| Based on the sterilizing effects of Pulsed Electric Field(PEF) on microorganisms, and the first-order kinetics among pulsed electric strength (E) and treatment time and relative survival rate of microorganisms (S), the theory of the thermal death time called F-value was introduced into non-thermal field which was Pulsed Electric Field (PEF) in this paper. Through the establishment of F-value theory of PEF, the decimal reduction time (D) and the PEF resistance constant (Z) of microorganisms were obtained, then the PEF death time (F) was calculated at a given E,which was expected to become an indicator for measuring the sterilizing effects of PEF on bottled water. And its ultimate goal was to guide the application of PEF in the bottled-water production. Meanwhile, in order to solve the issue of Bromate in bottled water, the comparion of Ozone technology with PEF was launched in the production of Bromate. All researches that were carried out were summarized briefly as follows:1. The major parameters of PEF device were as follows: voltage from 0 to 17 kV; pulse width 4μs; flow rate from 4.85 to 15.15 mL/s; pulse frequency from 0.1 to 3 kHz; bipolar pulse and square waveform. Under the condition of the PEF system with pulse frequency 3 kHz, pulse width 4μs, pulse number 24 and E=22.58 kV/cm, the sterilizing effects of different electrodes were compared between: 3 mm and 3 mm(with a piece of metal conductor ), 7 mm and 7 mm(with two pieces of metal conductor ). The results showed that: the sterilizing effects of electrodes with metal conductor were better than those without any metal conductor; the larger the diameter was, the worse the sterilizing effect was; the results of 7 mm with two pieces of metal conductor was very similar to that of 3 mm without any metal conductor.2. The co-field chamber with inner diameter 7 mm (with two pieces of metal conductor), and the PEF system with pulse frequency 3 kHz, pulse with 4μs ,pules number 24 were used in the research. Escherichia coli, Saccharomyces cerevisiae, Penicillium citrinum, Staphylococcus aureus and salmonella were treated by PEF with 22.58 kV/cm,29.03 kV/cm,35.48 kV/cm,41.94 kV/cm and 48.39 kV/cm respectively. The results showed that: these microorganisms in bottled water could be sterilized effectively by PEF; after the treatment of PEF with 48.39 kV/cm during different treatment time, Escherichia coli, Saccharomyces cerevisiae and Penicillium citrinum had not been detected; the number of active Staphylococcus aureus and salmonella were decreased by 6.6 and 5.5 circles of log10. At a given E=48.39 kV/cm and the same other parameters of PEF,these microorganisms were also treated by PEF with treatment time 94μs, 114μs, 155μs, 203μs and 295μs respectively. The results showed that: the PEF sterilizing effect was enhanced significantly with the extension of treatment time. Taken E. coli for an example, through the variance analysis it could show that both treatment time and E were very significant to PEF sterilization,especially the electric strength (E) which played a leading role in this field.3. By referencing the concept of thermal death time, F-value of PEF technology was introduced to show the death time of different microorganisms at different E. A relationship model between E and death time of microorganisms was established to become an indicator for measuring sterilizing effects of PEF used in bottled water firstly. The decimal reduction time (D) and the PEF resistance constant (Z) of microorganisms were obtained from the microorganism survivor curve and PEF resistance curve respectively. And D and Z were combined to indicate clearly the different tolerance of various microorganisms to PEF (the same in thermal field). In this article, a reference electric field strength (Er) was supposed to be the E at which micoorganisms could be reducted by 5 circles of log10. And the lethal rate (LR) formula LR=10(E-Er)/Z was introduced to estimate lethal rates of microorganisms at any E. The area under the lethal rate curve was referred to as lethality, then the PEF death time (F) was calculated by LR*F=1*Fr at a given E. By studying F-value of different microorganisms and exploring their different death time at any E, it was expected to guid the application of PEF in the bottled-water production. Both the analysis of relevant research at home and abroad and this experiment results could show that the death time of microorganisms calculated by the theory of F-value was basically similar to that of the experimental value or reference value, which had a certain guiding significance on the application of PEF in the bottled water production.4. Currently bottled water is usually sterilized by ozone, which can oxidizes Bromide in water to Bromate designated to be 2b potential carcinogens by the International Agency for Research on Cancer (IARC). Higher requirement is put forward for water enterprises, it is necessary to adopt new technologies to substitute ozone sterilization technology to reduce Bromate content. PEF is a novel non-thermal food processing technology, which inactivates microorganisms by instantaneous voltage in liquid food. Not only it has good sterilization effect, but also it is impossible to generate Bromate.The experimental results showed that: PEF had a least reduction of 5.5 circles of log10 in bottled water sterilization;when the bottled water was treated by Ozone,the output of Bromate was increased as the dosage of ozone concentration increasing,with a constant bromide ion concentration; No matter how much bromine ions the water with:0.54 mg/L or 70.74 mg/L(because the higher the concentration of bromide ions was, the greater the bromate would be produced) , or the water was treated by 30 kV/cm or 50 kV/cm of E, there was not any bromate to be generated. Therefore, PEF could be successful in controlling the production of bromate and applying in the production of bottled water in the future, with the expansion of the electrode diameter and the increase of sterilizing effect.
|
PDF Full Text Request