Detection of adulteration in walnuts with edible nuts using bottom-up proteome analysis

The present study identifies the adulteration of walnut products by other edible nuts, specifically cashew, hazelnut, and peanut by bottom-up proteome analysis. The protein extracts of the samples were proteolytically digested and analyzed by micro-flow liquid chromatography ion mobility quadrupole time-of-flight mass spectrometry (microLC-IM-QTOF) with subsequent peptide profiling. Guided peptide identification in the non-target LC-MS/MS data was optimized using a custom protein sequence database, leading to the identification of 11 selective marker peptides for the examined matrices cashew, hazelnut, and peanut. Adulteration levels down to 1% (w/w) could be reliably detected in spiked walnut samples. The authenticity of an independent test set with eight potentially adulterated samples was assessed in comparison to 27 authentic reference samples, achieving 100% accuracy. The acquired non-targeted LC-MS/MS data allow post-hoc re-processing with different protein sequence databases, in case other adulterants should be investigated.