Preparation Of PH-sensitive Starch-based Microcapsules And Their Effect On The Release Characteristics Of Vitamin E

With the rapid development of economy and the improvement of living standard,people's demand for functional food is increasing daily.However,functional factors have problems such as poor water-solubility,poor stability and low bioavailability at the absorption site,which limit their use in food.In recent years,studies on protection,in-vitro and in-vivo stabilization of functional factors have been widely reported.Among them,the pH-sensitive delivery system can respond significantly to pH changes in human digestive tract and enable targeted,quantitative release of functional factors to increase their bioavailability.While,pH-sensitive carriers are generally low-toxic polymeric materials,which are not suitable to be used in food systems.Starch contains multiple hydroxyl groups as a natural macromolecule substance,which is easy to be designed as functional material after modifications.Therefore,it has certain research significance and development prospects to apply starch as a pH-sensitive carrier for encapsulating and protecting functional factors.In this paper,difference between the pH value of human digestive tract was taken as the starting point.The acid hydrolysis-carboxymethyl starch(H-CMS)was prepared by double modifications of acid hydrolysis and carboxymethylation using normal corn starch as raw material.Then,pH-sensitive starch-based microcapsule carriers without toxicity were constructed by complexing H-CMS with anti-acid,anti-enzymatic xanthan gum(XG).By changing the degree of substitution(DS)of H-CMS,the ratio of wall materials and optimizing the microcapsule preparation process,Vitamin E,which was chosen as a model of hydrophobic functional factor in our study,could be released in the upper part of small intestine to improve its bioavailability.The main contents of this paper are as follows:Firstly,H-CMS with low,medium and high DS were successfully prepared and their characteristics were studied.Fourier transform infrared spectroscopy and scanning electron microscopy showed that carboxymethyl groups were successfully introduced into the native starch granules.Zeta-potential further proved that the normal corn starch and acid-hydrolysis starch were pH-independent,while surface charge of H-CMS changed significantly with the change of pH(p<0.05),which showed that H-CMS materials had pH sensitivity.The safety of H-CMS with different DS were systematically evaluated from three aspects,including chloride residue,sulfate residue and cytotoxicity.Results showed that the pH-sensitive H-CMS materials prepared by our subject were safe and suitable to be used in food systems.Secondly,pH-sensitive mechanism of the complex carriers,which were constructed by H-CMS and XG,were investigated.Light transmittance and particle size of the complex carriers in different pH solutions were measured in this part.Results showed that as the pH of the solution increased,light transmittance increased and the particles swelled,indicating that the XG/H-CMS complex carriers had pH-sensitive properties;Besides,we found that XG had a certain hindrance to the swelling behavior of carriers in the high pH environment.Thirdly,O/W emulsions were prepared using pH-sensitive complex carriers as the continuous phase and Vitamin E as the dispersed phase.Based on the emulsion stability and in-vitro release characteristics,the optimal O/W emulsion preparation conditions were a solid content of 15 g/100 g and a core-to-wall ratio of 1:6,respectively.Emulsion stability and steady flow measurements showed that H-CMS with a high DS had better compatibility with XG,and the emulsion prepared by them had a higher stability.Finally,Vitamin E as core material was encapsulated by XG/H-CMS complex carriers and characteristics of microcapsules were investigated.Results showed that the DS and the ratio of wall materials had no significant effect on the encapsulation efficiency and loading capacity of microcapsules;While,there was a significant positive correlation between the DS of H-CMS,XG/H-CMS compatibility and thermal stability of microcapsules(p<0.05);The DS of H-CMS and the ratio of wall materials had a great influence on the simulated release behaviors of microcapsules.When the DS of H-CMS was 0.6074 and the ratio was 1:20,microcapsules had the most obvious pH-sensitive release characteristics,showing that the release rate reached to 51.01%(m/m)in the first 30 min after transferring microcapsules from simulated gastric fluid(pH 1.2)to simulated intestinal fluid(pH 6.8),and the final cumulative release rate was as high as 100.00%(m/m);Fitting paraments of release kinetic equation showed that the release of Vitamin E from microcapsules mainly depended on Fick diffusion,Fick diffusion-carrier erosion in simulated gastric fluid,while in simulated intestinal fluid,the release of Vitamin E mainly depended on carrier erosion.In order to further study the in-vivo release behavior of microcapsules,transportation and release of Vitamin E in the gastrointestinal tract of rats were observed and determined by fluorescent labeled tracer method,which indicated that the carrier system constructed by our subject could effectively protect Vitamin E through the stomach and improved its bioavailability in the upper part of small intestine.