UNRAVELING THE CHEMICAL MECHANISM OF MND FORMATION IN RED WINE DURING BOTTLE AGING : IDENTIFICATION OF A NEW GLUCOSYLATED HYDROXYKETONE PRO-PRECURSOR

Abstract

During bottle aging, the development of wine aroma through low and gradual oxygen exposure is often positive in red wines, but can be unfavorable in many cases, resulting in a rapid loss of fresh, fruity flavors. Prematurely aged wines are marked by intense prune and fig aromatic nuances that dominate the desirable bouquet achieved through aging (Pons et al., 2013). This aromatic defect, in part, is caused by the presence of 3-methyl-2,4-nonanedione (MND). MND content was shown to be lower in nonoxidized red wines and higher in oxidized red wines, which systematically exceeds the odor detection threshold (62 ng/L). Concentrations up to 340 ng/L were evidenced in the most oxidized red wines as well as MND content up scaling was observed whatever the oxidation level. Very recently, we identified two new hydroxyketones (2-hydroxy-3-methylnonan-4-one) associated with MND distribution in aged red wines. We demonstrated that in red wine, their oxidation can produce MND (Peterson et al., 2020). To date, the origin of these precursors were not studied. During preliminary experiments, the presence in wine of a glycosylated form of this hydroxyketone was suggested by hydrolysis experiments. Based on the literature, we hypothesized the presence of a corresponding glucosylated precursor and developed a strategy for its organic multi-step synthesis. First, the MND hydroxylated precursor of MND was synthesized by aldolization (Crévisy et al., 2001). Then, based on literature, we optimized strategies for the O- glycosidation step. For this, the tetrabenzylated glucose was activated by imidation reaction (Chatterjee et al., 2018). Several deprotection methods for the glucoside moiety were then experimented. Finally, the use of palladium on carbon for the hydrogenolytic debenzylation lead to the target compound. A multi-step purification process (LC, HPLC) was carried out to reach sufficient purity. Glycosylated standard was characterized by Nuclear Magnetic Resonance (NMR) and by High Resolution Mass Spectrometry (HRMS) and then used to develop an LC-MS/MS for its identification in grapes and wines. The first analytical results lead to look deeper into the search for glucosylated compounds in various oenolo-gical samples (grapes, musts, red wines), affected or not by the nuances of “dried fruits”.