Submitted to: Analytica Chimica Acta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/19/2007
Publication Date: 6/1/2008
Citation: Cohen, S.D., Tarara, J.M., Kennedy, J.A. 2008. Assessing the Impact of Temperature on Grape Phenolic Metabolism. Analytica Chimica Acta. 621:57-67. Interpretive Summary: Phenolic compounds are critical to the quality of grapes and wine, but not enough is known about how the vineyard environment affects their formation or metabolism while the grapes are developing on the vine. This experiment was designed to control the temperature of a number of grape clusters during their development in the vineyard so that the effect of grape temperature on the resulting phenolic compounds at harvest could be measured by two methods of analytical chemistry. The phenolic compounds of most interest were tannins, which impart astringency and some bitterness to red wines. Some grape clusters were cooled during the day to limit high temperatures, some were heated at night to limit low temperatures, and others were both heated and cooled to moderate grape temperature throughout ripening. The results suggest that the more heat that the grapes accumulate, the more tannins are present in the fruit at harvest. The nighttime-heated grapes produced the highest concentration of tannin. Moderating both day and night temperatures led to smaller sized tannins from the grape skins, or shorter chains of the base tannin molecules. Because this study reports results from one season, the experiments will continue so that year-to-year effects can be accounted for.
Technical Abstract: This study assessed the impact of fruit temperature on the phenolic metabolism of grape berries (Vitis vinifera L. cv. Merlot) grown under field conditions with controlled exposure to sunlight. Individual cluster temperatures were manipulated in situ. Diurnal temperature fluctuation was damped by daytime cooling and nighttime heating of clusters. Daytime-only and nighttime-only temperature controls were applied for comparison. Berry temperatures were recorded continuously to compare to chemical data. Samples collected at véraison indicated that damping the diurnal temperature fluctuation advanced the onset of ripening. Those berries were larger (double-damped: 0.753±0.015 vs control: 0.512±0.034 g/berry) and more colored than all others. Development of phenolic metabolites was followed by two reversed-phase HPLC methods and gel permeation chromatography. These methods provided information on anthocyanins, proanthocyanidins, flavonols, flavan-3-ol monomers, and polymeric material. Damping the diurnal temperature fluctuation reduced proanthocyanidin mDP (double-damped: 21.8±1.0 vs control: 28.0±1.7). Proanthocyanidin accumulation at véraison was linearly related to heat summation over the developmental period with nighttime heating yielding the highest concentration and daytime cooling yielding the lowest (night-heat: 1.46±0.13 vs day-cool: 0.97±0.09 mg/berry). Damping the diurnal temperature fluctuation had a marked effect on the rate of fruit development whereas total heat summation had more of an effect on phenolic metabolism alone. The results provide insight on the direct effect of temperature on phenolic metabolism.