Studies On Sterilization Dynamics Of Pulsed Electric Field And Co-field Chamber Improvement

Based on the killing effect of Pulsed Electric Field(PEF) on microorganisms, the first order kinetics between PEF strength and treatment time and relative survival rate of microorganisms respectively were established with the critical PEF strength Ec and regression coefficients k . Then PEF was applied on killing Escherichia coli O157:H7 and pathogenic salmonella and the effects of PEF and thermal sterilization on orange juice quality were studied. At last, the Laplace equation was used to understand the distribution of electrical field strength in the treatment chamber of a pulse electric field process and improve the co-field chamber. Their effects on PEF sterilization were also investigated. The brief introduction of whole research was as follow:1. The pilot device of PEF was made up of high voltage pulse generator, co-field chamber, peristaltic pump, temperature sensors, temperature recorder, material tank and cooling system. The major parameters of PEF device were as follow: voltage from 0 to 15 kV; pulse duration from 5 to 20μs; flow rate from 5 to 100 mL/min; pulse frequency from 10 to 1000Hz; monopolar pulse and square waveform.2. The co-field chamber with inner diameter 2.00mm, and the PEF system with pulse frequency 32Hz, pulse duration 17μs was used in the research. Saccharomyces cerevisiae CICC 1001, Penicillium citrinum CICC4010 and Escherichia coli CGMCC1.90 were treated by PEF strength 2.5kV/cm, 5.0kV/cm, 7.5kV/cm, 10.0kV/cm, 12.5kV/cm and 15.0kV/cm respectively. The results were as follow:(1) With the strength and pulse number increasing, the killing effect of PEF was improved.(2) Different target microorganisms had different tolerance to PEF. The killing effect of Gram-negative Bacteria was better than Gram-positive Bacteria. The tolerance to PEF ranged from big to small as follow: Penicillium citrinum, Staphylococcus aureus, Escherichia coli and Saccharomyces cerevisiae.(3) The influence of different mediums on PEF kill effect on Escherichia coli CGMCC1.90 was investigated. The result showed that acid medium could benefit for killing bacteria.(4) When the medium temperatures were 25℃, 30℃and 35℃respectively, the effect of temperature combined with PEF on sterilization was not significant. (5) The growth period of target microorganism would also influence the killing effect. The microorganism staying in arrearage growth period was more sensitive to PEF than that in stationary or logarithmic growth period.3. By establishing the mathematical model of PEF strength and the survival rate of Saccharomyces cerevisiae, Penicillium citrinum and Escherichia coli, the effect of PEF strength on sterilization could be described as a piecewise functionIf the strength (E) was below a critical value (E_c),no lethal effect can be exerted to the microorganisms, while E was above E_c, there would be lethal effect. The fitting curve of lethal effect could be gained after series of mathematical operation. The critical field strength E_c was 1.9893kV/cm, 1.8085kV/cm and 0.3339kV/cm for P. citrinum CICC 4010, E. coli CGMCC1.90, and S. cerevisiae CICC1001, respectively. And the sensitivity coefficient k was 0.1139, 0.1629 and 0.4339 for P. citrinum CICC 4010, E. coli CGMCC1.90, and S. cerevisiae CICC 1001, respectively. E_c represented the tolerance to PEE and a large k value would suggest that the microorganism be more sensitive to the change in the field strength. In this research, with the same medium, k was relative to the kind of microorganism, furthermore to the size and the construction of microorganism.4. As the mathematical model of treatment time and survival rate could be describedas s = k₁×e^-k₂t , the results showed that the tolerance of microorganisms to thetreatment time was different. The sensitive parameter k2 was 0.0036, 0.0142 and 0.0153 for P. citrinum CICC 4010, E. coli CGMCC1.90, and S. cerevisiae CICC 1001, respectively. A large k₂ value would suggest that the microorganism be more sensitive to PEF.5. Under the room temperature, the co-field chamber with inner diameter 3mm and the distance between two electrodes 4.00mm and the PEF system with strength 12.5kV/cm, pulse frequency 128Hz, pulse duration 17μs, pulse number 20 was adopted for experiment. The killing effect of PEF on Escherichia coli O157:H7 and pathogenic salmonella was studied. The result showed that Escherichia coli O157:H7 decreased for 2.12 log-value while pathogenic salmonella went down for 4.00 log-value. However it's a long way for commercial sterilization because in our experiment the strength and treatment was limited. In another point, it's hard to get more than 10² pathogenic bacteria in orange juice which was verified in test. In the matter of fact, PEF could be used to kill the pathogenic bacteria in acid medium.6. PEF can effectively kill microorganisms in the fresh orange juice under conditions of 5, whose effect was inferior to the thermal sterilization. Restricted by the equipment, the electric filed strength was only 12.5kV/cm in this experiment, which was not intense enough. After 120 days storage, the total bacterial, yeast and mold colonies conformed to the national standards. That probably because comparatively low pH value about 4.0 combined with low temperature preservation can effectively inhibited the growth of some microorganisms. Compared the effect on soluble solid content, total acid and Vc content in fresh orange juice by PEF treatment with by thermal sterilization, the result showed that either PEF or thermal sterilization had little effect on the changes the soluble solid content and total acid, and the two indexes didn't change much during the storage; but Vc content decreased to 7.7% after thermal sterilization compared to 5% after PEF treatment, Vc content decreased fastest in untreated group followed by thermal sterilization, and PEF group was the slowest after 120 days storage. It is concluded that PEF treatment could effectively reduce Vc content losses compared to thermal sterilization during storage.7. Laplace equation solved by finite difference method was adopted to investigate the electric fields distribution in co-field chamber. The flow rate was 30mL/min, the pulse electric frequency was 128Hz, the square pulse duration time was 17μs and the treatment time was 3.92 ms. Saccharomyces cerevisiae CICC1001, Penicillium citrinum CICC4010 and Escherichia coli CGMCC1.90 were treated by PEF strength 2.5kV/cm, 5.0kV/cm, 7.5kV/cm, 10.0kV/cm, 12.5kV/cmand 15.0kV/cm respectively. And effects of unimproved (the inner diameter was 18.0mm and the distance between two electrodes was 4mm) and improved (the inner diameter was 18.0mm, the distance between two electrodes was 4mm, and 4 pairs of stainless steel conductors with thickness of 1 mm were included) chamber on relative survival rate were studied. Under the same PEF conditions the inactivation effect of improved chamber was much better than the conventional chamber design. When the electric filed strength was up to15.0kV/cm, relative survival rates in the improved chamber were 0.00, 0.0 and 0.05, respectively, for Saccharomyces cerevisiae CICC 1001, Escherichia coli CGMCC1.90 and Penicillium citrinum CICC4010, compared to the rates of 0.30, 0.77 and 0.76 in the unimproved chamber.