NEW ISOTOPIC CRITERIA FOR THE SHORT-TERM DATING OF BRANDIES AND SPIRITS

As a result of the atmospheric atomic experiments carried out in the 1 960s a method for short-term dating of wines and spirits could be base d on the determination of the C-14 radioactivity of ethanol. However, due to the turnover of atmospheric (CO2)-C-14 this method is no longer applicable to the most recent samples. The aptitude of stable isotope s of carbon and hydrogen to provide new probes for dating brandies suc h as cognacs and armagnacs is evaluated. The overall carbon isotope ra tio of ethanol is measured by isotope ratio mass spectrometry and the site-specific hydrogen parameters of the methyl, (D/H)(1), and methyle ne, (D/H)(II),,, sites are determined by the SNIF-NMR method. The isot opic results can be interpreted in terms of fractionation phenomena as sociated with thermo-kinetic evaporation undergone in the casks during the ageing period. A theoretical estimation of the vintage founded on this isotopic evolution requires the knowledge of the isotope ratios of the starting product. In principle, these values can be estimated o n the basis of the meteorological parameters associated with the growi ng period of the corresponding vine. However, although the combined co ntributions of climatic effects and of evaporation phenomena convenien tly explain the general isotopic behaviour, the influence of oenologic al practices such as ullage (replacement of the amount of a liquid los t in a cask by evaporation) must also be considered. Further progress towards an unequivocal dating procedure using stable isotope ratios is expected from the access to isotope databanks of the wine precursors. (C) 1998 SCI.