Improved detection of sugar addition to apple juices and concentrates using internal standard 13C IRMS

In order to improve the detection of sugar addition in apple juice using isotopic methods, malic acid has been defined as a suitable internal standard for the determination of the carbon 13 content. After a clean-up step, sugars and organic acids were separated from each other by an anion exchange process, and pure malic acid was isolated by preparative reversed-phase liquid chromatography (LC). This method has been applied to the stable isotope analysis of apple juice samples of different origins and years. A correlation between the carbon isotope ratios of sugars and malic acid has been observed, and cutoff points concerning the difference of δ13C values between those metabolites have been defined. In the case of the addition of sugar from C4 plants (such as corn and cane), it has been demonstrated that the detection limit can be as low as 5% whereas it is often higher than 10% while using the conventional carbon 13 method on the whole juice.