Authentication and geographical characterisation of Italian grape musts through glucose and fructose carbon isotopic ratios determined by LC IRM

The authenticity of grape musts is normally checked through the stable isotopic analysis of carbon (δ13C) after fermentation and distillation by following the official OIV MA AS-312-06 method. Unfortunately, it presents some issues that are difficult to over come. Grape must samples can only be analysed after they have been fermented to obtain ethanol. The process must be carried out under careful control of the fermentation to avoid the presence of unwanted by-products arising from a premature fermentation interruption. Moreover, if the musts have been preserved by the addition of sulphur dioxide (SO2 ), they must undergo an additional step to eliminate the SO2 , which would affect the fermentation. Once the product has been fermented, the ethanol must be separated using specific distillation columns (such as the Cadiot ones) making it possible to obtain ethanol free of isotopic fractionation with a minimum alcohol degree of 95% vol. In this study, the alternative use of a technique based on δ13C isotopic analysis of the major sugars of the grape must by liquid chromatography coupled with isotope ratio mass spectrometry (LC-IRMS) is provided. In LC–IRMS, analytes are separated on an LC system and consecutively oxidized in an online reactor to CO2 , which is required for the determination of compound-specific carbon isotopic ratios. This technique has been already used in the study of matrices such as wine [1], ethanol [1,2], glycerol [2], and honey [3] to detect fraudulent alterations of their natural composition such as the addition of exogenous sugars to the products. The LC-IRMS allows a single separation of the individual components of a sample and makes it possible to determine their δ13C values online, avoiding both the disadvantages of off-line methods and the disadvantages of methods requiring a derivatization step (such as GC-C-IRMS), causing the addition of extra carbons. In order to discriminate between musts from different areas of Italy, a preliminary dataset was considered; the δ13C isotopic ratios of glucose and fructose of around 100 authentic Italian must samples from 16 different sampling regions were analysed. In addition, the δ13C variability in authentic and fake must (added with increasing percentages of exogenous sugars) has been explored and tested to verify their validity as fraud detectors. The two analysed parameters, ranging from −29.8‰ to −21.9‰, are well correlated (R2 = 0.7802) and the northern Italian regions showed significantly more negative δ13C values for both sugars than the rest of the dataset (Figure 1). By using the LC-IRMS technique, the addition of exogenous sugars, such as fructose and glucose from C4 photosynthetic cycle plants, is easily detectable as it modifies the δ13C of the individual sugars