Study On The Preservation Effect Of Mulberry Leaf Polyphenols On Fresh-Cut Fruits And Vegetables And The Mechanism Of Controlling The Adhesion Of Food-Borne Pathogenic Bacteria

Mulberry leaves contain a large number of polyphenolic compounds such as phenolic acids and flavonoids.These active substances have antioxidant,antibacterial,hypolipidemic and immunomodulative effects.There are abundant resources of mulberry leaves in Chongqing,which also are "medicine and food homology" plant.However,the application of mulberry leaves is limited.Mulberry leaf polyphenol is an ideal natural antioxidant and antibacterial agent,which has the characteristics of safety,high efficiency and health.Cutting can cause mechanical damage,which will make the fruits and vegetables lose their original protective tissue.A series of physiological and biochemical reactions are exacerbated,such as respiration rate,ethylene production and browning.After cutting,the nutrients flow out from the fruit and vegetable tissues and the large cutting surface provide favorable conditions for the growth of pathogenic bacteria.Bacteria are easy to adhere on the surface of fresh-cut fruits and vegetables and further form mature biofilm.Biofilm is difficult to remove and endanger the health of consumers.Therefore,it is very important to improve the quality of fresh-cut fruits and vegetables and control the adhesion of food-borne pathogenic bacteria on the surface.At present,no researchers used mulberry leaf polyphenols as natural preservatives and antibacterial agents in the preservation of fresh-cut fruits and vegetables and the control of food-borne pathogenic bacterial adhesion.Therefore,this paper extracted polyphenols from mulberry leaves,studied the differences in composition and content of polyphenols from different varieties of mulberry leaves.Mulberry leaf polyphenols as preservatives and antimicrobial agents were applied in fresh-cut fruits and vegetables.The mainly studies and results are listed as follows:(1)The mulberry leaves of Naxi,Jialing20,Jialing40,Husang and Hongguo were used as experimental materials to extract polyphenols.The results showed that the content of polyphenols in Naxi was the highest,followed by Jialing20,Hongguo,Husang and Jialing40.The antibacterial activities of Naxi and Jialing20 mulberry leaf polyphenols against E.coli O157:H7 and Listeria monocytogenes were higher than the other three mulberry leaf polyphenols.The MIC of Jialing 20 against L.monocytogenes was 2.5 mg/m L,which showed the best bacteriostatic effect.7 main components were detected in Jialing20 mulberry leaf polyphenols,which were gallic acid,protocatechuic acid,chlorogenic acid,caffeic acid,rutin,isoquercetin and p-coumaric acid.6 main components were detected in Naxi mulberry leaf polyphenols,which were gallic acid,protocatechuic acid,chlorogenic acid,caffeic acid,rutin and isoquercetin.5,4 and 4 polyphenol monomers were detected in Hongguo,Husang and Jialing40,respectively.(2)The preservation effects of Naxi and Jialing20 mulberry leaf polyphenols on fresh-cut cantaloupe were studied.The results showed that Naxi and Jialing20 mulberry leaf polyphenols effectively maintained the quality of fresh-cut cantaloupe by reducing weight loss,maintaining color,firmness,TTA and SSC content.Naxi and Jialing20 mulberry leaf polyphenols effectively inhibited the reduction of total phenol content,DPPH free radical scavenging activity and reducing power.The antioxidant capacity of Naxi mulberry leaf polyphenols in fresh-cut cantaloupe was significantly higher than that of Jialing20 mulberry leaf polyphenols(p < 0.05).Naxi and Jialing20 mulberry leaf polyphenols significantly inhibit the increase of total number of colonies on fresh-cut cantaloupe during storage(p < 0.05).The antibacterial activity of Jialing20 mulberry leaf polyphenols on fresh-cut cantaloupe was significantly higher than that of Naxi mulberry leaf polyphenols(p < 0.05).(3)The preservation effects of Naxi and Jialing20 mulberry leaf polyphenols on fresh-cut lettuce were studied.The results showed that Naxi and Jialing20 mulberry leaf polyphenols effectively maintained the quality of fresh-cut lettuce by reducing weight loss,maintaining the brightness and chlorophyll content.Naxi and Jialing20 mulberry leaf polyphenols effectively inhibited the browning of fresh-cut lettuce by controlling the increase of total phenol content,PPO,POD and PAL activities.Naxi and Jialing20 mulberry leaf polyphenols significantly inhibited the increase of total number of colonies on fresh-cut lettuce during storage(p < 0.05).The antibacterial activity of Jialing20 mulberry leaf polyphenols on fresh-cut lettuce was significantly higher than that of Naxi mulberry leaf polyphenols(p < 0.05).(4)Due to the strongest antibacterial ability of Jialing20,Jialing20 mulberry leaf polyphenols with low concentration was used to control the adhesion of E.coli O157:H7 and L.monocytogenes on fresh-cut lettuce.The development stages of E.coli O157:H7 and L.monocytogenes biofilm on lettuce leaves are determined: 0-2 h was adhesion stage,2-6 h was proliferation stage,6-8 h was maturity stage and 8 h later was dispersion stage.Mulberry leaf polyphenols of 1/16 MIC did not influence the growth of planktonic bacteria but inhibited the adhesion of E.coli O157:H7 and L.monocytogenes on fresh-cut lettuce.Mulberry leaf polyphenol significantly inhibited the motility and extracellular polymer(EPS)content of E.coli O157:H7 and L.monocytogenes(p < 0.05).Mulberry leaf polyphenol mainly inhibited the production of protein in E.coli O157:H7 EPS,but it inhibited the production of protein and polysaccharide in L.monocytogenes EPS.Mulberry leaf polyphenols controlled bacterial adhesion mainly by inhibiting the expression of flagella and pili-regulated genes(fim A,csg B and ppd D)in E.coli O157:H7 and the expression of flagella-regulated genes(fli G and fli M)and glycoprotein synthesis genes(mog R)in L.monocytogenes.