Wireless, passive, resonant-circuit sensors for monitoring food quality

The inductor-capacitor (LC) sensor, comprised of a thick-film printed LC resonant circuit the resonant frequency of which can be remotely detected with a loop antenna, has been used for the monitoring of temperature, humidity, atmospheric pressure, salt concentration, and complex permittivity, as well as the detection of bacteria hi a liquid medium based upon changes in the complex permittivity due to the bacteria growth. Due to its low unit cost and wireless detection, the LC sensor is potentially suitable for commercial scale monitoring of food quality. This paper includes the operational principles and design criteria of the LC sensor, and illustrates the monitoring of bacteria growth in milk, meat, and beer.