Effect Of Chitosan On Postharvest Green Asparagus Quality And Antimicrobial Mechanism Against Pathogen In Vitro

Chitosan is a natural basic polysaccharide polymer, exhibited film-forming properties and antimicrobial actions and has been used for shelf life extension of fresh fruit and vegetables. Green asparagus (Asparagus officinalis L.) is one of the popular fresh vegetables and eaten worldwide due to its high nutritional values. However, respiration rate makes it particularly susceptible to postharvest perish then leading to less of a delicacy than it once. The quality of the edible coatings based on chitosan on postharvest green asparagus, the pathogens were isolated and identified from decayed asparagus, and antifungal activity and mode of action of chitosan against pathogen were investigated. The main contents and conclusions were listed as follows.(1) The quality of0.5%(v/v) carboxymethyl chitosan (CMC), water-soluble chitosan (WSC) and acid-soluble chitosan (ASC) coatings on postharvest green asparagus stored at2℃was investigated. At the35th day, the weight loss was observed in the control sample, it reached12.2%, whereas, that of CMC treated batches was9.0%. Ascorbic acid retention of asparagus treated with CMC was12.6mg/100g FW, whereas control sample was only8.5mg/100g FW. Moreover, CMC treatment ensured lower color variation, retard the firmness, the loss of chlorophyll, presenting better quality of asparagus than other treatments and prolonging a shelf life of postharvest green asparagus.(2) Three strains were isolated and purified from decayed asparagus, Based on pathogenicity tests, culture characteristics, physiological and biochemical tests and16S rDNA gene sequencing and comparison, two soft rot bacterial pathogens were identified as Enterobacter sp. and Pectobacterium carotovorum subsp. carotovorum. The wilt pathogen based on pathogenicity tests, culture characteristics, morphological observation and ITS gene sequencing and comparison, the results showed the fungal pathogen was Fusarium concentriccum.(3) The in vitro antifungal activities of CMC, WSC, ASC and N,N,N-trimethyl chitosan (TMC) against Enterobacter sp., P. carotovorum and F. concentriccum were investigated. The MIC of TMC against Enterobacter sp., P. carotovorum and F. concentriccum were0.08,.16and0.02mg/mL, the MBC were0.16,0.32and0.08mg/mL, the MIC of WSC were0.50,1.00and0.06mg/mL, the MBC were4.00,8.00and0.25mg/mL, the MIC of ASC were0.13,0.25and0.13mg/mL, the MBC were0.50,1.00and0.50mg/mL. CMC showed no significant antifungal activity against three pathogens. WSC efficiently inhibited the radial growth of F. concentricum, and both of WSC and ASC appeared to be more effective in inhibiting spore germination.(4) The mechanisms of action of ASC and WSC to pathogens were examined. Results showed that ASC may be due to the membrane disruption of bacterial pathogens, entered the interior of cell, membrane destruction via lipid peroxidation was proposed as a mechanisms. ASC destroyed its ordered metabolism and inhibit Enterobacter sp. and P. carotovorum growth. WSC mainly could increase the permeability of the fungal cell membrane and induce the activity of chitin en2yme of F concentricum cell wall. The profound ultrastructural alteration of F. concentricum with WSC was the disruption of the internal structure of cell, the inhibition of mycelial growth increased.