Identification of γ-nonalactone precusor in merlot and cabernet sauvignon grapes

Wine flavor results on complexes interactions of odorous components, which come from different aromatic families like esters, thiols, aldehydes, pyrazines or lactones. Varietal lactones identified in red wines contribute to cooked fruity flavors such as dried peach, apricot, figs and dried prune. Recent studies have demonstrated the key impact of the harvest date on the lactone content in wine. The influence of the temperature during grape ripening was also underlined. Many lactones have been detected in wines, but one of them, γ-nonalactone, possesses a low detection threshold (D_th 27 µg/L), and has been detected at high concentration in wine (up to 200µg/L;). Thus, it contributes directly to the cooked peach flavors in red wines.

All these observations led the research group to investigate the chemical and biochemical mechanisms associated with γ-nonalactone formation in must and wine. Based on a previous work realized in beer, they hypothesized that 4-oxononanoic acid was a potential precursor of γ-nonalactone in wine. To study its occurrence in musts from Merlot and Cabernet Sauvignon grapes, a GC-(NCI) MS method was developed and validated. 4-oxononanoic acid is identified for the first time in must samples whereas its concentration was ranged from some µg/L to more than 60 µg/L. Additionally, in order to demonstrate the impact of alcoholic fermentation on the formation of γ-nonalactone, the researchers synthesized labelled d₆–4-oxononanoic acid and observed a positive correlation between d₆–4-oxononanoic acid concentration added and d₆–γ-nonalactone formed in spiked samples.

In conclusion, the results demonstrated the presence of 4-oxononanoic acid in must and its biotransformation to γ-nonalactone during alcoholic fermentation of red grape varieties. They also validated for the first time its role of precursor of the odourous γ-nonalactone in wines.

Poster presented at Macrowine virtual (June 23-30, 2021)