Theotetical And Experimental Study On Non-Thermal Pasteurization Food By Pulse Electric Fieleds

Pulse electric fields (PEF) for inactivation of microorganisms is one of the more promising technologies in the field of non-thermal food processing, which can maintain the original nature of the food for as long as possible. Heretofore, this technology was not systematically proved on theory yet, for the drive of practicability, some developed countries have paid much attention to the research on testing system and hardware. In order to develop PEF technology to the point of industrial application in our country, some theoretical and experimental research works are completed in this paper.For analyzing pulse electric fields effects on microorganism systemically, firstly, dielectric model of microbial cell is established from its structure, composition and function in the paper. In addition, the membrane polarization time is gained from cell's simplified equivalent circuits and approved by system time constant of the state space equations of cell suspended system. Secondly, Laplace equations of the ideal model are built based on Faraday's Law, Gauss's Law, and charge conservation to analyze and calculate the amplitude and variation of transmembrane potential detailedly. Then the best PEF electric parameters for food processing are summarized from these equations. Afterward, radial pressure upon cell is calculated out through the methods of Maxwell stress tensor and Coulomb virtual work. Lastly, the mechanisms of non-thermal effects by PEF are summarized. According to the amplitude of transmembrane potential, three different effects: membrane dielectric electrical breakdown effects, electric field force effects, transmembrane information disorder effects are developed to enrich the theory about PEF.Designing and building high precise, high voltage pulse generator is expected to be the key technologies for PEF to cold pasteurize liquid food. Historically, there were some ways to generate high voltage generator, such as: using a pulse capacitor to store energy, discharged by trigger vacuum to generate high voltage pulse, or in combination with pulse forming networks (PFNs) to generate high voltage pulse, which can not meet the requirements of PEF technologies. A high precise, high voltage pulse generator, of which trigger signals are generated by a complex programmable logic devices (CPLD) and controlled by a computer, made up of high-power IGBT and pulse transformer isproposed in this dissertation. These technologies, which overcome the shortcomings (voltage and current sharing equally or gate drives highly synchronizing) of cascading power electronic devices in series or parallels, make PEF more practical. A mathematical model to estimate sterile rate (relating to pulse electric field strength, pulse width, pulse numbers) is concluded from lots of experiments on the common microbial bacteria in liquid food (Saccharomyces, E. Coli, Bacillus, Asp. Niger Van Tieghem) treated by the trial PEF system. These experiments also validate the effects proposed to explain non-thermal effects before.In the end, through the experimental data, experience and the analysis of existent continuous PEF systems, a new design plan mainly composed of high power IGCT and pulse transformer is proposed, which has a continuous treatment chamber with about l50L/h capability.