Study Of Lethal Effect On Microorganisms Induced By Coupled Multiphysics In Pulsed Electric Field Treatment Chamber

The safety issue of liquid foods is always attracting widespread interest. The quality and safety of products, including not only fruit and vegetable juice, but also milk products and alcoholic liquor, largely depend on that if the pasteurization producer is reliable. Although the traditional thermal pasteurization method is capable to inactivating the microorganisms, the physical and chemical properties of treated food products would change. It causes the variation of color, flavor, taste and nutrients, occurring of chemical reactions, and degrading of freshness, which seriously affects the quality of the food. As an innovative nonthermal processing technology mainly for liquid and pumpable foods, pulsed electric field (PEF) technology is predominantly used for the inactivation of microorganisms and enzymes at ambient or mild temperature, thereby preserving the fresh flavor, functional properties, and integrity of heat-sensitive compounds. It presents an attractive prospect in the field of liquid food processing with excellent treatment efficient and low operating costs.Utilizing the knowledge of different fields, such as numerical simulation, computational fluid dynamics, microbial metrology, chemometrics and fluorescent optical fiber sensing technology, this research focuses on the impacts of configuration parameters of treatment system and operating parameters of PEF processing on the inactivation of pathogenic and spoilage microorganisms. The multiphysics coupled models with different geometries were developed for analyzing the influence of chamber configuration on the physical environment in the treatment chamber. The impact of flow rate and initial temperature of treated medium was studied to calculate the fall time of square pulses. The relationship between microbial morphological characteristics and PEF treatment efficiency was also analyzed. The kinetic models of microbial inactivation were established base on PEF experiments to study the effects of operating parameters on the microorganisms. In order to describe the microbial reduction based on the distribution discipline of various physical fields and processing operation parameters, the quantitative prediction model is constructed through a combination of both two models mentioned above:the multiphysics coupled model and the kinetic model for microbial inactivation. This study is to prove the feasibility of optimizing the configuration of treatment chamber and preferring operating parameters of PEF processing based on numerical simulation method. It may provide new ideas and theoretical basis for the wide application of PEF technology in the liquid food processing. It has profound effects on improving processing efficiency and reducing production costs.The main results and conclusions were listed as follows:(1) The influence of different configurations on the distribution of physical field in the treatment chamber was analyzed. Based on the numerical simulation method, multiphysics coupled models for each kind of chamber were developed. The results indicated that:1) In the parallel plate static treatment chambers, the electric field distribution was uniform, however, the temperature of treated medium obviously raised as the increasing of treatment time.2) In the coaxial continuous treatment chambers, the distribution of electric field was not homogeneous. The uniformity of electric field depended on the radial difference between inner and outer electrodes. With the axial distance increasing, the temperature in the coaxial chamber increased linearly. However, the temperature fluctuation was weak in the radial direction. Besides, the turbulent kinetic energy in the coaxial chamber became lower with the increasing of electrode distance.3) In the co-field continuous chambers, the axial distribution of electric field strength along the centerline was more uniform than that at0.1mm distance from the wall of the insulator. The high field intensity peaks were observed in the vicinity of edge interfaces of the electrodes and insulator. Both the relative electric field strength and the relative specific energy input linearly increase with increasing radius to gap ratio(r/d). The temperature increased with the flow direction of treated medium, which in the first treatment zone was higher than the second treatment zone. At each outlet of treatment zone, the temperature increased significantly in the vicinity of edge interfaces of the electrodes and insulator, where the peak temperature was observed. The results indicate that based on the multiphysics coupled models, the physical variables in the PEF treatment chambers could be analyzed, achieving the relationship between geometric parameters of electrodes and electric field, energy input, as well as temperature. (2) The influence of insulator shape on the on the distribution of physical fields in the co-field treatment chamber was analyzed under laminar and turbulent flow conditions. Three geometries of the insulator configuration of the optimized co-field treatment chambers were modeled:rectangular, chamfer edge, and circular arc. The results indicated that:1) Under laminar flow condition, the intensity distribution of physical fields was not improved in the optimized co-field chambers, on the contrary, both electric field and temperature peak values were higher than those in the original configuration.2) Under turbulent flow condition, the distribution of electric field strength was improved in all three optimized treatment chambers. The electric field peaks still existed at the same location, however, the strength was significantly weakened. Compared with laminar flow, the flow rate in the chamber was more homogeneous, the difference of which along the centerline and at0.1mm distance from the wall of the insulator was reduced significantly. The effect of turbulent kinetic energy on the temperature controlling was obvious, and the temperature in the treatment chambers (B) and (C) was much lower than original configuration.3) Under both laminar and turbulent flow conditions, both the relative electric field strengthand the relative specific energy input depended on the ratio of inset depth to electrode radius (i/r), as well as the ratio of electrode radius to gap distance(r/d).4) Under both laminar and turbulent flow conditions, the relative temperature increasing was a function of gap distance (d), the ratio of inset depth to electrode radius (i/r), and the ratio of electrode radius to gap distance (r/d). The goodness of fitting of relative temperature increasing under turbulent condition (0.921<R2<0.943) was better than that under laminar condition. The results indicate that the distribution of physical fields in the optimized treatment chambers would be improved under turbulent flow condition. Based on the multiphysics coupled models, the relationship between geometric parameters of insulators and electric field, energy input, as well as temperature could be achieved.