Study On The Effect Of Self-assembled Nanoemulsion On The Preservation Of Chilled Pork Based On The Antibacterial Properties Of Thyme Essential Oil

Pseudomonas is a common foodborne spoilage bacterium,mainly found in meat products and drinking water.It is one of the superiority strains in refrigerated pork,which can easily cause food spoilage and harm human health.A safe and effective method is urgently exploit to control the evolution and spread of Pseudomonas.Thyme（Thymus monoglious Romm）is a herb of genus thyme in the labiaceae.Thymol as a dominant compound exists in the chemical profiles of Thyme essential oil（TEO）.TEO and thymol have a wide range of bactericidal,mildewproof,antiseptic,antioxidant properties,they can be used as a safer and more effective natural alternative to antibiotics and chemical agents.However,the practical applications in food industry of TEO and thymol are greatly limited due to their high volatility,insolubility and poor oxidation stability.In particular,the unique odor and bitterness of TEO and thymol can adversely affect the sensory quality of the food products.Thus,in this study,we first selected Pseudomonas aeruginosa（P.aeruginosa）as the standard test strain to explore the antibacterial and anti-biofilm activities and possible mechanism of action of TEO and its main component thymol.Also,we prepared TEO-CSs and T-CSs by loading TEO and thymol to chitosan nanoemulsions,and characterized their structure and antibacterial properties.Then these two emerging fresh-keeping materials were applied to refrigerated pork to reveal their fresh-keeping potential.The major results are as follows:1.GC-MS results showed that thymol was the major component of TEO.TEO and thymol could effectively combat P.aeruginosa and directly change the cell structure.The integrity of the cell membrane was destroyed,which was reflected as increased permeability of the inner and outer membrane of the cell and leakage of the cell contents（DNA）.The study further confirmed that TEO and thymol could cause the accumulation of reactive oxygen in cells and lead to DNA damage.Thymol could interfere with the normal function of DNA by being embedded directly into the DNA and even cause bacteria to die.In addition,TEO and thymol also affected the formation of biofilm,limiting bacterial swimming and swarming motilities;2.The average particle size of the prepared TEO-CSs and T-CSs were 139.47 nm and 123.30 nm,respectively.The encapsulation efficiency of them were 80.99%and 70.72%,and the absolute values of Zeta potential were 30.87 mV and 32.42 mV.The results of FTIR,XRD and TGA showed that chitosan was physically mixed with TEO and thymol,respectively,and there was no chemical reaction.Moreover,the TEO and thymol embedded in chitosan had better thermal stability.In addition,TEO-CSs and T-CSs effectively inhibited the activity of Staphylococcus aureus and Escherichia coli,controlled the formation of biofilm,and destroyed mature biofilm;3.The prepared TEO-CSs and T-CSs were used to soak refrigerated pork stored at 4℃.Then,the soaked pork was determined the color indicators（L*,a*,b*and AE values）,pH value,volatile constituents and total viable count（TVC）of bacteria at the certain time interval（0,3,6,9 and 12 d）to analyze the effects of TEO-CSs and T-CSs on the quality of refrigerated pork during storage.The results showed that the appearance deterioration and pH rise of the treated pork were significantly delayed,and the shelf life of the treated pork was effectively extended by more than 6 days after the treatment of TEO-CSs and T-CSs.Furthermore,based on 16S rRNA gene sequencing technology and HS-SPME-GC-MS analysis,the changes of microflora during the storage of refrigerated pork were studied.With the prolonging of storage time,the microbial abundance of pork under different treatments decreased.The superiority strain of untreated pork during storage period was Pseudomonas,while the TEO-CSs and T-CSs treatments had significant effects on the microbial community.The superiority strain of the treated pork was basically Lactobacillus in the later storage period.The results of HS-SPME-GC-MS analysis showed that the odor of spoilage was related to superiority strains and their metabolites during cold storage.The odor of pork during the middle and late period of storage mainly came from sulfur compounds,which was related to the growth and metabolism of Pseudomonas and Escherichia coli.However,the low odor in the treatment groups possibly was related to the decrease in the proportion of Pseudomonas.