Abstract The association of sinapine with the fishy or crabby odour present in eggs laid by certain brown egg laying hens when fed diets containing rapeseed meal indicates the desirability of removing sinapine from the original rapeseed. A method is presented whereby seeds and cotyledons of Brassica napus and B. campestris may be selected for low levels of sinapine. The method, based on microextraction, rapid chromatographic separation of sinapine and visual estimation of its fluorescence allows detection of 0.025 μg sinapine and enables 300‐400 seed samples to be screened daily.