HEAT BERRY : Sensitivity of berries ripening to higher temperature and impact on phenolic compounds in wine

Abstract

The grapevine is an important economical crop that is very sensitive to climate changes and microclimate. The observations made during the last decades at a vineyard scale all concur to show the impact of climate change on vine physiology, resulting in accelerated phenology and earlier harvest (Jones and Davis 2000). It is well-known that berry content is affected by the ambient temperature. While the first experiences were primarily conducted on the impact of temperature on anthocyanin accumulation in the grape, few studies have focused on others component of phenolic metabolism, such as tannins. Due to their chemical structure they are involved in the sensation of astringency / bitterness and play a key role in the quality of wines (Downey, Dokoozlian, and Krstic 2006). In a context of climate change and global warming, it is necessary to ask the question of the effect of temperature on the metabolism and its impact on wine quality. One of the goals of the HEAT BERRY project is to understand the physiological basis of the response of secondary metabolism to heat and microclimate, in grape berries, and their resulting effects on wine chemistry and organoleptic properties. A passive heating system made of polycarbonate screens has been set up at the vineyard. Field experiments conducted on Cabernet Sauvignon showed that this system induces 1 to 3°C increase in berry temperature. The heating system was set up in June at fruit set, and berries juices were sampled and used for ripening analysis between veraison and harvest time. The results of ripening agreed with previous studies: phenolic maturity and extractability of anthocyanins were decreased on heated berries. Nowadays, it is well-known that tannins nature within different berry tissues has an impact on bitterness and astringency (skins tannins mainly participate to the sensation of astringency, while seeds tannins also contribute to bitterness) as well as the percentage of each berry part. But nothing was shown about the importance of a temperature increase on these aspects. Microvinifications were performed on ripe (and over-ripe) berries. First, tastings demonstrated significant differences between wines from heated berries and non-heated berries in accordance with ripening process and berry compounds. Second, phenolics compounds (tannins levels, Dpm, tannins composition) were analyzed in berries and in wines. The analytical results will be compared with the sensory analysis in order to better understand the qualitative impact of heat treatment on berries and wine properties. Overall, the aim of HEAT BERRY is to address the poor knowledge of the effects of high temperatures on berry composition and wines. This should provide useful clues for the adaptation of viticulture to climate change.