Liquid Food Sterilization By Ultrasound-assisted Pulsed Electric Field And Its Treatment Chamber And Pulse Generator

High-voltage pulsed electric field(PEF)liquid food sterilization is a non-thermal sterilization technology,which kills microbial cells by irreversible electroporation.It can inactivate microorganisms in food at a lower temperature than other techniques while maintaining the original nutrition and flavor of the food.The electric field strength is the determining factor for the efficiency of PEF sterilization.A standalone PEF technology requires a higher electric field strength(above 30 k V/cm)to meet industrial applications.However,as the intensity of the electric field increases,the risk of food contamination will also increase because of electrochemical reactions in the processing chamber.At the same time,numbers of studies have proved that it is easier to induce electrodes discharge or even breakdown under high electric field strength,which will cause serious damage to the pulse generator.In order to reduce the electrochemical reaction and the risk of dielectric breakdown in the PEF treatment chamber in the practical applications,two tactics were used in the research.On the one hand,ultrasound was introduced to assit moderate PEF for a higher inactivation rate.On the other hand,the pulse generator of the system was promoted,which can withstand higher voltage and current,by using series-parallel connection of IGBTs.The main research content and conclusions are as follows:(1)A novel electro-sonoporation microdevice that can simultaneously apply ultrasound(US)and pulsed electric field(PEF)in a flow-through scheme was developed,and was used to investigate the combined effects of two hurdles for Saccharomyces cerevisiae inactivation in the phosphate buffer solution.A synergistic effect for S.cerevisiae inactivation(4.26 log)was observed when simultaneous combined treatment of US(25 °C,400 W,8 s)and PEF(25 °C,12 k V/cm,3200 μs)was applied,while only 1.28 log and 2.44 log reduction were observed when the two hurdles were applied individually.(2)Simulation and experiment methods are used to analyze the current sharing characteristics of 4 parallel IGBTs,and 4 parallel IGBTs components were built.The parameter discreteness of three 1200 V-class IGBTs were evaluated,and the IGBT with the most concentrated parameters was selected as the follow-up research object.A half-bridge IGBT driving circuit based on capacitive isolation was designed.Its output peak voltage was up to 14 A,which can simultaneously drive 4 parallel IGBTs.Numerical modeling of the selected IGBTs was used to study the static or dynamic uneven current characteristics caused by the asymmetry of the drive circuit and power circuit layout.The simulation structure was used to optimize the grid layout of the parallel circuit and used to design a circularly arranged 4-pipe parallel assembly,with a dynamic unbalanced current factor of less than 0.18.(3)Using the parallel connection of 4 IGBTs as the foundation,the problem of the series voltage equalization of 64 series connected IGBT components was studied.A high-power solid-state switch composed of 256 IGBT single-tube stacks in 4 parallel and 64 series was successfully developed.To solve the problem of unsynchronized switching of the series components,the multi-channel signal synchronization methods using fiber optics were explored.A multi-channel trigger signal synchronization link based on laser diodes and optical splitters was designed,and 64 channels of drive signals were designed.The maximum delay time was controlled within 25 ns;16-channels of 12 V/800 m A series isolated auxiliary power supplies were designed to provide an isolated power supply for each series-connected board.Finally,a test of the performance of the constructed high-power solid-state switch proved that it can work stably at 35 k V/150 A,and the current rise time was 632 ns.The pulse generator based on the solid-state switch can output adjustable quasi-square wave pulses with the amplitude of 1 k V ～35 k V,pulse width of 3 μs ～ 10 μs,and frequency of 100 Hz ～ 1000 Hz.(4)A pilot-level ultrasound assited high-voltage pulsed electric field liquid food sterilization system has been developed based on the synergestic effect of the two hurdles for a higher thoughput.A co-field ultrasound assited pulsed electric field treatment chamber was designed to realize the simultaneous coupling of two sterilization technologies,i.e.,an US and a high-voltage PEF operating together.The entire system was functionally tested using Na Cl solutions with different conductivities as the processing medium.The entire system was functionally tested using Na Cl solutions with different conductivities as the processing medium.Though the sensors and self-developed monitor software,the results showed that all components in the system can work effectively,and the microbial cells in the effective processing area were simultaneously subjected to US and PEF.