| Non-thermal sterilization and anti-microbial packaging technology is a newly developed technology in food application in recent years.Nanoparticles of metallic oxides both TiO2 and Fe2O3 are caused more and more attention, for their low-cost, safe, non-toxic, and high photocatalytic inactivation activities. At present, studies about applications of metallic oxides nanoparticles in food packaging were carried on. However, because of light requirement for their inactivation activities, application of metallic oxide nanoparticles in the packaging appears to be limited. High Voltage Electric Field Cold Plasma (HVEF-CP), is a novel sterilization technology for food, and studies about its application in food was just started. It can excite the media around food to generate light, ions and active free radicals, which can attack cells and cause their death.This study were mainly carried on following contents:photocatalytic performance of metallic oxide nanoparticles TiO2/Fe2O3, anti-bacterial activities targeting on Escherichia coli and photocatalytic kinetics analysis, nanoparticles photocatalytic inactivation mechanism on spoilage bacteria, sterilization of HVEF-CP on spoilage bacteria of chicken, and the inactivation effect and application of plasma combined with nanoparticles on MAP packaged chicken. Our mainly findings are as follows:1. Photocatalytic inactive effect of nano TiO2 and Fe2O3Photocatalytic tests were carried out with the system setup by ourselves, methylene blue (MB) was used as model to analyze the photocatalytic activity of nano TiO2 powder, coating film on glass, or PVA-TiO2 film irradiated by UV-light, then the disinfection activities of nanoparticles on E.coli were also studied. The main results are as follows: The photocatalytic activity was detected only under UVA light for TiO2. Nano-TiO2 showed photocatalytic degradation effect on MB only under light, after degradation the absorption peak of MB was blue-shift. When TiO2 powder in aqueous suspension, it showed highest photocatalytic activity, the degradation of MB was as much as 50% in 25min under 800uw/cm2 UV-light. Increasing TiO2 content to 0.4g/l resulted in increasing photocatalytic effect. E.coli was taken as model bacteria to study the photocatlaytic inactivation of nanoparticles, TiO2 showed a higher inactivation to E.coli than Fe2O3 in aqueous suspension. During 150min,0.4g/l TiO2 cause the reduction of concentration (C/Co) of E.coli was 10-4, while the reduction of Fe2O3 was only 10-2 under same condition. The most effective ratio of ZnO:TiO2 was 1:4, it reduced the concentration of E.coli from 7.4 to 2.2 log in 150 min, indicated that appropriate ZnO could enhance the photocatalytic inactivation effect of TiO2 significantly (P<0.05).2. Photocatalytic inactivation effect of Nano-TiO2 on chicken spoilage bacteria and its photocatalytic kinetics studyBoth E. coli and Pseudomonas aerugionsa were taken as test bacteria, with the same photocatalytic system, the photocatalytic inactivation effect of TiO2 was carried out through three different factors, and the photocatalytic inactivity kinetic were also carried out. The results showed that It took about 60 min for TiO2 to show antimicrobial activity after the TiO2 powder was added into reaction solution, absorption balance between nanoparticles and bacteria cells was build during this time.The inactivating kenetics of TiO2 on E. coli and P. aeruginosa were fit the equations log both R2 were higher than 0.999.The photocatalytic kinetics parameter 1/KT was proportional to initial concentration of bacteria, and the equation was pseudo-one order kinetics. The react rate constant of E.coli and P.aeruginosa was 7.668 X 106 cfu/(ml · min) and 5.655 X 106 cfu/(ml · min), respectively, and their absorption balance constant was 1.053×10-8 ml/cfu and 1.438×10-8 ml/cfu, respectively. E.coli was more sensitive to TiO2 treatment than P.aeruginosa, even though changes in both bacteria populations behaved similarly during the treatment.3. Antimicrobial effects and mechanism of nanoparticles TiO2/Fe2O3 on chicken spoilage bacteria studyBoth Pseudomona fluorescens (G-) and Macrococcus caseolyticus (G+) were taken to test the photocatalytic inactivation effect of nano TiO2/Fe2O3. In order to study the antimicrobial mechanism of nanoparticles on bacteria, TEM was used to observe the shape changes of cells during reacting processing, K+ leakage