Isoflavones quantification in rainbow trout muscle by QuEChERS tecnique and liquid chromatography coupled with mass spectrometry

•QuEChERS is used to extract free and conjugated isoflavones from filet of trout.•A LC–MS/MS method is validated according to the Decision 2002/657/EC.•The limit of quantification was 0.2 μg kg−1 for all the isoflavones monitored.•The method was applied to 26 rainbow trouts fed with or without soy and derivates.•The new method is able to evaluate isoflavones occurrence in foods of animal origin.