Study On Developing The Growth Model Of Main Microorganisms In Raw Milk By Impedance Method

The microbial number is an important index of raw milk's quality and safety and lots of rapid microorganism determination methods have developed. However, effective prediction of microbial numbers is far better than conventional detection in terms of quality control and cost saving. Given the specific environment condition (temperature, pH, et al), instead of detection, the microbial number can be predicted according to the predictive models. The establishing of predictive models is based on the growth curves of microorganism. As the standard method, plate count method is too labor and time consuming, and the data is limited. Therefore, alternatives such as impedance methods and absorbance method have caused more attention from scientists. Impedance method, as a rapid microbial detection method was recognized as the industrial standard method for the determination of total microbial numbers in raw milk. The studies of using conductance/impedance curve of microorganism for predictive models establishing are limited. In this thesis, the impedance technology was used for modeling the growth of Acinetobacter lwoffi and Pseudomonas fluorescens, and the results were compared to those of plate count method. The research contents and conclusions are as follows:(1) The impedance detection system was constructed. The system includes precision impedance analyzer, sample cell, temperature controller and computer with data recording software. The testing frequency (1kHz) and voltage (20mV, 10mV) were determined.(2) The models (primary and secondary model) for predicting the specific growth rate (μm) of A. lwoffi in the broth medium under different temperature (5-30℃) and pH (5-8.8) were established by plate count method and impedance method, respectively. The primary models showed a good fit (R2>0.9 and>0.98) to a Gompertz equation to obtain theμm of A. lwoffi and conductance/admittance of each condition. The cubic models were established to describe the relationship between the μm of conductance/admittance of broth medium (x) and theμm of A.lwoffi (y) (R2=0.859 and 0.806). To analyze the individual effects and the interaction of these factors, quadratic polynomial models (the secondary model) were developed. The three quadratic polynomial models were found to be significant (Pr<0.05) and the predicted values were found to be in a fair agreement with experimental values. Through response surface analyzing, the maximalμm of A. lwoffi could be 0.276h-1 under 22.9℃and pH=6.68.(3) The models (primary and secondary model) for predicting the specific growth rate (μm) of P. fluorescens in the broth medium under different temperature (5-30℃) and pH (5.6-8.2) were established by plate count method and impedance method, respectively. The primary models showed a good fit (R2>0.96 and>0.985) to a Gompertz equation to obtain theμm of P. fluorescens and conductance/admittance of each condition. The S models were established to describe the relationship between theμm of conductance/admittance of broth medium (x) and theμm of P. fluorescens (y) (R2=0.954 and 0.955). The three quadratic polynomial models were found to be significant (Pr<0.05) and the predicted values were found to be in a good agreement with experimental values. Through response surface analyzing, the maximalμm of P. fluorescens could be 0.248h-1 under 25.0℃and pH=7.89.