Dielectric Properties of Raw Milk as Functions of Protein Content and Temperature

Abstract

To understand the effect of protein content on dielectric properties (dielectric constant e ′ and dielectric loss factor e ″) of raw milk and to provide information for developing protein content detector being suitable for routine laboratory analysis or real-time quality monitoring, the values of e ′ and e ″ of raw cow’s milk with the protein content of 3.21–7.12 % were measured over the frequency range from 10 to 4500 MHz at temperatures from 25 to 75 °C by using a vector network analyzer and an open-ended coaxial-line probe. The results showed that the e ′ decreased with increasing either frequency or temperature. e ″ decreased linearly with frequency in a log-log plot at low frequency end and had minimums at about 2000–3500 MHz. The minimums increased with temperature. Below about 600 MHz, e ″ increased with increasing temperature and decreased above 1000 MHz. e ′ increased linearly with an increase of protein content below about 150 MHz and decreased linearly above 600 MHz, and e ″ increased linearly with increasing protein content over full investigated frequency range. The developed third-order polynomial models could describe the dielectric properties of raw cow’s milk as functions of protein content and temperature at a given frequency exactly. If the dielectric properties and temperature of milk can be obtained, its protein content could be sensed.