IMPROVED DETECTION OF SUGAR ADDITION TO APPLE JUICES AND CONCENTRATESUSING INTERNAL STANDARD C-13 IRMS

In order to improve the detection of sugar addition in apple juice usi ng isotopic methods, malic acid has been defined as a suitable interna l standard for the determination of the carbon 13 content. After a cle an-up step, sugars and organic acids were separated from each other by an anion exchange process, and pure malic acid was isolated by prepar ative reversed-phase liquid chromatography (LC). This method has been applied to the stable isotope analysis of apple juice samples of diffe rent origins and years. A correlation between the carbon isotope ratio s of sugars and malic acid has been observed, and cutoff points concer ning the difference of delta(13)C values between those metabolites hav e been defined. In the case of the addition of sugar from C4 plants (s uch as corn and cane), it has been demonstrated that the detection Lim it can be as low as 5% whereas it is often higher than 10% while using the conventional carbon 13 method on the whole juice.