| Thyme oil is extracted from Thymus mongolicus, which exhibit wide antimicrobial activity and antioxidant activity. Thus, it possesses the potential application prospect in the field of cosmetic, medicine and food. However, as a kind of natural food preservatives,thymel oil is extremely volatile in adverse conditions, so its application is limited in many fields. In this paper, the thyme oil microencapsulation process was studied, and the collected optimum conditions were obtain. In addition, the structural identification and stabilization of thyme oil microcapsules was analyzed. The article was also explore further the antibacterial activity and inhibitory mechanism. The results provided the research ideas and methods for developing an efficient, safe and stable natural food preservative. Meanwhile lay the foundation for the practical application of thyme oil. The major results are as follows.1. Chitosan/sodium alginate microcapsules encapsulating thyme oil were prepared by complex coacervation using ultrasonic assistive technology. The emulsion particle size was affected by kinds and dose of emulsifier. Finally, the compound emulsifier was the optimum choice, and the best HLB and the suitable ratio of emulsifier / thyme oil was 13,1:4 respectively. The ratio of 2.0 was decided as the optimal parameters of sodium alginate/chitosan. The optimum process conditions gained from orthogonal tests were as follows: ratio of core material to wall 0.5, temperature 40 °C, p H value 3.0 and time 20 min, where the practical encapsulation efficiency was 85.17 %.2. Infrared spectroscopy and differential scanning calorimeter were adopted to characterize the thyme oil microcapsules, followed by the measurement of partice diameter. The results shown that the average particle diameter of microcapsules is about531.17 nm. The results from infrared spectroscopy indicated that no specific peak value was found by comparison to the figure of sodium alginate and chitosan, futher confirms the formation of complexes promoted by physical interaction such as electrostatic interaction. The thermal property were measured by differential scanning calorimeter and it is observed that the thyme oil microcapsules in less than 53.1 °C can better protect the core material.3. At a different temperature, in a different storage time, the retention rate of thyme oil microcapsules was measured. The analysis shown that thyme oil retention rate at 4 °C,25 °C and 40 °C were 49.37 %, 46.63 %, 43.88 %, respectively, with the storage days beyond ten at different temperature. When thyme oil microcapsules stored 30 days,compared with its storage days of 10, retention rate of thyme oil at 4 °C, 25 °C and 40 °C fell by 10.16 %, 9.64 % and 10.08 %, respectively. By the statistical analysis after ten days in storage, the difference between the retention rate of thyme oil was not significant,that was to say, embedded thyme oil was affected little by temperature. The retention rate of thyme oil declined to 20.01 %, 25.88 % respectively, when thyme oil microcapsules were exposed to light or avoided light with the storage ten day, and in storage after 20 days, retention rate fell to 12.86 %, 15.24 %. Experiment results show that keeping the microcapsules in dark place is beneficial to reduce thyme oils loss. So it is more advantageous to extend the microcapsules slow-release under the conditions of protection from light.4. Inhibition rate was used as the main evaluating index, the antimicrobial activity of thyme oil microcapsules against Staphyloccocus aureus and Escherichia coli were studied.Results showed that the antimicrobial effect on Staphyloccocus aureus was better than that of Escherichia coli. The antibacterial properties analysis, including before and after encapsulating thyme oil. The studies found that the inhibition rate of the same amount of chitosan and thyme oil are far below that of compound microcapsules. And it deeply studies the inhibitory rate of thyme oil microcapsules at different growth stages of Staphyloccocus aureus.The microcapsules at 1.00 mg/ml exhibited inhibition rates of98.24% against Staphyloccocus aureus druing the logarithmic phase which were 9.44%and 12.50% higher than that druing the stationary and decline phase, respectively.5. The stability of the bacteriostatic have analyzed the effect of temperature and visible light. Temperature tests found that the inhibition rate of microcapsules preserved at below60 °C for 1 h had no obvious difference. But at high temperature of 80 °C and 100 °C treatment for 1 h, the inhibition rate fell to 29.58 %, 5.83 % respectively. In addition,compared to thyme oil, when thyme oli microcapsules were treated with 20 °C, 40 °C,60 °C, 80 °C and 100 °C for 1 h, the inhibition rate of thyme oil were 24.00 %, 11.00 %,7.00 %, 4.00 %, 0.50 % respectively and the inhibition rate of encapsulating thyme oil were relatively stable at below 60 °C. So microencapsulation of thyme oil can maintain the stability of the antibacterial activity. The visible tests found that thyme oilmicrocapsules exposed to light for 1 day still showed a high antibacterial activity against Staphyloccocus aureus with the inhibition rate of 97.34 %, only less 1.08 % than avoiding light, while inhibition rate of the amount of thyme oil were 21.80 % and 28.43 % in order.By research, the inhibition rate of thyme oil microcapsules and the amount of thyme oil declined to 0.30 %, 0.18 % respectively when stored at light for 20 days, that were29.14 % and 5.38 % lower the light-interrupted. All in all, the inhibition rate of thyme oil encapsuling by sodium alginate/chitosan were higher than that of without embedding no matter on light or dark. And the inhibition rate of the light-exposed microcapsules had been consistently lower than the light-interrupted microcapsules.6. Preliminary study on inhibitory mechanism of thyme oil microcapsules against Staphyloccocus aureus from cell walls. Compared with the control, the absorbance value of Staphyloccocus aureus determinated with 1.80 mg/m L thyme oil microcapsules for 120 min increased 0.10. That indicated the cell walls of the Staphyloccocus aureus ruptured after treated with the thyme oil microcapsules and the material in the cell leaked. By scanning and transmission electron microscopy can be clearly observed thyme oil microcapsules altered surface roughness of Staphyloccocus aureus and broken up the cell wall of Staphyloccocus aureus in logarithmic phase. Respiratory metabolism analysis shows that respiratory inhibition rate of Staphyloccocus aureus determinated with 1.80mg/m L thyme oil microcapsules was 47.31 %. And the overlay rate with typical respiratory inhibitors iodoacetic acid, malonate, sodium phosphate were, in order,21.92 %, 25.60 % and 26.88 %. The results showed that thyme oil microcapsules mainly restrained the embden-meyerhof-parnas pathway and broken the balance between anabolism and catabolism in Staphyloccocus aureus, leading to death in the end. |
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