The hydrogen isotope ratios of the methyl [(D/H)I] and methylene [(D/H)II] groups in ethanol can be used as geographical origin tracers of wines. These ratios reflect not only the deuterium content of sugars but also that of water in the grape medium, and hence are influenced by the irrigation, the degree of ripeness at harvest, and other viticultural and enological practices. Thus, the isotopic redistribution coefficients between water in the grape must and the methyl and methylene sites of ethanol were determined and then used to compute an isotopic ratio [(D/H)Is] representing the contribution of the fermentable sugars to (D/H)I. To evaluate the discrimination power of (D/H)Is, the ethanol D/H ratios of 78 Lebanese wine samples from different vineyards were determined using the SNIF-NMR method. An improved classification at the subregional level was obtained using (D/H)Is which exhibits an inverse linear correlation with the annual precipitation amount. Furthermore, the variations of the ethanol D/H ratios with the degree of grape ripeness and with the juice fractions obtained from the inner and peripheral pulps of the grape berries were studied on a small sample set. These factors should be considered when using the SNIF-NMR results to characterize the geographical origin of wines.
Keywords: (2)H SNIF-NMR; Degree of ripeness; Enological practices; Ethanol; Geographical origin; Grape juice fractionation; Hydrogen isotope ratios; Lebanese wines.
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