Impedance sensor for rapid enumeration of E. coli in milk samples

Detection of pathogenic bacteria in dairy farms and processing plants is important for milk quality control. In this study, a simple in situ method for determining the bacterial concentration in milk was developed with an interdigitated microelectrode sensor. This method was based on a punctual measurement of the electrode-milk interface impedance change due to bacterial metabolism in the milk. The interface impedance varied predominately at low frequencies, attributed to the change on the double-layer capacitance. The detection time T_D, defined as the time required to attain a -10% impedance change at 10 Hz, became a practicable means for the estimation of initial E. coli concentration in milk samples. Regression analysis resulted in a third-order polynomial relationship between the detection time and the logarithm of the initial E. coli concentration (N₀, cells/ml) in a milk sample, i.e., log N₀ = 11.48 - 2.33 T_D + 0.288(T_D)² - 0.0154(T_D)³ with R² = 0.9988. The sensor was able to detect E. coli in milk of initial concentrations as low as 7 cells/ml. The proposed impedance sensor is suitable for determining bacterial contamination at dairy farms and processing plants.