(3) The influence of flow rate and initial temperature of treated medium on the waveform of square pulse was analyzed. The results indicated that:1) Based on the multiphysics model for co-field treatment chamber, the impacts of flow rate and initial temperature on the electrical conductivity of treated medium, the current through the cross-section of efficient treatment area, and the equivalent parallel resistance of treatment chamber were studied. The simulated results indicated that the variation tendency of electrical conductivity of treated medium and current through the cross-section were similar, both of which increased gradually with the increasing of initial temperature, but decreased as the fluid flow rate increased. On the contrary, the fall time of pulse was directly proportional to flow rate but inversely proportional to initial temperature.2) The relationship between equivalent resistance and two tested operating parameters was established. The results indicated that the fitting goodness of the model was available whether for single treatment zone or for parallel-connected chambers, and the determination coefficients were higher than0.980, as well as the RMSE values were lower than0.003.3) The real waveform was acquisited using oscilloscope and NI SignalExpress software, and the fall time of square pulse was calculated and compared to the simulated results. The variation trend of experimental fall time was consistent with simulated results, however, the experimental values were slightly larger than the calculated values.4) The practical significance of shorten fall time of pulse was proposed, which was benefit to controlling the temperature increasing and reducing the energy consumption. The results indicate the fall time of square pulse is related to the flow rate and initial temperature of treated medium.(4) The relationship between microbial morphological characteristics and PEF treatment efficiency was also analyzed.1) The influence of cell size and shape as well as the membrane thickness on the transmembrane potential of different microorganisms such as Staphyloccocus aureus, Escherichia coli DH5a and Saccharomyces cerevisiae were studied by means of numerical simulation method. The results indicated that with the same intensity of electric field, S. cerevisiae presented a largest transmembrane potential, E. coli DH5a the second, S. aureus the third.2) The PEF resistance of three strains of microorganisms in grape juice was investigated by applying treatments ranging from12to24kV/cm and from30to180μs at an initial treatment temperature of30℃. In agreement with the simulation results of transmembrane potential, S. cerevisiae exhibited the least resistance to PEF treatments, and S.aureus presented least sensitivity to PEF.3) The larger cell size could increase the transmembrane potential and induced electric field strength in the cell, hence led to more sensitivity to PEF treatment.4) With the same major-axis and minor-axis diameters, the rod cells had higher transmembrane potential and induced field strength than elliptical cells.5) As the membrane thickness increased, the transmembrane potential increased slightly, however, the induced electric field strength decreased obviously. The concentrated electric field in the cell membrane enhanced the sensitivity to PEF treatment, which caused more reduction of microorganisms. The results indicate that numerical simulation could be applied to calculate the transmembrane potential in different microorganisms and predict the resistance to PEF treatment.5) The lethal effects of various PEF processing parameters on the microorganism were analyzed.1) PEF was applied at the initial temperature of25-35℃, the electric field strengths of12-21kV/cm, with respective treatment times of30-180μs to evaluate the efficiency of this technology on the inactivation of spoilage yeast S. cerevisiae commonly associated with rice wine. The results indicated that the highest inactivation value was approximately5.5-log cycles.2) The results from atomic force microscope imaging of yeast cells indicated that PEF treatment induced the destruction of cell membrane structures, which supported by the decrease of yeast number, the blurring of images, and the flattened yeast border.3) Based on fitting with log-linear model, Weibull model and polynomial regression model, the count of survival yeast was fitted. The model validation results showed that the predicted count of survival yeast calculated from Weibull model (Af=1.082and Bf=1.019) was slightly accurater than polynomial regression model (Af=1.152and Bf=0.961), and the5-D values calculated from the Weibull model (Af=1.169and Bf=0.996) provided more accuracy than those from the log-linear model (Af=1.773and Bf=1.773). The results suggest that both Weibull model and polynomial regression were more suitable for calculating the survival ratio of PEF-treated spoilage microorganisms in Chinese rice wine.(7) The feasibility of estibalishing the quantitative prediction model through a combination of both multiphysics coupled model and kinetic model was demonstrated. In order to describe the microbial reduction based on the distribution discipline of various physical fields and processing operation parameters, the quantitative prediction model is constructed through a combination of both two models mentioned above:the multi-physics coupled model and the kinetic model for microbial inactivation. Selecting S. cerevisiae in Chinese rice wine as research subject, the kinetic model suitable for model conbining was developed based on polynomial regression method, and was transferred to the partial differential equation about the time variation. Comparison of the simulated and measured count of survival yeast showed that implementation was successful in predicting the treatment efficiency. It could be concluded that coupling the inactivation field to the three physical fields (electric field, thermal field, and fluid flow field) using a finite element method was feasible.