Identifying the extension of fruit juices with water is a great challenge. In this study, Brazilian researchers used the isotope ratio (18O/16O) as the authenticity marker for the addition of exogenous water in grape juice. The development and validation of the assay were performed using Isotope Ratio Mass Spectrometry (IRMS), and the effect of temperature and evaporation of δ18O in experimental juices was evaluated, to determine their reference values.The influence of the juice industrial production process on 18O values was verified, and commercial juices were evaluated in relation to the reference values of the addition of water. The temperature and evaporation parameters did not influence the results of the 18O of the juice, as they presented differences lower than the method uncertainty. The heat exchanger system did not influence the proposed method either. Hence, the reference values for juice is derived from the grape musts, without affecting the interpretation of the final results. The method was tested on 30 commercial samples of whole grape juices, of which 9 had exogenous water, 3 proved to be reconstituted juices, and 18 were considered to have no exogenous water.
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