Detection of electrical properties of soybean milk based on AC impedance method

Abstract: In order to rapidly detect soybean milk quality and to optimize the frequency of power for ohmic heating device, the electrical impedance characteristics of soybean milk with different solid contents (ranging from 1.01% to 9.58%) at different temperatures (ranging from 30 ?C to 85 ?C with 5 ?C interval) were measured with an impedance analyzer at a frequency ranging from 20 Hz to 12 MHz. The impedance analyzer equipped with a commercial conductivity electrode (DJS-10) was operated at a measurement voltage of 50 mV. The temperature of the soybean milk contained in a jacket beaker was controlled with water flowing through the jacket at a constant temperature. The results showed that the impedance amplitude of the soybean milk decreased with the increase in frequency at a low frequency (f<300 Hz), and the impedance amplitude of the soybean milk did not change in the middle-frequency range (300 Hz 300 kHz). The impedance phase angle decreased with the increase in frequency at the low frequency range, and it tended to be zero in the middle-frequency range, while it showed the trend of increase in the high frequencies range. The results showed that the electrical impedance of soybean milk was significantly influenced by the temperature and solid content of soybean milk. There were clear distinctions among the Nyquist plot of electrical impedance for soybean milk with different solid contents at different temperatures. The electrical impedance amplitude of the soybean milk decreased with the increase in temperature and solid content in full frequency range. The impedance phase angle of the soybean milk increased with the increase in temperature and solid content in the low frequency range, while it showed the trend of decrease in the high frequency range. An equivalent electrical circuit of soybean milk which consisted of a resistor a capacitor and a constant phase element (CPE) was built for simulating the impedance data acquired during experiments. The electrical parameters of the equivalent electrical circuit were acquired with the ZSimpWin software. By analyzing the change in the impedance of the soybean milk with the frequency, combining with the equivalent circuit model, it was concluded that the frequency of the power for ohmic heating should be in the range from 300 Hz to 300 kHz. The experimental results also showed that the equivalent resistance (R) of soybean milk decreased with the increase in the temperature and the solids content, respectively. By the regression analysis on the data, it was found that the relationship between the equivalent resistance (R) and the temperature could be described by a negative exponential function (the coefficient of determination R2 was 0.996 at the solid content of 9.58%, P<0.01), the relationship between the equivalent resistance (R) and the solid content could be also described by a negative exponential function (the coefficient of determination R2 was 0.992 at 40 ?C, P<0.01). The value of Q (one of the parameters for describing the CPE) decreased with the increase in temperature; however, if the solid content of soybean milk was more than 3.5%, the value of Q increased sharply. The value of n (the other parameter for describing the CPE) increased with the increase in temperature, but it did not changed significantly with the solid content (ranged from 1.01% to 9.58%). The results in this research should be taken as a reference for selecting the frequency of the power for designing an ohmic heating system. In addition, the results also founded a method for detecting rapidly soybean milk quality on the analysis of electric parameters of soybean milk.