from cell cytoplasm and lipids oxidation products MDA content were also test. All the results showed:TiO2 and Fe2O3 showed the best inactivation effects to both bacteria with a content 0.4g/l and 0.8g/l, respectively. However, M. caseolyticus was more sensitive to nanoparticles than P. fluorescens under our experimental condition. Nanoparticles photocatalytic inactivation of microbes appeared to be separated into three stages at cell levels. First, nanoparticles attached to outside bacterial cell walls, then nanoparticles started damaging cell surrondings under light; and at last, cytoplasm leaked out and cell death. K+ leakage significantly increases in reaction solutions, indicating that the permeability of cell membranes was affected by the treatment. The amount of MDA, a product of lipid oxidation, increased first and reduced quickly after reached the maximum value during the photocatalytic process. Both K+ leakage and MDA content of M. caseolyticus were higher than that of P. fluorescens during the processing, indicating that the outside walls of M. caseolyticus was easier to be destroyed than that of P. fluorescens.4. Study of disinfection effects of HVEF-CP combined with nano TiO2 photocatalytic on chicken spoilage bacteriaBoth P. fluorescens (G-) and M. caseolyticus (G+) were taken as model, plasma generation voltage and gases, treat time were taken as different factors to study plasma disinfection effects, effects of plasma combined with nanoparticles on both bacteria were also studied. The results were as follows:when the generation voltage was between 55-80kv, More ozone was generated inside of packaging, as electric voltage and treat time increased, and more bacteria were killed inside, but the content of ozone could not always increased for the packaging size limited. Storage of samples at 4℃ for 24h after treatment resulted in more reduction in bacterial populations than sampling immediately after treatment, and the pH of reaction solution were appeared reduction trends, indicating that there are significant residue effects in the treated packages. The response of bacteria to treatment conditions varied as bacteria strains, in our study the G" P. fluorescens was more sensitive to plasma treatment than G+ M. caseolyticus, these may be caused by the different structure of cells outside wall. The plasma, generated from gases contained oxygen, showed better disinfection effect than that generated from pure Argon (Ar) and Helium (He), no matter what status the samples treated. Nano TiO2 did not enhance-the disinfection of plasma, in contrast, it showed a protection effect on bacteria regardless the sample status.The nano-particles may result in reduced antimicrobial activity of plasma by reacting with the active particles in plasma.5. Study of disinfection effect of HVEF-CP combined with nano TiO2/Fe2O3 on MAP packaged chickenFresh chicken fillets were taken as material, plasma generation voltage and treat time, packaging gases were taken as test factors to study the disinfection of plasma on chicken, effects of plasma combined with nanoparticles on packaging chicken were also carried out. The main results are as follows:Increased voltage from 55 to 80 kv significantly increased ozone formation in packages, but the total neutral flora of chicken surface was no difference after treatment, quality of chicken surface also has no changes. When the treatment time of packaged chicken increased from 3 min to 9 min under 80 kv, long time treatment could reduce the visible quality. When meat was packed in modified atmospheric air (MA) (65% O2/30% CO2/5% N2), HVEF-CP treatment increased ozone formation by six times more compared to packages with air, significantly reduced mesophiles and psychrophiles flora of chicken surface about 1 log more than that of air packaging, it declared that more oxygen contained in packaging gases, more ozone generated in packaging, better disinfection effect was generated. The shelf life of MA-packaged chicken meat was at least 14 days compared with 7 days for air-packaged samples, there was no strong spoilage smell generated inside, when samples were treated at 80 kv for 3min. When metallic oxide nanoparticles were added on packaged chicken meat that was inoculated with bacterium, no enhanced HVEF-CP antimicrobial effect was noticed with nano TiO2 particles. |
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