Comparative study of C-13 composition in ethanol and bulk dry wine using isotope ratio monitoring by mass spectrometry and by nuclear magnetic resonance as an indicator of vine water status

The potential of wine C-13 isotope composition (delta C-13) is presented to assess vine water status during grape ripening. Measurements of delta C-13 have been performed on a set of 32 authentic wines and their ethanol recovered after distillation. The data, obtained by isotope ratio monitoring by mass spectrometry coupled to an elemental analyser (irm-EA/MS), show a high correlation between delta C-13 of the bulk wine and its ethanol, indicating that the distillation step is not necessary when the wine has not been submitted to any oenological treatment. Therefore, the ethanol/wine delta C-13 correlation can be used as an indicator of possible enrichment of the grape must or the wine with exogenous organic compounds. Wine ethanol delta C-13 is correlated to predawn leaf water potential (R (2) = 0.69), indicating that this parameter can be used as an indicator of vine water status. Position-specific C-13 analysis (PSIA) of ethanol extracted from wine, performed by isotope ratio monitoring by nuclear magnetic resonance (irm-C-13 NMR), confirmed the non-homogenous repartition of C-13 on ethanol skeleton. It is the delta C-13 of the methylene group of ethanol, compared to the methyl moiety, which is the most correlated to predawn leaf water potential, indicating that a phase of photorespiration of the vine during water stress period is most probably occurring due to stomata closure. However, position-specific C-13 analysis by irm-C-13 NMR does not offer a greater precision in the assessment of vine water status compared to direct measurement of delta C-13 on bulk wine by irm-EA/MS