Evidence for terroir effect associated with botrytisation relatively to compounds implicated in typical aromas of noble rot sweet wines

Abstract

Recent studies have demonstrated the role of certain lactones, particularly 2-nonen-4-olide, and volatile thiols (3-sulfanylhexan-1-ol) in the over ripped aromas of noble rot sweet wines (Stamatopoulos et al. 2014ab). These compounds are partly formed during the maturation and under the activity of B. cinerea on grapes. This research was carried out in the vineyard of Sauternes with aim to better understand their genesis depending on the grape over-ripening on two different soil types during 3 vintages. Thus, the study was conducted, with the Sémillon grape, during vintages 2012, 2014 & 2015, at 4 stages of over-maturation of the grapes (healthy, pourri plein, pourri roti, pourri roti + 15 days) considering two vineyard plots with different soil characteristics (calcosol & peyrosol) planted with the 315 Sémillon clone and grafted on 101-14 rootstock respectively in 1981 and 1980 and cultivated with the same vineyard management. Volatile lactones were assayed by liquid-liquid extraction followed by GC/MS analysis and the precursors of 3-sulfanylhexanol by an adaptation of the method by Capone et al. 2010 (SPE-UPLC/FTMS). All the analyses were performed in grape juice. In agreement with Miklosy & Kerényi (2004) and Sarrazin (2007), the quantification of lactone compounds has confirmed the important role of B. cinerea in increasing their content during the over-ripening of the grapes. However, our observations indicated an evolution that is not similar and depends on the molecular structure of lactones. While the concentrations of saturated lactones i.e. γ-nonalactone continued to increase in the last stage (pourri roti + 15 days), the contents of unsaturated lactones i.e. 2-nonen-4-olide and massoia lactone reached their maximum at the “pourri roti” stage. Moreover, this study also showed differences in the concentration of lactones between the different vintages (2012, 2014 & 2015) and between the vineyards plots, with always higher content for both saturated and unsaturated lactones on the peyrosol plot. Given the sensory impact of 2-nonen-4-olide and massoia lactone, these could be considered as molecular markers for the quality of the noble rot sweet wines. Concerning the quantification of the 3-sulfanylhexan-1-ol precursors, the results showed differences depending on the vintage, the stage of B. cinerea maturation and the vineyard plot. First, depending on the stage of noble rot development, increasing content of precursors is associated with the impact of B. cinerea as previously observed by Thibon et al. (2009). Regarding the vintage, higher levels of S-conjugates precursors were observed in 2014 in comparison with 2012 vintage. Finally, as for lactones, the level of S-conjugates precursors seems to be dependent on the nature of soil, with differences between the vineyards plots and higher content on the peyrosol plot. Keywords: B. cinerea, sweet wines, lactones, S-conjugates aroma precursors, aroma.