Validation of methods for H, C, N and S stable isotopes and elemental analysis of cheese: results of an international collaborative study.

Rationale: PDO cheeses, such as Parmigiano Reggiano and Grana Padano, which cost more than double generic similar cheeses, must be protected against mislabelling. The aim of this study was to validate the methods for the isotopic and elemental analysis of cheese, in order to support official recognition of their use in authenticity assessment.

Methods: An international collaborative study based on blind duplicates of seven hard cheeses was performed according to the IUPAC protocol and ISO Standards 5725/2004 and 13528/2005. The H, C, N and S stable isotope ratios of defatted cheese determined using Isotope Ratio Mass Spectrometry (IRMS) and the content of Li, Na, Mn, Fe, Cu, Se, Rb, Sr, Mo, Ba, Re, Bi, U in cheese after acid microwave digestion using Inductively Coupled Plasma Mass Spectrometry or Optical Emission Spectrometry (ICP-MS or -OES) were measured in 13 different laboratories.

Results: The average standard deviations of repeatability (sr) and reproducibility (sR) were 0.1 and 0.2 ‰ for δ(13)C values, 0.1 and 0.3 ‰ for δ(15)N values, 2 and 3 ‰ for δ(2)H values, and 0.4 and 0.6 ‰ for δ(34)S values, thus comparable with results of official methods and the literature for other food matrices. For elemental data, the average RSDr and RSDR values ranged between 2 and 11% and between 9 and 28%, respectively, consistent with methods reported by the FDA and in the literature for cheese.

Conclusions: The validation data obtained here can be submitted to the standardisation agencies to obtain official recognition for the methods, which is fundamental when they are used in commercial disputes and legal debates.