Stable isotope analysis in grape products : 13C-based internal standardisation methods to improve the detection of some types of adulterations

Abstract Correlations between δ 13 C of total sugars, heterosides and the sugar fraction from heterosides were investigated, as well as those between amino acid fractions isolated from musts and relevant wines and, in this frame, between alcohol and amino acids in wine. The target of this study was to find internal standardisation elements which could reduce the uncertainty limit to detect possible adulteration in wine, such as addition of C4-plant sugar. As for the investigation on heterosides, isolated both from experimental and industrial musts and relevant concentrated rectified musts, a highly significant linear correlation between δ 13 C value of sugar from heterosides and total sugar (δ 13 C sugar = −1.57 + 0.87 × δ 13 C heterosides sugar; r  = 0.88; N  = 26) was found. It is worth mentioning the mean 13 C difference of about 1.7‰ between free glucose and glucose from heterosides, as well as the resulted predictability of at least 10% of added C4-plant sugar. As for the correlation between δ 13 C of the amino acid fraction of musts and relevant wines referring to 70 mostly microvinified white and red renowned varietal grapes, a highly significant linear relationship was obtained. Considering δ 13 C values of alcohol and the amino acid fraction of wines, measured in the same samples and in more than 50 wines from the official Italian isotope data-bank, it resulted that the 13 C content of alcohol cannot be higher than that of amino acids. This tendency was observed also in foreign wines as, e.g. in Argentinean products, while for the Uruguayan ones, where addition of C4-plant sugar was allowed until 10% of total alcohol, most samples showed an inverse isotopic situation confirming the validity and sensitivity of this approach.