The present invention relates to a fryer (1) for frying food products comprising a frying vessel (11) for accommodating a frying oil (10), comprising a guided wave radar sensor (21) and an analysis unit (24) for determining a permittivity of the frying oil (10) accommodated in the frying vessel (10) based on measurement data of the guided wave radar sensor (21). The invention further relates to a method for frying food products in a frying vessel (11) of a fryer (1), wherein the frying vessel (11) contains frying oil (10) or melted frying fat, in particular wherein at least one conveyor (2) conveys the food products to be fried through the frying vessel (11), wherein an analysis unit (24) determines a permittivity of the frying oil (10) accommodated in the frying vessel (11) based on measurement data of a guided wave radar sensor (21).