The Effect Of Cold Plasma Activated Water On Fresh Cut Lettuce During Storage

Fresh cut vegetables have become increasingly popular among consumers during these years because of their convenience and high edible rate.Lettuce,rich in a variety of vitamins,calcium,iron and other trace elements and dietary fiber,is one of the green leafy vegetables that account for a large percent consumption in China.So,lettuce has a high nutritional value and lettuce processing has a broad market prospect.However,the tissue of fresh-cut lettuce was damaged in the process of processing,and the respiration was strengthened,which led to the rapid rate of water evaporation and rapid browning reaction.Thus,it is easy for fresh cut lettuce to be contaminated by pathogenic bacteria and then result in a series of adverse problems such as shortened shelf life in the circulation process,which pose a threat to the health of consumers.As a new cold sterilization technology,cold plasma(CP)has been shown to sterilize different types of microorganisms,including bacteria,fungi,spores and viruses.Through plasma discharge in water or water surface,a large number of active substances produced by CP can be combined with aqueous solution to produce plasma activated water(PAW),which shared the similar broad-spectrum bacteriostatic property.PAW plays an important role in fruit and vegetable preservation because it is free of chemical reagents,easy to degrade and environmentally friendly.The purpose of this paper is to study the effects of PAW treatment on the safety and nutrient quality of fresh-cut lettuce during storage,and to study from the aspects of phenylpropane metabolism,active oxygen metabolization-related enzyme activity and antioxidant anabolism,so as to provide theoretical basis for the industrial application of cold-source plasma cold sterilization technology in fresh agricultural products.The main research results are as follows:1.Effect of plasma activated water on microbial decontamination and quality of freshcut lettuceThe cold temperature dielectric barrier discharge plasma equipment prepared PAW by treating distilled water for 5 min at voltages of 35 kV,55 kV and 75 kV.Then wash the lettuce with PAW for 5 min,store it under 4? and 85%RH for 10 days,and test the microbe(total number of colonies,coliform group)and physiological and biochemical indicators(weightlessness,color,vitamin C,chlorophyll and polyphenol oxidase)of the samples every 2 days.The results showed that the bactericidal effect of PAW on lettuce significantly increased with the increase of voltage(P<0.05).At the end of storage,the total number of bacterial colonies and the number of coliform bacteria in the 75kv treatment group were 1.151g(CFU/g)and 1.381g(MPN/100g)lower than the control group.Compared with the control group,PAW with different treatment voltages could effectively inhibit the activity of polyphenol oxidase,inhibit the rise of L*value and the decline of total color difference value(P<0.05).The 35 kV and 55 kV treatments had no significant effect on the contents of ascorbic acid and chlorophyll in the samples,but the voltage increased to 75 kV would have an adverse effect.Therefore,the optimal handling voltage of PAW for fresh-cut lettuce is 55 kV.2.Effects of PAW treatment on antioxidant system of fresh-cut lettuceFresh-cut lettuce was treated with a voltage of 55 kV PAW and washed with pure water as a control to study the change of antioxidant system of lettuce stored at 4? for 10 days.The results showed that PAW treatment was beneficial to the increase of total phenol content and flavonoids content in fresh-cut lettuce during storage,and significantly increased the activity of PAL enzyme and promoted the secondary metabolism of lettuce.At the same time,PAW treatment can make fresh-cut lettuce maintain high SOD,CAT,APX and GR enzyme activity during storage,inhibit superoxide anion generation and H2O2 content in the later stage of storage,and keep the cell membrane of fresh-cut lettuce well.PAW treatment can further improve the DPPH free radical scavenging ability and ABTS·+ free radical scavenging ability of fresh-cut lettuce,thus improving the antioxidant ability of fresh-cut lettuce.3.Analysis of quality changes of fresh-cut lettuce during storage based on metabolomicsMetabolomic analysis showed that PAW treatment mainly had an effect on fresh-cut lettuce quality in the early stage of storage.Compared with the 0th day,a total of 135 differential metabolites were found after PAW treatment.After comparison with the enriched metabolic pathways,a total of 6 marker differential metabolites were screened out.It showed that PAW produced oxidative stress on fresh-cut lettuce in the early stage of storage,promoted the sample to produce higher energy accumulation and promoted the secondary metabolism such as phenylpropane metabolism,and improved the comprehensive antioxidant capacity of fresh-cut lettuce by improving the synthesis of phenols and other antioxidants.