A processing apparatus that includes: a pump; a microwave chamber; and at least one solid-state radio frequency source. The pump pumps a diary product through the microwave chamber; the solid-state radio frequency source includes one or more antennas and/or waveguides; the processing apparatus has a sensor and a control system, the sensor is configured to measure at least one property of the dairy product and at least one property of radiation reflected from the diary product, and a signal from the sensor is utilized by the control system to control the solid-state radio frequency source; a chamber is arranged between the microwave chamber and the solid-state radio frequency source so that the solid-state radio frequency source is free from contacting a wall of the microwave chamber, the chamber is cooled by a cooling unit that is configured to cool the radio frequency source.