Preparation Of Natural Preservatives From Plant Essential Oils And Research In Fermented Vegetables Application

Plant essential oils are highly volatile secondary plant metabolites obtained by processing and extracting from different parts of the plant,and are rich in a variety of active ingredients with a variety of physiological activities such as antibacterial,antiinflammatory and anti-viral.However,its volatility,low water solubility and strong odour have limited its application in food products.Therefore,this project aims to develop a green plant-derived preservative that can inhibit three common food contaminants,viz.Escherichia coli,Salmonella and Staphylococcus aureus,and one usually beneficial species,viz.Lactoplantibacillus plantarum.Investigate the mechanism of bacterial inhibition of this preservative initially,and study the application of this preservative in industrial fermented vegetable products,so as to provide a theoretical basis and data support for the development and application of plant essential oil-based preservatives.The main findings are as follows:(1)Screening and compounding of essential oils: Three essential oils with strong antibacterial effects were screened by agar diffusion method and determination of viable bacteria,namely cinnamon essential oil,peppermint essential oil and lemon essential oil.The MIC and MBC of E.coli CMCC44496,Salmonella CMCC500041,S.aureus CMCC26003 and L.plantarum NCU001495 were determined;secondly,the diameter of the inhibition circles of the three essential oils after treatment with different temperatures and UV irradiation times were not significantly different from those of the control(P<0.05);furthermore,the optimal ratio of cinnamon,peppermint and lemon essential oils was 1.62:2.93:3.92,which was determined by one-way and response surface experiments.Under these conditions,the antibacterial tests were carried out on kimchi liquid and indicator bacteria,and the average number of colonies in kimchi liquid and indicator bacteria were 0.38 and 1.94 Lg CFU/m L,respectively.(2)Preparation and characterization of composite essential oil microcapsules: The ratio of the composite preservative was obtained by single-factor,orthogonal experimental design and information entropy weighting method,with the addition of gelatin at 20 mg/m L,chitosan at 4 mg/m L and core-to-wall ratio of 1:2,which resulted in an encapsulation rate of 56.26% and an overall score of 63.49.The microcapsules were characterised with particle size of 14.96 μm,zeta potential of-7.04 m V and moisture content of 5.45%,and were stable over a 56 d storage period;FTIR results showed that the essential oils was successfully encapsulated in the gelatin-chitosan complex and had a strong inhibitory effect on all four indicator bacteria.(3)Investigation of the mechanism of bacterial inhibition by microcapsules of essential oil: microcapsules of essential oil significantly increased the concentration of extracellular nucleic acid and protein,reduced LDH enzyme activity and increased organic acid content of the indicator bacteria;SEM and PI staining showed that the morphology of bacteria in the treated group of microcapsules of essential oil was wrinkled and deformed,aggregated and adhered,and the red fluorescence was significantly stronger than that of the control group.The experimental results showed that the microcapsules of essential oil could disrupt the microstructure of bacterial surface,which in turn changed the integrity and permeability of cell membrane,inhibited LDH activity and led to the accumulation of citric acid in the TCA cycle and further led to bacterial apoptosis.(4)Application of essential oil microcapsules in fermented vegetables: the essential oil was applied to fermented radish and the p H acidity,salinity,hardness,color and microorganisms were monitored during the 60 d storage period.In addition to the compounds treated with the pasteurisation,preservatives and control groups,DLmenthol,cinnamyl alcohol,D-limonene,cinnamaldehyde and other components unique to essential oils were also detected,and the masking effect of essential oil microcapsules on their odour needs further study.