Effects of EGCG on the properties of starch-based emulsion gels

Here food-grade emulsion gels were prepared using waxy maize starch hydrogels in the presence of epigallocatechin gallate (EGCG). A systematic investigation was implemented to explore the effects of EGCG contents on the the structural, rheological, and emulsifying properties of starch hydrogels. The potential of starch hydrogels with different additions of EGCG was examined using both a food-grade emulsifier and a gelling agent. The properties of emulsion gels were evaluated to stabilize by starch hydrogels with the additions of EGCG. The microstructure of starch hydrogels with the additions of EGCG was measured via cryo-scanning electron microscopy (cryo-SEM). The rheological behavior of starch hydrogels with the additions of EGCG was determined by a rheometer. The results showed that the pore size of the starch hydrogel network increased significantly, as the addition of EGCG increased from 6% to 18%. The reason was the bridge functions of EGCG between amylopectin. The share stress and viscoelastic properties of the starch hydrogels also increased with the EGCG additions increasing from 6% to 18%. There were also interactions between starch molecules and EGCG. An outstanding change was obtained in the emulsion gels that stabilized by starch hydrogels with the additions of EGCG. The micromorphology of emulsion gels was also characterized using a laser confocal fluorescence microscope (CLSM) and cryo-SEM. The CLSM observation showed that there was no adherence of the starch hydrogels to the soybean oil droplets, without EGCG additions. While the starch hydrogels with EGCG covered the oil droplets, the coverage of starch hydrogels was significantly different variation in the different additions of EGCG. The interaction between starch hydrogels and the dispersed oil droplets in the emulsion gels was analyzed by cryo-SEM. Among all samples of emulsion gels, there was quite a large size of oil droplets that stabilized by starch hydrogel matrix. A large crack was observed between the starch hydrogel matrix and oil droplets, indicating the low hydrophobic of starch hydrogels without a stabilize emulsion gels. However, the starch hydrogels with the addition of EGCG from 6% to 18% were observed to coat the soybean oil droplets, where the coating structure of starch hydrogels varies with different EGCG contents. The thick starch hydrogels were covered over the oil droplets, and 5% EGCG was added. While the homogeneous ordered hydrogel network was coated with the oil droplets, when the added EGCG reached 12%. The coverage of starch hydrogels on oil droplets decreased, as the addition of EGCG increased to 18%. The physical stability of emulsion gels was evaluated by emulsification index during a year of storage. There was significant creaming in emulsion gels that stabilized by starch hydrogels just for one day of storage. However, the emulsion gels that stabilized by starch hydrogels with different additions of EGCG (from 6% to 18%) showed a higher emulsification index during long-term storage, indicating their excellent physical stability. As both excellent emulsifiers and gelling agents, EGCG-starch hydrogels can be expected to serve as promising materials in the food-grade starch-based emulsion delivery system. It is of great importance to explore the effect of EGCG on the starch-based emulsion gels. The finding can be beneficial for the food and pharmaceutical industry, in order to develop novel bioactive component delivery vehicles.