A Study Of Experiment On The Relationshiop Between Dielectric Properties And Fat Content Of Raw Milk

Milk fat is the most important nutrients, which is also the key quality index of milk. In order to establish the mathematical equation between dielectric properties of raw milk and fat content, to provide a theoretical basis for applications of radio frequency/microwave dielectric properties technology in milk quality rapid detection as well as to identify abnormal milk and to control of parameters during milk processing, the influence of temperature (25℃～45℃)and frequency (20MHz-4500MHz) on the dielectric properties of raw milk was studied in this thesis. The relationship between dielectric properties of raw milk and fat content (2.47%～3.58 and 0.06%～4.04%) in the temperature (25℃～45℃) and radio frequency/microwave frequency range (20MHz-4500MHz) was also studied by two different fat content change methods(hand-skimmed and centrifugation-skimmed. The relationship models between dielectric properties (relative dielectric constant ε’ and dielectric loss factor ε") of raw milk and fat content was established, and the influence mechanism of various factors on dielectric properties as well as penetration depth were analyzed. The main conclusions were as follows:1) Over the test frequency range of 20MHz～4500MHz, the relative dielectric constant ε’ and dielectric loss factor ε" of raw milk decreased with the increasing temperature, the linear correlation coefficient R2>0.95). When the frequency was greater than 1GHz, the penetration depth Dp of raw milk increased with increasing temperature, the linear correlation coefficient R2>0.90. Relative permittivity of raw milk increased monotonically as the frequency increased and dielectric loss factor decreased firstly and then increased with increasing frequency (the change point at about 2GHz time).2) Over the test frequency range of 20MHz～4500MHz, the relative dielectric constant ε’ of hand-skimmed/adjusted milk decreased with increasing fat content with a significant linear relationship (R2>0.95). Dielectric loss factor ε" decreased with increasing fat content, especially in the low frequency band the ε" decreased faster than that in the high frequency band, with a linear relationship (R2>0.95). The penetration depth Dp increased with the increasing fat content and with a significant linear relationship (R2>0.95).3) Over the test frequency range of 20MHz～4500MHz, the relative dielectric constant ε’ of centrifugation-skimmed/adjusted milk decreased with increasing fat content with a significant linear relationship (R2>0.96). Dielectric loss factor ε" decreased with increasing fat content, especially in the low frequency band the ε" decreased faster than that in the high frequency band, and there was a linear relationship (R2>0.95). The penetration depth Dp increased with the increasing fat content, there showed a significant linear relationship (R2>0.95) and the curve tended to coincide at high frequency end.4) Nearly identical change rules were observed about dielectric properties parameters (ε’, ε" and Dp respectively) of hand-skimmed/adjusted milk and centrifugation-skimmed/adjusted milk, but there are still tiny differences between corresponding relation models, due to the content of various ingredient of milk change with the fat content changed.This study provides basic data for understanding the dielectric properties of raw milk at a different temperature, frequency and fat content. It also provides an experimental basis for the development of rapid detection method based on dielectric properties for the fat content of milk and helps to develop the parameter control tools for milk heating processing process using radio frequency/microwave energy.