Dielectric spectroscopy in agriculture

Reported measurements of the dielectric properties or permittivities over broad frequency ranges for some agricultural materials are cited, and graphical data from the cited literature are presented. They include 50-kHz to 12-GHz frequency-domain permittivity data for adult rice weevils and hard red winter wheat, time-domain reflectometry data for the same insects from 30 MHz to 1 GHz, frequency-domain data from 200 MHz to 20 GHz for adults of the lesser grain borer, 200-MHz to 20-GHz data for apple juice and fresh peaches, and 10-MHz to 1.8-GHz data, as a function of temperature, for whey protein gel and navel orange tissue. Interpretation of the data showed that selective dielectric heating of rice weevils in wheat could be expected between 10 and 100 MHz but not at microwave frequencies. For fresh fruit and vegetable tissues, temperature dependence of the dielectric constant was minimal at some frequency in the 20–120 MHz range with a positive temperature coefficient below that frequency and a negative temperature coefficient at higher frequencies. Between 10 and 300 MHz, the loss factor can be very well expressed as a linear function of the log of frequency and temperature. With further study dielectric spectroscopy may be useful in detecting product quality factors.