The sampling plan was designed in order to cover as much as possible the different food commodities listed in the Annex III of Regulation 1334/2008 (EU, 2008). Twelve food categories were selected and five samples for each group were collected and analysed (see Table S1 inSupplementary information). Samples were collected in Dutch and Belgian supermarkets and stored as specified on the label until further processing.
Fish-, meat-, bakery-, breakfast-, processed-, and mint confectionary products, chewing gums, and soups were homogenised using liquid nitrogen to obtain a fine powder and stored in the freezer. Ready-to-eat products were milled and stored at room temperature. Dairy products and sauces were stored in the fridge, while beverages were kept at room temperature.
Tea samples were analysed as ready-to-drink beverages. As the brewing time has been demonstrated to have a considerable influence on the extraction efficiency of thujone (Walch et al., 2011), all the tea samples were brewed according to the following protocol. A bag of 2 g was immersed into 150 mL of water at 80 °C and left for 5 min. The bag was retrieved and the sample was cooled to room temperature before extraction.