利用电学阻抗技术检测巴氏杀菌乳的物理特性

阻抗特性作为宏观的电学参数可以实现对牛乳的理化性质评估。在20 Hz～12 MHz频率区间采用带螺旋测微仪的平行板测量系统连接阻抗分析仪研究了3类巴氏杀菌的全脂乳、低脂乳和脱脂乳在不同稀释比例及开封后室温储藏时的阻抗特性。提出巴氏杀菌乳的RC等效电路并采用ZSimpWin软件拟合得到了乳样的电学元件参数，发现低脂乳的稀释比例和其等效电阻呈指数关系，决定系数为0.8337(P<0.05)。各类巴氏杀菌乳的阻抗幅值随频率的提高而降低，相位角则呈现先减少后增加的趋势，其阻抗特性与脂肪含量无明显规律性，但与稀释比例存在显著性关系，20 Hz和1033 Hz时的全脂乳和脱脂乳的相位角与稀释比例具有良好的指数回归关系，决定系数为0.9887和0.9493(P<0.01)，而低脂乳于11.4 kHz下相位角与稀释比例呈线性回归，决定系数为0.9846(P<0.01)。室温下开封储藏的乳品内部发生复杂的生化反应且伴随有机物分解，全脂乳和脱脂乳存放时间与其阻抗特性无显著性关系，但低脂乳在207 Hz的激发电场频率下的相位角与储藏时间呈现对数回归关系，决定系数为0.889(P<0.05)。该研究为实现生产中巴氏杀菌乳的浓度和储藏时间分析评估提供了快速而可靠的方法。

Impedance characteristics measurements to assess physical indicators in pasteurized milk

Abstract: The knowledge of electrical properties measurements may hold a potential to develop a rapid technique for quality evaluation or determination of concentration and freshness of foods. Especially impedance characteristics measurements in a frequency range focus on unstructured food materials which do not exhibit a structure with insulating cell membranes including milk, edible oil, juice, whole egg products, or homogenized food. Previous researches have not been reported on two plate electrodes method for detecting pasteurized milk adulteration and deterioration. In this research, the impedance characteristics of UHT whole, low fat and skim milk at room temperature were investigated over the frequency range from 20 Hz to 12 MHz. The measurements were conducted with a impedance analyzer was connected to a specially constructed parallel-plate capacitor sample holder at the constant voltage 200 mV. The parallel-plate capacitor with micrometer was designed with height 1mm by diameter 18 mm.One series of measurements was made on successive dilutions of three kinds of pasteurized milk. Meantime three samples were left at room temperature in parafilm covered beakers for two weeks. The spectrums of these were measured over this period as the milk spoiled. The results showed no obvious correlation between impedance parameters and fat content was observed. The impedance amplitude was greatly affected by the water content. The increasing water content triggered a increase in impedance values of pasteurized milk, irrespective of the measuring voltage frequency. An equivalent electrical circuit of pasteurized milk was deduced for the measured impedance data. By using of ZSimpWin software, the electrical parameters of the equivalent electrical circuit were extracted and the relationships between these electrical parameters and the quality indicators of pasteurized milk were analyzed. Exponential correlations between dilution ratio and equivalent resistance were characterized by R2 values of 0.8337. The impedance amplitude of pasteurized milk decreased monotonically and yet phase angle decreased first and then increased as frequency increased. Each combination of concentration and freshness levels were statistically analyzed at 20 Hz, 121 Hz, 1033 Hz, 11.4 kHz and 12 MHz. Exponential regression relationship between impedance amplitude and dilutions ratio were characterized by R2 values of 0.9887 (whole milk at 20 Hz) and 0.9493 (skim milk at 1033 Hz). However, obvious linear correlations between dilution ratio and phase angle were found in low fat milk with coefficients of determination of 0.9846 at 11.4 kHz. Logarithmic regression relationship between phase angle and storage time were founded in low fat milk, with coefficients of determination of 0.889. The components in milk are decomposed into various acids and other inorganic materials with the growth of microorganisms during storage in room temperature. The freshness was not the most significant factor affecting the impedance characteristics of pasteurized milk. Impedance amplitude, phase angle and equivalent resistance of these electrical parameters can quantify physical indicators of pasteurized milk in the characteristic frequency. This works provide interesting insights aimed at developing a rapid method for determination of physical properties of unstructured food.