Correlations of carbon isotope ratios of wine ingredients for the improved detection of adulterations I. Organic acids and ethanol

The δ13C-values of organic acids and their correlations to those of the sugar and ethanol, respectively, from 57 EU data bank wines of the Rheinpfalz area (years 1991 – 1993) and from some of their corresponding musts have been determined. In addition to the well established difference between fermented sugar and ethanol (Δδ13C = –1.7±0.2‰), a new constant correlation was found in wine for ethanol and citric acid (Δδ13C = +2.4±0.4‰). From this result a fixed δ-value difference for citric acid in wine to the fermented sugar of +0.7±0.6‰ can be deduced. The δ13C-values of L-malic acid and L-tartaric acid in must were not altered by the alcoholic fermentation; they should therefore directly provide access to the δ13C-value of the natural sugar in must. However, in non-adulterated wines the expected δ13C-value differences between these acids and ethanol showed unsatisfactory correlation coefficients. For L-malate this is attributed to the secondary (partial) degradation of this acid by the malolactic fermentation; a corresponding correction is envisaged in order to make L-malate available as an internal standard. As a reason for the unsatisfactory correlation between L-tartaric acid and ethanol, it is supposed that the time of its maximum biosynthesis period does not coincide with that of glucose in the grape ripening period.