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 ε′ and dielectric loss factor ε″) 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 ε′ and ε″ 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 ε′ decreased with increasing either frequency or temperature. ε″ 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, ε″ increased with increasing temperature and decreased above 1000 MHz. ε′ increased linearly with an increase of protein content below about 150 MHz and decreased linearly above 600 MHz, and ε″ 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.

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