Effects of tea polyphenols on the properties of starch ester nanoparticles and their stabilized Pickering emulsions

Pickering emulsions can be expected to serve as a promising method against coalescence during storage in the food industry. Food-grade oil-in-water Pickering emulsions are stabilized by the solid particles on the oil-water interface, where a dense film can be formed against the aggregation of droplets. In this study, the food-grade oil-in-water Pickering emulsion was prepared using Octenyl Succinic Anhydride (OSA) modified waxy maize Starch Nanoparticles (SNPs) in the presence of Tea Polyphenols (TPs). An investigation was also made to clarify the effects of TPs on physicochemical properties and emulsifying capacity of OSA-starch nanoparticles (OSA-SNPs). The potential of TP-OSA-SNPs was examined after the combination of OSA-starches with TPs. The ethanol precipitation was then selected as a kind of particle emulsifier. An analysis was also conducted for the properties of Pickering emulsions that stabilized by OSA-SNPs and TP-OSA-SNPs at different concentrations. The mean particle size of OSA-SNPs and TP-OSA-SNPs was measured by the laser diffraction particle size analyzer and Scanning Electron Microscope (SEM). The results showed that the mean particle size of TP-OSA-SNPs (222.00±1.40) nm was larger than that of OSA-SNPs (173.20±0.57) nm. The absolute value of Zeta potential for the TP-OSA-SNPs (|-19.10±1.30| mV) was less than that of OSA-SNPs (|-24.20±3.18| mV). The contact angle of TP-OSA-SNPs (78.60°±0.84°) was also smaller than that of OSA-SNPs (83.20°±1.02°). It infers that there was a weaker hydrophobicity of TP-OSA-SNPs, compared with the OSA-SNPs. Furthermore, the structural properties of OSA-SNPs and TP-OSA-SNPs were analyzed by Fourier Transform Infrared spectroscopy (FT-IR). Specifically, the characteristic absorbance peak at 3 600-3 200 cm-1 of TP-OSA-SNPs was shifted towards the lower wavenumber, compared with the OSA-SNPs. Moreover, the FT-IR spectra of TP-OSA-SNPs appeared a peak at the same wavenumber, indicating that the hydrogen bonds were formed in the interactions of TPs with OSA-SNPs, compared with the absorbance peak at 1 700 cm-1of TPs. There were the decreased intensities of peaks for the TP-OSA-SNPs at 1 720 and 1 569 cm-1, indicating the hydrophobic association between TPs and OSA-SNPs. There was an outstanding change in the Pickering emulsions that stabilized by OSA-SNPs and TP-OSA-SNPs at different concentrations. The mean droplet size of the OSA-SNPs emulsions increased slightly, as the concentration of OSA-SNPs increased from 0.5 g/mL to 2.0 g/mL. By contrast, the mean droplet size of the TP-OSA-SNPs emulsions decreased with the increasing TP-OSA-SNPs additions from 0.5 g/mL to 2.0 g/mL. The droplet size distributions of the TP-OSA-SNPs emulsions at different concentrations were changed from the bimodal to the unimodal distribution. The trend was attributed to the various compositions of interfacial structure with the different particle emulsifiers. The morphologies of Pickering emulsions were visualized by Laser Confocal Fluorescence Microscopy (CLSM). Specifically, a large quantity of OSA-SNPs was absorbed closely on the surface of oil droplets, inhibiting the droplets from gathering. There was a weaker absorbing capacity of TP-OSA-SNPs, compared with the OSA-SNPs. The reason was the smaller quantities of absorbed TP-OSA-SNPs on the oil-water interface. Additionally, an interfacial droplet-compacted structure was formed in the TP-OSA-SNPs emulsion with a concentration of 2 g/mL, which was in favor of inhibiting oil droplets migration. A rapid oxidation test was carried out to determine the oxidative stabilities of OSA-SNPs and TP-OSA-SNPs emulsions. The Peroxide Value (POV) was also selected to evaluate the formation of primary oxidative products in the emulsions. There was an increase in the POV of the OSA-SNPs and TP-OSA-SNPs emulsions during storage. However, the POV descended with the increasing concentration of OSA-SNPs or TP-OSA-SNPs in the emulsion. Furthermore, the POV values in the TP-OSA-SNPs emulsions were smaller after 15 days of storage, compared with the OSA-SNPs emulsions. It infers that the oxidation stabilities of the TP-OSA-SNPs emulsions were better than that of the OSA-SNPs emulsions. Consequently, the interactions between TPs and starch-based nanoparticles emulsifier can greatly contribute to developing the better carrier for the bioactive substances. The finding can also provide a potential application to fabricate the delivery system of food-grade starch-based Pickering emulsion.