Screening Of Blueberry Natural Preservatives And Research On Preservation Technology Of Composite Coatings

Blueberry is a small berry fruit with respiratory climacteric,a relatively concentrated picking period,and it is extremely resistant to storage after harvest.The loss of blueberry fruit during storage and transportation is serious,however,the current cold chain logistics system in our country is imperfect and the processing foundation is weak.The lack of green and efficient preservation technology has become a bottleneck restricting the development of the blueberry industry.The corruption caused by the infection of pathogenic microorganisms is one of the main factors that affect the storage time and shelf life of blueberries.The most effective means to control the postharvest diseases of fruits and vegetables is the treatment of chemical bactericides,but the residue of chemical bactericides will endanger food safety and pathogenic bacteria are likely to produce resistance to chemical bactericides.Therefore,there is an urgent need to develop efficient and pollution-free preservative products and technologies to replace the extensive use of chemical bactericides.The compound preservative of natural plant extracts and chitosan combines the antibacterial activity of plant source preservatives and the film characteristics of edible polysaccharides.The new green composite coating preservation technology can not only be more broad-spectrum and efficient to inhibit the infection of pathogenic microorganisms in the storage and logistics of fruits and vegetables,but also to regulate the physiological metabolism of the harvested fruits and vegetables,delay the aging process,reduce the loss of water inside the fruit,prolong the storage and preservation period,and which has become a hot spot for research on preservation of green fruits and vegetables.In this paper,the method of tissue separation was used to separate the pathogenic bacteria from the natural rotten blueberry fruit during storage,and the main pathogenic fungi were classified and identified by morphological and molecular biological methods.The effective active substances were extracted from natural herbs and spices by ultrasonic assisted ethanol extraction,and the high concentration extract of plant extracts was prepared.In this study,21 kinds of plant crude extracts were used to investigate the antimicrobial activities of 3 pathogenic fungi isolated from blueberry during storage and 2 common foodborne pathogenic bacteria in the logistics by Oxford cup method and the minimal inhibitory concertration(MIC)and minimal bactericidal concentration(MBC)of these plant extracts were determined.Besides,the effect of concentration of extracts on the inhibition rate of pathogens was studiedand the bacteriostatic activity of the compound extracts were tested.Based on the above research,clove and schisandra-chitosan compound preservative with mass fractions of 0.5% and 1.0% respectively were used to soak the postharvest rabbit blueberries(powder blue)for 5 minutes(compared with distilled water treated).After drying,the treated fruits were stored under the environment with temperature of4 ? 0.5℃ and relative humidity of 80% to 85% as a storage and preservation test for42 days.Finally,the effect of composite coating preservative on the storage quality of blueberry was studied.All the research results are as follows:(1)Three main pathogenic fungi causing the deacy and deterioration of blueberry fruit during natural storage were iolated and identified.According to the characters of pathogen morphology and sequences analysis of 18 S rDNA,the three isolated strains were identified as Botrytis cinerea,Alternaria alternata and Penicillium adametzioides respectively.Among them,Botrytis cinerea,Alternaria alternata are two important diseases commonly found in the low temperature storage and preservation of blueberries.(2)The effective active substances were extracted from 21 natural herbs and spices,and the extract of plant extracts was obtained by rotating evaporation,and then the natural plant source preservative was prepared.The results obtained that extraction rates of four plant extracts such as rosemary,green pepper,green tea,and clove were higher than 30%,oregano,honeysuckle,scutellaria,star anise,schisandra,and forsythia were the next,18% to 27%.In this study,the extraction rate and cost value were considered to screen plant resources that are suitable for the research and development of preservatives.(3)Clove extract has the strongest inhibitory ability,and the average inhibitory circle diameters of Botrytis cinerea,Alternaria alternata and Penicillium adametzioides were 47.58,39.68,40.54 mm,respectively,and MICs of clove against these 3 fungi were 15.63,31.25,31.25 mg/mL,the MBCs were 31.25,62.5,62.5mg/mL.The antibacterial effect of schisandra chinensis against Staphylococcus aureus and Escherichia coli was obviously higher than the other extracts,the average inhibitory circle diameter on the 2 kinds of bacteria was 42.08,28.49 mm,the MICs were 31.25,62.5 mg/mL,the MBCs were 62.5,125 mg/mL.Two extracts of clove and Schisandra chinensis,which were the strongest inhibitory effect on blueberry pathogenic fungi and bacteria,respectively,were screened out and the change of these two kinds of extract concentration gradient inhibition effect on different pathogens are different,the larger effect of high concentration,besides,clove and schisandra compound ratio about 5:5 can be used as a better blueberry preservative application reference.(4)Clove and schisandra-chitosan compound preservative treatment can effectively reduce the weight loss rate and decay rate of blueberry fruit,inhibit the decline of hardness,keep the integrity of the membrane structure of the blueberry,effectively inhibit the increase of fruit electrical conductivity and malondialdehyde,postpone the decline of DPPH radical scavenging capacity,and improve the antioxidant and antibacterial activity of total phenol and total flavonoids in blueberry fruit.The results confirmed that 1% clove and Schisandra chitosan compound preservative had better preservation effect on the blueberry in storage.It had the advantages of simple operation,safety and innocuity,and had good application prospect of fruit and vegetable preservation.