Stability, rheological behavior and microstructure of Pickering emulsions co-stabilized by soy protein and carboxymethyl chitosan

Modifying proteins to enhance their emulsifying properties is critical for expanding the applications of emulsions in food manufacturing. In this study, we investigated the possibility of improving Pickering emulsion stability using a combination of protein particles (soy protein isolate (SPI) or SPI treated with a pH 12-shifting treatment (SPIS)) and carboxymethyl chitosan (CMCS). Pickering emulsions were prepared with different interfacial structures: (I) SPI or SPIS alone, (II) SPI-CMCS or SPIS-CMCS complexes, and (III) SPI-CMCS or SPIS-CMCS bilayers. The results obtained showed that the pH 12-shifting treatment decreased the particle size of SPI and increased its affinity to CMCS through multiple forces. The addition of CMCS induced particle size distributions of SPI/SPIS that shifted towards higher values of particle size. When the ratio of SPI/SPIS to CMCS was 1:0.2, its particle size distribution corresponded most closely to SPI/SPIS. pH 12-shifting treatment, compounded CMCS, or a combination of both significantly influenced the microstructure and properties of Pickering emulsions, with the combination of both showing the largest effect. Rheological measurements showed that Pickering emulsions formed an elastic-like structure (G′>G'′) and displayed a shear-thinning behavior. Microstructure analysis showed that SPI/SPIS adsorbed at the interface to form a thicker interface layer, which led to the formation of a three-dimensional network that maintained the stability of Pickering emulsions. In vitro digestion and oxidation results showed that the combination of both could be used for the slow release of free fatty acids and enhanced oxidation stability of Pickering emulsions. However, no differences were noted in all Pickering emulsions except for SPI. Moreover, this led to a significant improvement in stability for thermal and pH treatment, while the benefit on freeze-thaw stability was not significant. Importantly, Pickering emulsions stabilized by complexes or bilayers provided better stability, with SPIS-CMCS bilayer being a more effective approach to enhancing Pickering emulsions' stability.