Effect of solids-not-fat content on dielectric properties of skim milk

Dielectric constant (epsilon') and loss factor (epsilon '') of skim milk samples at different total solids-not-fat contents were measured over the frequency range of 20-4500 MHz at 25 degrees C using a vector network analyser. The results showed that epsilon' of skim milk decreased as frequency increased, and epsilon '' changed from decreasing to increasing and had minimums at about 2000 MHz. There was a linear relationship between epsilon' and solids-not-fat content with correlation coefficient higher than 0.940 below 50 MHz and keeping at -0.990 above 150 MHz. epsilon '' had positive linear relationship with solids-not-fat content, and their correlation coefficient was higher than 0.970 at all investigated frequencies. Penetration depth of electromagnetic wave in milk decreased as increased frequency or solids-not-fat content. The study is helpful to know the relationship between solids-not-fat content and dielectric properties, and it provides information for developing solids-not-fat detector could be used in on-line detection for skim milk production.