| Fresh-cut fruits and vegetables are ready-to-eat or ready-to-use fresh fruits and vegetables that have been classified,trimmed,cleaned,cut,hearted(cored),dressed,preserved,packaged and other procedures.Fresh-cut fruits and vegetables have the characteristics of convenient,fresh,nutrition and safe.The waste material produced by fresh-cut processing can also be recycled and reused,which has the characteristics of reducing domestic waste and the environmental protection.However,cutting causes the fruit and vegetable losing its protective tissue and suffering mechanical damage,which greatly increases the sensitivity and safety risk of fruit and vegetable to microorganism,especially for foodborne pathogen.It has restricted the development of fresh-cut industry.Plant essential oils has natural,safe,high volatility and broad spectrum of antibacterial properties,and was widely applied for bacteriostasis and preservation of food.However,few studies have been reported on the application of plant essential oils on fresh-cut fruits and vegetables,especially the antibacterial mechanism is unclear,which restricts the application of plant essential oils on fresh-cut fruits and vegetables.In this study,plant essential oils with the strongest antibacterial activity was screened and the antibacterial mechanism was evaluated.The influence of plant essential oils incorporated into alginate composite coating on the quality and safety of fresh-cut fruit was studied.It provides technical support for the safety of fresh-cut fruits and vegetables during the processing,production and logistics.The main research results are as follows:1.Screening the plant essential oil with the strongest antibacterial activity.15 plant essential oils were chosen as antibacterial agents,namely thyme,cinnamon,oregano,lemongrass,mint,rosemary(2 kinds),clove,eucalyptus,lavender,tea tree,blumea,valerian,atractylodes and zingiber.Four kinds of foodborne pathogens were chosen as the target strain,namely Listeria monocytogenes(L.monocytogenes),Salmonella Typhimurium(S.Typhimurium),Staphylococcus aureus(S.aureus)and Escherichia coli O157:H7(E.coli O157:H7).The plant essential oil with strongest antibacterial activity was screened from 15 plant essential oils through the inhibition zone diameter,minimum inhibitory concentration(MIC)and the growth curve of foodborne pathogen.Results showed that thyme,cinnamon and oregano oils inhibited L.monocytogenes,S.Typhimurium,S.aureus and E.coli O157:H7,and the inhibition zone diameter range was 14.07-23.60,13.07-24.00 and 12.27-21.87 mm,respectively,all of which were moderately to highly sensitive.The inhibition zone diameter range of the other 12 types of essential oils was 6.00-14.40 mm,which were low to moderately sensitive or no inhibition.The MIC of thyme,cinnamon and oregano oils against four kinds of foodborne pathogens were 0.31,0.63 and 0.63-1.25?/mL,respectively.Thyme,cinnamon and oregano oils of MIC,2MIC and 4MIC almost completely inhibited the growth of four kinds of foodborne pathogens.The growth curve of L.monocytogenes treated with thyme oil has long delay period and the highest inhibition rate among the three essential oils of 1/2 MIC and 1/4 MIC.It demonstrated that thyme oil has the best antibacterial effect.2.The antibacterial mechanism of thyme oil against L.monocytogenes was analyzed from protein level.The constituent compounds of thyme oil were evaluated by gas chromatography-mass spectrometry.The cellular structure of L.monocytogenes treated with thyme oil was observed by scanning electron microscopy(SEM)and transmission electron microscope(TEM).L.monocytogenes was treated with Treatment-1(0.28 ?L/mL)and Treatment-2(0.31 ?L/mL),and was investigated at the protein level using tandem mass tag-based quantitative proteomic analysis.Results showed that thyme oil contains 28 kinds of composition.Phenolics is the main compounds in thyme oil,thymol is 47.23%,p-cresol is 20.37%and 2,6-dimethylphenol is 16.26%.The cell membrane of L.monocytogenes treated with thyme oil shown deformation,wrinkle and rupture,which causing cell damage in SEM and TEM.A total of 100 differentially expressed proteins including 57 up-regulated and 43 down-regulated proteins were identified in L.monocytogenes subjected to Treatment-1 vs control.The ratio of up-regulated and down-regulated proteins was 57%and 43%,respectively.The high proportion of up-regulated protein indicated that Treatment-1 might induce the stress regulation of L.monocytogenes.A total of 745 differentially expressed proteins including 220 up-regulated and 525 down-regulated proteins were identified in L.monocytogenes subjected to Treatment-2 vs control.The ratio of up-regulated and down-regulated proteins was 30%and 70%,respectively.The high proportion of down-regulate protein indicated that Treatment-2 might interfere the stress regulation and physiological metabolism of L.monocytogenes.The differentially expressed proteins were functionally categorized using gene ontology(GO)enrichment,Kyoto Encyclopedia of Genes and Genomes(KEGG)pathway,and STRING analyses.It revealed the antibacterial mechanism of thyme oil against L.monocytogenes:Thyme oil penetrated and incorporated into cell membrane after passing through the cell wall of L.monocytogenes.It damaged the permeability and integrity of cell membrane.Phenolics disturbed energy metabolism and the transcription,translation,RNA degradation and repair of DNA in L.monocytogenes,caused the loss of motion and bacterial resistance.The growth of L.monocytogenes was inhibited.3.To investigate the effect of Thyme Oil-Alginate-Based Coating(TO AC)on the quality and safety of fresh-cut apples.Fresh-cut apple as the experimental material were treated with Thyme Oil-Alginate-Based Coating containing different concentrations(0.05%,0.35%and 0.65%,v/v)of thyme oil.The effect of TOAC on respiration rate,weight loss,firmness,color and sensory quality were evaluated on fresh-cut apples.The change of total plate counts,total coliform counts,yeast and mould counts,and lactic acid bacteria counts on fresh-cut apples treated with TOAC was determined.The antibacterial effect of TOAC against L.monocytogenes,S.Typhimurium,S.aureus and E.coli O157:H7 on fresh-cut apples was evaluated.Uncoated and alginate-based coated samples were served as the control and coating treatment.Results showed that the 0.05%thyme oil incorporated into alginate-based edible coating could inhibited significantly the increase of respiration rate,maintained the weight loss rate,firmness,color,and the sensory evaluation score of fresh-cut apples was above 5(p<0.05).TO AC inhibited the growth of naturally occurring microorganisms and four kinds of foodborne pathogens inoculated artificially on fresh-cut apples.4.To investigate the effect of Thyme Oil-Alginate-Based Coating(TOAC)on the quality and safety of fresh-cut cantaloupes.Fresh-cut cantaloupe as the experimental material were treated with TOAC.The experimental method is the same as that of fresh cut apple.Results showed that the coating treatment of 0.05%thyme oil inhibited the increase of respiration rate and maintained the quality of fresh-cut cantaloupes.TOAC inhibited the growth of naturally occurring microorganisms and foodborne pathogens inoculated artificially on fresh-cut cantaloupes.In this study,the antibacterial mechanism of thyme oil against L.monocytogenes was investigated.The TOAC preservation agent for control foodborne pathogens on fresh-cut fruits was developed,which can effectively control the infection of foodborne pathogens and maintains the sensory quality of fresh-cut fruits and vegetables during the process of packaging,storage,logistics and sales. |
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