For the visual express-control over animal product freshness, the sensor devices based on conducting polymer films (polyaniline, poly-o-methoxyaniline, and poly-o-toluidine) have been proposed. Sensor action is based on a change in the spectral properties and the corresponding color transitions in conjugate polymer films on the transparent surface at a low partial pressure of ammonia (NH(3)) which is evolved at the loss of protein product freshness.