Effects of milk concentration and freshness on microwave dielectric properties

The knowledge of dielectric properties may hold a potential to develop a new technique for quality evaluation of milk. The dielectric properties of water-diluted cow’s milk with milk concentrations from 70% to 100% stored during 36 h storage at 22 °C and 144 h at 5 °C were measured at room temperature for frequencies ranging from 10 to 4500 MHz using open-ended coaxial-line probe technology, along with electrical conductivity and pH value. The raw milk had the lowest dielectric constant when the frequency was higher than about 20 MHz, and had the highest loss factor at each frequency. The highest linear coefficient of determination, 0.995, between the milk concentration and the loss factor at 915 MHz was observed. The change tendency of the loss factor was inversed to pH during milk storage with the best linear correlation (R2 = 0.983) at 1100 MHz. The loss factor can be an indicator in predicting milk concentration and freshness.