An untargeted strategy based on UHPLC-QTOF-HRMS metabolomics to identify markers revealing the terroir and processing effect on thyme phenolic profiling

Thyme is one of Europe's most consumed aromatic herbs and represents a matrix susceptible to intentional mislabelling and food frauds. In this study, a phenolic profiling approach based on UHPLC-QTOF-HRMS untargeted metabolomics was used to trace its geographical origin, as well as to assess the effect of post-harvest processing by comparing sterilized vs non-sterilized thyme. Both unsupervised and supervised statistics led to reliable sample clustering, high-quality model parameters, as well as the identification of a total of 45 differential compounds (markers) for discrimination purposes. The phenolic signature was markedly affected by environmental conditions related to the region of production, leading to an overall higher abundance of flavonoids in Moroccan thyme (from Fez), flavanols in Polish one (from Lublin), and tyrosols and other phenolics in thyme cultivated in Spain (from Castilla-La Mancha). The processing was also shown to play an important role in phenolic profiling, noticing not only the decrease of thermolabile phenolics (such as flavonoids) but also the enhancement of other phenolic subfamilies in response to sterilization. This study opens the path to novel metabolomics-based strategies to support the integrity of thyme and possibly other spices, scarcely studied so far.