| Essential oils are important ingredients commonly used in the food,cosmetic,textile,agriculture and pharmaceutical industries and are indispensable in human life.However,many components of essential oils are volatile and can be easily damaged by heat,oxygen,light,strong acid and alkali,which restricts the application of essential oils in industry.Microencapsulation especially complex coacervation is an effective strategy to protect sensitive substances Complex coacervated microcapsules are not only resistant to high temperature and high humidity,but also have high yield,efficiency and better controlled release performance.Moreover,complex coacervated microcapsules are very suitable for industrial production because of simple operation,mild preparing conditions and low cost.Even so,once the external conditions(such as p H,ionic strength,temperature,etc.)drastically change,the stability of the microcapsules will be greatly damaged since the main binding force of complex coacervation is electrostatic interaction.Gelatin-CMC were applied for eugenol loading to prepare low environment-sensitive double-layer microcapsules.Firstly,single-layer microcapsules were chosen as the subject.The effect of gelatin-CMC ratio,p H of complex coacervation,core-wall ratio and concentration of tannic acid on turbidity,thermal stability,microscopic morphology and yield was analyzed.Measurement of turbidity showed that the highest peak(95%)appeared at the gelatin-CMC ratio of 7:1.TGA analysis reaveled that the thermal stability of the microcapsules was better at the gelatin-CMC ratio of 7:1 and 8:1.The microscopic morphology of single-layer microcapsules prepared at different p H and core-wall ratio were observed.Results showed that spherical microcapsules with good morphology can be obtained when the p H of complex coacervation were 4.6,4.5,and 4.4.When p H droped to 4.3,Adhesion of microcapsules ocurred.The particle size of the microcapsules with core-wall ratio of 2:1 was significantly larger than that of 1:1.However,both of them showed a gradual increase in size with the decrease of p H.Finally,the influence of the concentration of tannic acid on the yield was investigated.Results showed that the highest yield was achieved when the concentration of tannic acid was 0.4%.According to the results of single-layer microcapsules,complex coacervated microcapsules were prepared at different p H(4.3,4.4,4.5,4.6),and then chitosan was added to preparedouble-layer microcapsules.Morphology observation indicated that the double-layer microcapsules maintained good shape after being coated with chitosan when the p H of complex coacervation was 4.4,while other single-layer microcapsules had varying degrees of flocculation after chitosan addition.Therefore the p H of complex coacervation was determined to be 4.4.Accordingly,the optimal conditions for the construction of chitosan protective layer were clarified.The effect of gelatin-CMC ratio on the amount of chitosan added was analyzed by potentiometric method.The highest saturation adsorption quantity(the amount of chitosan completely coating the gelatin-CMC is the saturation adsorption quantity for chitosan)for chitosan under system p H 4.5,5.0,5.5,6.0 were 25%,30%,30%,and 40%,respectively(the mass of chitosan/total mass of gelatin and CMC).Then,the appearance of double-layer microcapsules at different coating p H(4.5,5.0,5.5,6.0)and core-wall ratio was observed.The double-layer microcapsules at core-wall ratio 1:1 and coating p H 5.0 were uniform and the thermal stability was good.In addition,the capacity of the gelatin-CMC mixture for chitosan with different relative molecular mass(2×10~5,6×10~5 and 10~6)was studied.The results showed that chitosan with relative molecular mass of 6×10~5 had the highest saturation adsorption quantity(30%at coating p H 5.0).In summary,the optimal preparation conditions of single-layer microcapsules were determined as follows:the ratio of gelatin and CMC is 7:1,the total concentration of wall material is 1%,the complex coacervation p H is 4.4,and the ratio of core to wall is 1:1.The optimal conditions for the construction of the chitosan protective layer were:the relative molecular mass of chitosan is6×10~5,the coating p H is 5.0,and the amount of chitosan is 30%of the total amount of gelatin and CMC.The difference of encapsulation yield and efficiency(above 90%)between single-layer and double-layer microcapsules prepared with the optimal conditions were not significant.The morphology and structure of single-layer/double-layer microcapsules were further characterized and the mechanism of microcapsules was analyzed.The results of CLSM and SEM confirmed the formation of the protective layer of chitosan.FTIR showed that electrostatic interaction played a major role in the binding of gelatin,CMC and chitosan.The results of CD showed that the secondary structure of gelatin was transformed from a flexible structure to an ordered structure such asα-helix andβ-sheet due to the addition of CMC and chitosan.Based on the above studies,the processing stability(high temperature,extreme p H and ionic strength)of single/double layer microcapsules was investigated.TGA analysis showed that the severe decomposition temperature of the double-layer microcapsules was292℃,which was nearly 30℃higher than that of single-layer microcapsules.As the p H increased from 2 to 10,the retention of eugenol in single-layer microcapsules decreased from65%to 20%while it remained above 70%in double-layer microcapsules.When the ionic strength of the system was in the range of 0-200 mmol/L(p H 4.0 and 10.0),the retention rate of eugenol in the double-layer microcapsules remained at 70%-80%.Eugenol was not oxidatively degraded after being encapsulated and treated under harsh conditions.In addition,the storage stability of the double-layer microcapsules at different temperatures(4℃,25℃and 45℃)was higher than that of the single-layer microcapsules. |
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