Contamination-free process measurement is essential for efficient monitoring of fermenters. This contribution introduces the use of high-frequency sensors for the contactless measurement of the permittivity and conductivity of fermentation media. As microwaves can penetrate through plastic materials without much attenuation, it is possible to achieve measurement setups that can measure the media under test without any direct contact and with long-term stability. The sensor can be installed directly to the outer surface of the fermenter or can be clamped onto tubing.