Incidence of a vine protection using a commercial formula of Bordeaux mixture on the Sauvignon grapes maturity and the wines varietal aroma (results of a 3-year study)

Abstract

The incidence of cupric sprayings using a commercial formula of Bordeaux mixture (containing 20% of copper sulphate or an organic fungicide), on Sauvignon grapes composition and wines varietal aroma, has been studied in three areas of the Bordeaux region (Entre-deux-Mers, Graves, Médoc). Copper content has been noticed to increase in the clarified juices in all cases after these treatments, values ranging from 0,3 mg/l to 14,5 mg/l, whereas natural copper content did not exceed 0,4 mg/l. The value of residual copper by treatment was found to be significantly different from one year to the other (2 to 3,5 mg/l in 1993, 0,3 to 0,9 mg/l in 1994 and 0,7 to 0,9 mg/l in 1995) mainly due to the grapes composition and the different climatic conditions, whereas for the same year in the three different areas, this value was almost the same. The operation known as «skin contact» during which juices were left with their skins under CO2 at 10°C, during 18 hours before pressing, was found to decrease copper content in the clarified juices from 60 p. cent to 80 p. cent, whereas immediate pressing of grapes led to a decrease of only 20 p. cent ; the combination of copper during skin contact with glutathione, which is present in Sauvignon grapes, is a possible explanation.

Under the ripening conditions of Bordeaux's oceanic temperate climate, we have noticed that even one single spray with a commercial formula of Bordeaux mixture at 3 000 g/ha, between the stage of closing of the cluster and the post-veraison, is sufficient to significantly reduce the content in 4-mercapto-4-methylpentan-2-one (4-MMP), a main component of the characteristic Sauvignon wines aroma. Increase of the introduced copper quantities in the must as well as of the number of applications, results in a greater reduction of the content in this thiol, as 4-MMP reacts with copper residues during alcoholic fermentation while its precursor is revealed into an aroma. Nevertheless, the experiments with a polymer that eliminates copper from the juice prior to alcoholic fermentation, showing in some cases no recovery in the wines 4-MMP content, lead us to the hypothesis that copper might as well intervene during ripening of the grapes, causing thus a limitation of the aroma's precursor synthesis. By the sensory analysis, Sauvignon wines produced from copper treated vines showed much less characteristic aroma than those produced from not treated vines (having nevertheless been protected by an organic fungicide) and were lower graded. While the average grade for the last ones was 7,9/10, this grade was lowered to 2,9/10 and to 2,8/10 for the samples produced after one and two or three vine's copper treatments respectively. Grapes sugar concentration can also be affected by the vine's cupric treatments. Throughout the 3-year study, differences in sugar concentration up to 40 g/l have been observed, especially concerning the treatments made between stages of closing of the cluster and veraison. Nevertheless, sugar content seems highly dependent on the number of cupric applications, the period of their application as well as climatic conditions during vine sprayings.