Stable isotopes for the differentiation of eggs from barn, free-range and organic farming in the German food retail market

The National Reference Centre for Authentic Food (NRZ-Authent), which supports the official food control authorities in Germany in the fight against food fraud, is responsible for developing analytical authentication methods and providing databases with reference data sets. High profits can be made by labelling conventional eggs as organic, as consumer prices for organic eggs are 2-3 times higher in food retail. Previous studies on eggs using δ¹³C and δ¹⁵N values showed good approaches to differentiation, but no clear allocation of conventional and organic eggs. However, these studies did not consider potential variations in isotope values due to seasonal changes in feed composition. Fluctuations may be due to natural variations in vegetation and insect abundance, but also to the application of seasonal feeding practices. The present pilot study aimed to improve the identification of eggs from all three farming systems (barn, free-range, organic) considering seasonal variations as well as phases of compulsory stabling (during avian influenza) through high-resolution monthly sampling for the first time. A total of 180 egg samples from Schleswig-Holstein and Bavaria were analysed for their CNS isotopes, which showed a high variability over the studied 15-month period. The results will serve as a basis for an IRMS database supporting food German control authorities of the federal states and will be expanded in further studies with authentic samples from additional locations.