Detection of added citric acid

Citric acid (in various chemical forms) is one of the most widely used additives for fine-tuning acidity in fruit products.

Since the natural ranges of citric acid concentrations in fruits are very wide, fairly large additions may well be undetected. Isotopic methods present a satisfactory solution to the discrimination between endogenous and exogenous sources of citric acid. Two decades ago, isotope ratio mass spectrometry (IRMS) determination of the 13C/12C isotope ratio was proposed as a means of detecting the addition of citric acid from C4 plant sources (cane or maize) in C3 fruits such as citrus. However, many other sources remained undetectable.

More recently it has been shown that the simultaneous use of the 13C/12C ratio of citric acid and of the 2H/1H of its non-exchangeable sites enables the detection of all commercial sources of citric acid in fruit juice.

A method based on the transformation of citric acid to its calcium salt followed by a pyrolysis-IRMS measurement has been developed in Eurofins' Nantes laboratory and peer-tested in the context of an EU-funded project. The method is applicable to all types of fruit juices containing high concentrations of citric acid (citrus, pineapple, red fruits, etc). Preliminary market observations tend to indicate that citric acid derived from cheap C3-plant sources is sometimes used without declaration.