Existing theory of high voltage pulsed electric field technology
The sterilization principle of high-voltage pulsed electric field is to produce instantaneous high-voltage pulsed electric field between two electrodes to sterilize food. Its basic process is to treat low-temperature cooled food placed between the poles with instantaneous high pressure. After 40 years of discussion, the sterilization mechanism of high-voltage pulsed electric field has formed the following representative views:
① "Cell membrane perforation effect" theory: When external electric field acts on cells, the cell membrane of food microorganisms is induced to produce transmembrane potential under its action. When the potential of the whole membrane reaches the limit value (about lV), the membrane breaks, which makes the membrane structure enter a disordered state, forms pores and enhances permeability. Therefore, the permeability of cells can be increased according to the intensity of external electric field.
② Electrolyte product theory: When an electric field is applied at the electrode point, the electrolyte in the medium near the electrode ionizes to produce anions, which are extremely active under the action of strong electric field, and pass through the cell membrane with improved permeability under the action of electric field to combine with cell active substances such as protein and ribonucleic acid to denature them.
③ Ozone effect theory: Under the action of electric field, liquid medium electrolyzes to produce ozone, which can effectively kill bacteria at low concentration.
The food industry has always used heat treatment to sterilize, but it has a great influence on the color, aroma, taste and nutritional components of food. However, new non-heat treatment methods, such as adding preservatives, irradiation sterilization, physical treatment and refrigeration, have their limitations. Pulsed electric field (or discharge) can effectively kill bacteria in liquid. Three problems must be solved in practical application: ① whether it causes new pollution; ② Whether it has obvious economic advantages over traditional methods; (3) realize scale. The strong pulsed electric field, especially the strong shock wave generated by high-voltage pulse discharge, and the comprehensive action of ultraviolet light, strong current and ozone can kill microorganisms more effectively than DC current and low-frequency AC voltage, and the energy utilization rate is higher. Electrolytic loss, chemical products of electrolysis, the decomposition of food components by shock waves and ultraviolet rays may all introduce new pollution to food, so continuous discharge arc sterilization is not suitable for food processing industry. It is generally believed that high voltage pulsed electric field (PEF) with short duration (1 ~ 100μ s) can minimize the unfavorable factors such as electrolysis and decomposition. Electric power/span > ? The nuclear magnetic resonance (EPR) test showed that the increase, generation and introduction of new free radicals were not found in the treated samples.
Compared with heating treatment and radiation treatment, pulsed electric field sterilization has obvious economic advantages. When the input energy is the same, comparing the processing efficiency of square wave pulse, exponential decay pulse, oscillation decay pulse and bell pulse, the square wave pulse is the best and the oscillation decay pulse is the worst. When the energy of each pulse is 252.2J, the energy efficiency of square wave pulse can reach 9 1%, while the energy efficiency of exponentially decaying pulse is only 64%. Bipolar square wave pulse is superior to unipolar pulse and is an ideal power supply.
The electric field for effective sterilization is generally ≥20 kV/cm, and the instantaneous value of the output voltage of the high-voltage device is limited, so the electrode gap is generally several mm, and the pulse energy also limits the electrode area, so the volume of the discharge treatment chamber.
So far, the research is still in the experimental stage, and the effects of pulsed electric field on food physics, chemistry, microorganism, enzyme activity and nutritional characteristics need further study.