The Effect of Storage Time on Dielectric Properties of Pasteurized Milks and Yoghurt

Abstract

The aim of the study was to investigate the effect of storage time on complex dielectric permittivity of opened milk with different fat content and of a natural yoghurt. The measurements were conducted using a vector network analyzer and an open-ended coaxial-line probe in the frequency range from 200 MHz to 20 GHz at 9±0.5oC. The results showed that the real part of dielectric permittivity of the tested products decreased with an increase of frequency, while the imaginary part of dielectric permittivity increased below 14.3 GHz for yoghurt and 13.6 GHz for milks and decreased above those frequencies. During storage the real part of dielectric permittivity of yoghurt and milks slightly decreased. The value of the imaginary part of dielectric permittivity varied much more for milks during time storage. The changes of the imaginary part of dielectric permittivity of the tested products were associated with the changes of pH value. Finally, complex dielectric permittivity spectra of the measured products were modeled with the two-pole and three-pole Debye model. Three-pole Debye function was better fitting to the results.