| Atmospheric pressure non-thermal plasma (APNTP) is an innovative technology that has been widely used in several fields. Given its attractive features, such as low-temperature properties and high efficiency of microbial inactivation, non-thermal plasma is ideal for applications in agricultural production, food safety and crop protection, etc.Fresh produce is full of nutrients, vitamins and fiber, which is essential to human health. After traditional thermal pasteurization, these components may degrade, which seriously affects the nutritional value of the food. Raw consumption is a healthy eating habit which is widely accepted by customers. Thus, there is certainly a lot of attention received on the food safety issue. Many food safety issues caused by the contamination of pathogenic microorganism on the surface of fresh produce break out. Conventional preservation treatments, such as SO2 fumigation, ultraviolet light, electrolyzed water and ozone are costly, not highly effective for produce, or not very efficient inactivation of yeast and mould, and may lead to some residue. The use of pesticides in modern agricultural practices has made the crop production stable. Nevertheless, the environmental and food safety problems associated with the use of pesticide cannot be ignored. Therefore, there is certainly a lot of interest in the development of a novel crop protection solution and food processing technology, which should be high efficiency, rapid and no residue. Recently, it is found that plasma-activated water (PAW) can efficiently inactivate a wide range of bacteria. Due to the advantages of PAW (high efficiency, rapid and no residue), it would be ideal for the application in the crop protection, and reduce pesticide consumption. Therefore, there is certainly a lot of interest in the development of a novel food processing technology and crop protection, which should be high efficiency and no residue.In this paper, microbial metrology, scanning electron microscope imaging, fluorescence microscopy, transmission electron microscopy imaging and atomic emission spectroscopy analysis were employed to investigate the mechanism underlying the inactivation of E.coli by the non-thermal plasma agents and the application in food safety and crop protection. Colorimeter method and pH differential method were also employed to analyze the physical and chemical characteristics of the samples treated by plasma activated water. The main results and conclusions were listed as follows:(1) The influence of different working gases on the cellular morphological changes of microorganisms was investigated. The experimental results indicated that the cell membrane and cell wall of E.coli were totally destructed when the working gas was air. Whereas, no significant changes were observed when the working gas was pure argon and nitrogen.(2) The role of UV radiation generated in non-thermal plasma played in the inactivation of E. coli was investigated. OES analysis and single factor experiment were employed to investigate the role of UV radiation played in the inactivation of E. coli. Results indicated that the bactericidal effect of UV radiation could be ignored in the air or argon plasma treatment systems. Therefore, temperature and UV radiation did not play a major role in the inactivation of bacteria in this study. However, this conclusion may or may not apply to other non-thermal plasma treatment systems, and is applicable to a DC non-thermal plasma treatment system built in this study.(3) The mechanism behind the inactivation of E. coli y changed particles was investigated. The electrostatic force caused by the accumulation of charges on the outer surface of the membrane can overcome the tensile strength of the membrane, thereby leading to rupture. This effect is observed in Gram-negative bacteria because their membrane possesses an irregular surface. However, the experimental results in this study did not support this hypothesis, and the results demonstrated that the cytoplasmic contents of the bacteria were probably reduced after argon plasma treatment. This trend was probably caused by the change in membrane permeability and cell membrane damage, which resulted in leakage of the cytoplasmic content and bacterial death.(4) The mechanism behind the inactivation of E. coli y changed particles was investigated. The experimental results indicated that the cell walls were completely ruptured after air plasma treatment. However, neither obvious morphological changes nor the rupture of the cell wall were observed in E. coli after pure nitrogen plasma treatment. These results indicated that the oxidative stress of OH radicals, ozone, and nitrogen oxide was probably the inactivation mechanism of the reactive species. OH radicals and ozone can destroy the bacterial cell wall by their strong oxidizing property. The strong oxidative stress of OH radicals, ozone, and nitrogen oxide led to the disruption of the cell wall and membrane. This mechanism probably underlies the interaction of reactive species with bacteria in the indirect mode of a DC non-thermal plasma treatment system.(5) The influence of different plasma activated time on the temperature, conductivity, ORP and pH value was investigated. The experimental results indicated that the temperature, conductivity, ORP value of PAW increased and pH value decreased in a plasma activated time-dependent manner. The ORP value of the PAW decreased and increased rapidly within 30 min after plasma activation, and then increased and decreased gradually 30min after plasma activation. For PAW60, ORP reached the maximun at 15 min after plasma activation, and decreased to 378.7 mV at 30 min after plasma activation. The ORP reached a maximum (442 mV) at 120 min after plasma activation, and decreased to 326 mV gradually after the peak value. The pH value gradually increased from 4.18 to 5.0 over post plasma activated time.(6) The antimicrobial efficacy of PAW against S. cerevisiae (CICC1374) in suspension and inoculated on grapes was investigated. The experimental results indicated that the environment where yeast harbored had a significant influence on the antimicrobial efficacy of PAW. When S. cerevisiae was incubated in sterile water,2 log CFU/ml or greater reduction in fungal viability was achieved in 30 min exposures with PAW directly after plasma treatment. For the same treatment time, the antimicrobial efficacy of PAW against yeast inoculated on grapes was approximately 0.51 ±0.11 log CFU/ml reduction. Moreover, some experimental results indicated that the pH value and component of bacterial suspensions had an influence on PAW inactivation. In this study, the bactericidal effect of PAW against S. cerevisiae incubated in PBS (pH 7.4) can be ignored.(7) Changes in the surface color of grapes and anthocyanin stability after following PAW60 treatments of grapes were studied. Results of statistical analysis confirmed that total anthocyanins and surface color demonstrated no significant reduction after 30 min of PAW treatment (P> 0.05). chromatic aberration AE* is 0.62±0.38. Total anthocyanin before and after PAW60 treatment were 23.03±12.15 mg/L and 22.11±11.98 mg/L respectively. ANOVA results indicated that the population means of control and treatment groups were not significantly different (P> 0.05).(8) Inhibition effect of plasma activated water on conidial germination and mycelial growth of F. graminearum were studied. Results of statistical analysis confirmed that plasma activated water had a great inhibition effect on mycelial growth and conidial germination rate of F. graminearum. Inhibition effect increased with plasma activated time. The inhibition time of mycelial growth of PAW60, PAW45, PAW30 and PAW15 was more than 24 h and less than 36 h, more than 12 h and less than 24 h,12 h and less than 12 h respectively. Fisher LSD test indicated that the colonial diameters of treatment and control group in all the levels were significantly different (P< 0.05). PAW had a significant effect on the conidial germination rate of F. graminearum. After PAW15, PAW30, PAW45 and PAW60 treatments, the conidial germination rate of F. graminearum were 98.37±1.43%,96.52±1.32%,73.62±9.79% and 57.87±7.80% repectively. ANOVA results indicated that the population means of control and treatment groups were significantly different (P< 0.05). |
PDF Full Text Request