Authentication of lemon juices and concentrates by a combined multi-isotope approach using SNIF-NMR and IRMS

A methodology for measuring the natural deuterium distribution in citric acid by the SNIF-NMR method is presented. Triethyl citrate (TEC) is retained as a molecular probe for H-2 NMR. The method provides efficient criteria for characterizing the authenticity of lemon juices: it is a powerful approach to detect the addition of exogenous citric acid manufactured from beet sugar (or other C3 plants). The whole analytical chain includes the separation and purification of citric acid from lemon juices or concentrates via preparative HPLC, followed by the synthesis of TEC. The present procedure may also integrate a determination of the C-13 contents of sugars, citric acid, and L-malic acid. The correlation between delta(13)C values of sugars and of both acids is very specific and reinforces the discriminating power of the SNIF-NMR method.