Towards the geographical traceability of the Italian walnut: a case study based on stable isotopes signatures (δ(2H), δ(18O), δ(13C), δ(15N), δ(34S)) and elemental fingerprinting

Juglans regia (L.) is cultivated worldwide for its nutrient-rich nuts. Due to the beneficial properties of its nutrients, walnut is a highly recommended food in the human daily diet and a crop of high economic interest. Italy is among the top five importing countries of walnuts in shell due to the strong decline in the Italian walnuts production in the early seventies combined to a constantly growing demand. Most of the imported walnuts in Italy come from the USA (50%), followed by France (20%) and Chile (14%), and a lower part also from Argentina and Australia. The current context is therefore favourable to promote the development of a local high-quality Italian walnut production. In this respect, tools capable of tracing the origin of walnut can support the process of valorisation and promotion of this product, especially by protecting the authenticity of its geographical provenance. The main aim of this study was to investigate to what extent stable isotope data (H, O, C determined in the lipid and defatted fractions, N and S of the defatted fraction) coupled with 56-element profiles could be used to differentiate Italian walnuts on a regional and global level. A total of 122 walnut samples from six countries and two years of harvest were analysed. Principal Components Analysis suggested a strong effect of the geographical origin, particularly at the regional level. However, values of δ(2H), δ(18O) and δ(13C) and some mineral profiles were significantly influenced also by the harvest year. A preliminary analysis focussing on a few samples of international origin confirmed that the proposed approach could be potentially applied for the discrimination of Italian walnuts. Overall, the results presented support the effectiveness of stable isotopes and multi-element analyses for walnut traceability and local origin protection.