Effect Of Endophytic Bacteria On Storage Quality Of Fragrant Pear

Fragrant pear industry is the dominant pillar industry in Korla Region,which has a vital impact on Korla economy.Fragrant pear is easy to be infected by external microorganisms in the process of storage and transportation,causing fruit rot and serious economic losses.Compared with the equipment cost of physical storage,chemical storage method has the problems of residues and safety risks,and biological preservation has the characteristics of low cost,safety and non-toxic.Endophytic bacteria have the characteristics of rich species and high safety,which can be used as an important resource for biological preservation.The purpose of this paper is to screen out endophytic bacteria with good antibacterial effect and stable colonization from fragrant pear tree,optimize its fermentation conditions,and study the effect of antagonistic fermentation liquid on Fragrant Pear preservation.The results are as follows:(1)The results showed that Black spot and Penicillium were the main causes of pear decay during storage.Isolation of rot causing fungi:the rotten pear fruits were collected and the pathogens were isolated by tissue isolation method.Combined with its sequencing and morphological identification,two strains of rot causing fungi were isolated,namely Alternaria alternata XL2 and Penicillium expansum XL3.(2)Screening of antagonistic bacteria:leaves and branches of fragrant pear were collected,and antagonistic bacteria of Alternaria alternata XL2 and Penicillium expansum XL3 were screened by plate confrontation method.Results:8 strains of endophytic bacteria against XL2 and 5 strains of endophytic bacteria against XL3 were obtained.Oxford cup agar diffusion method was used to determine the resistance of antagonistic fermentation broth to XL2 and XL3.The results showed that NY2 and NY7 had the best inhibition on the growth of Alternata alternata XL2 on PDA medium,and the inhibition diameter was more than 23 mm;The growth of Penicillium expansum XL3 on PDA medium was inhibited by NY2,NY7 and NY9,and the inhibition diameter was more than 22 mm.The results showed that the inhibition rate of NY2 to XL2 was more than 80%and NY9 to XL3 was the best.The inhibition rate of NY9 to XL3 was more than 60%in the first 4 days.Finally,Bacillus velezensis NY2 was selected for further study.(3)Optimization of fermentation conditions of antagonistic bacteria:carbon,nitrogen and inorganic ions were screened by single factor experiment,and then the fermentation medium of NY2 inhibiting Alternaria alternata XL2 and Penicillium expansum XL3 was optimized by response surface methodology.Box-Behnken experiment was used to optimize the fermentation medium of XL2 inhibited by NY2.The results showed that the diameter of the maximum inhibition zone could reach 34.1mm.Box-Behnken experiment was used to optimize the medium of NY2 inhibiting XL3 fermentation.The results showed that the optimal formula had the best antibacterial effect,and the maximum inhibitory diameter was 32.5 mm.Finally,it has better antibacterial activity to XL2 and XL3.The final formula of fermentation broth is:yeast extract 10g/L,beef extract 3g/L,K₂HPO₄ 1g/L,Na Cl 0.5g/l,lactose 5g/L,glucose 5g/L,(NH₄)₂SO₄ 3g/L.(4)Application effect:Taking Korla fragrant pear as the research object,the effects of NY2 fermentation liquid treatment on defense enzyme activities of fragrant pear were studied.The results showed that:compared with CK control group,the activity of pal,cat,SOD and pod were significantly increased after 15 days storage,and the decrease of vitamin C content and hardness were delayed,but the polyphenol oxidase(PPO)was not significantly affected.