes for even short periods during ripening, for example, a few hours a day for a few days, were detrimental to the amount of anthocyanins in the grapes at harvest. Future work by food chemists will provide more information on which groups of anthocyanins impart specific, measurable qualities to the grapes or the wine. Growers then will be able to apply our new knowledge of the effects of the vineyard environment on anthocyanins to promoting the production of desired types of anthocyanins in their grapes.
Technical Abstract: Using a forced convection system, the temperatures of 'Merlot' grape clusters were monitored and controlled between veraison and harvest to produce a dynamic range of berry temperatures under field conditions in both sun-exposed and shaded fruit. Ten combinations of temperature and solar radiation exposure were used to quantify these effects on phenolic profiles (anthocyanins and flavonol-glycosides) and on total concentrations of skin anthocyanins (TSA) in the fruit at commercial maturity. Among flavonol 3-glycosides, quercetin 3-glucoside increased with exposure to solar radiation. Low incident solar radiation alone appeared not to compromise total anthocyanin accumulation, rather a combination of low light and high berry temperatures decreased TSA. Regardless of exposure to solar radiation, higher berry temperatures led to higher concentrations and a higher proportion of TSA comprised by the malvidin-based group, driven primarily by increases in its acylated derivatives. Under shade alone and under high temperature extremes in sunlit and shaded fruit, acylated anthocyanins represented a larger proportion of TSA than did non-acylated anthocyanins. At berry temperatures equivalent to those of shaded fruit and ambient air, exposure to solar radiation decreased the proportion of TSA comprised by the acylated (acetic and coumaric acids) forms of the five base anthocyanins (delphinidin, cyanidin, petunidin, peonidin, malvidin) and increased the proportion of TSA comprised by dihydroxylated or di-substituted anthocyanins (cyanidin and peonidin-based). Results indicate a complex combined effect of solar radiation and berry temperature on anthocyanin composition, synergistic at moderate berry temperatures and potentially antagonistic at high temperature extremes. Exposure of berries to high temperature extremes for relatively short periods during ripening appears to alter the partitioning of anthocyanins between acylated and non-acylated forms, and between dihydroxylated and trihydroxylated branches of the phenylpropanoid biosynthetic pathway.