| Fresh cut fruits and vegetables have attracted more attention because of their nutrition,convenience,freshness and 100%edible rate.However,the quality of fresh cut fruits and vegetables is decreased and easily polluted by microorganisms due to mechanical damage.As a nonthermal sterilization technology,UV-C has been used in fresh cut fruits and vegetables in recent years.In this paper,the main fungi causing the rotting of fresh-cut melon was identified and characterized;the effect of different doses of UV-C irradiation on the quality of fresh-cut Xizhoumi 25 melon and the optimal dose of UV-C irradiation was established;the secondary metabolis and oxygen metabolism pertinent to the optimal dose of UV-C treatment.The main results are as follows:(1)The methods of dilution plate and three-point were used to separate and purify the putrefactive fungi in fresh-cut cantaloupe.The DNA of putrefactive fungi was extracted by using the genomic DNA Extraction Kit of fungi.After PCR amplification,gel electrophoresis experiment was carried out to identify the DNA sequence.Eight kinds of main putrefactive fungi of fresh-cut cantaloupe were identified by morphology and molecular biology,among which 6 kinds were putrefactive fungi,which were arranged from large to small according to their putrefaction ability,including Aspergillus ochraceus,Cladosporium sphaerospermum,Didymella glomerata,Acremonium sclerotigenum,Aspergillus rugulosus,Aspergillus sydowii.There are two kinds of spoilage yeast,Cryptococcus magnus and Rhodotorula mucilaginosa.(2)UV-C was used to irradiate fresh-cut cantaloupe for 0,2.2,4.4.6.6 kJ/m2 respectively.Samples were taken at 0,4,8,12d.The optimal dose was selected and nutritional quality was studied by detecting quality indexes.The results showed that the hardness,soluble solid content and weight of fresh-cut cantaloupe could be well maintained in the UV-C irradiation group for 6.6 kJ/m2,so as to keep its original color,reduce its relative conductivity and maintain its respiratory rate in the early stage of storage.Compared with the CK,the shelf life of fresh-cut cantaloupe increased by 2 days.There was no significant difference in the contents of sucrose,fructose,glucose and three organic acids between the UV-C treatment group and CK group(P>0.05).UV-C treatment for 6.6 kJ/m2 can stimulate the increase of ascorbic acid content in fresh-cut melon at the later stage of storage(P<0.05).(3)Fresh-cut cantaloupe was irradiated with UV-C for 6.6 kJ/m2.Samples were taken every 2 days to determine the effects of UV-C on the activities of three key enzymes in phenylpropane metabolism and the contents of total phenols and flavonoids.The results showed that UV-C treatment significantly increased PAL,C4H and 4CL in the middle and later stage of storage compared with CK treatment.The activity can promote the rapid growth of total phenol content and effectively inhibit the decline of flavonoid content(P<0.05).Therefore,UV-C treatment can significantly improve the activity of three key enzymes in phenylpropane metabolism,thus promoting the accumulation of total phenols and flavonoids.(4)UV-C treatment was used to irradiate fresh-cut cantaloupe for 6.6 kJ/m2,and samples were taken every 2 days to study its effect on active oxygen metabolism.The results showed that UV-C treatment could increase POD activity in the whole storage period,CAT and APX activity in the early stage of storage,inhibit MDA production,increase DPPH free radical scavenging rate in the whole storage period and ABTS free radical scavenging rate in the middle and late stage of storage(P<0.05).Therefore,UV-C treatment can improve the activity of antioxidant enzymes,scavenge free radicals and inhibit membrane lipid peroxidation products. |
