Effect of pH on the formation mechanisms, emulsifying properties and curcumin encapsulation of oat protein isolate–high methoxy pectin complexes

Oat protein isolate (OPI) has high nutritional value but poor emulsifying properties. The purpose of modifying the OPI with high methoxyl pectin (HMP) was to improve emulsification. In this work, an oat protein isolate (OPI)–high methoxyl pectin (HMP) complex was constructed by changing the pH, and the interactions, formation mechanism and potential use of the OPI–HMP complex as a food emulsifier were explored. The presence of HMP caused structural changes in the OPI, and the changes were pH dependent. When the OPI to HMP mass ratio was 3:1, the critical pH for the phase behavior of OPI–HMP was 5.0. When the pH was greater than 5.0, the OPI and HMP were codissolved in the composite system mainly through electrostatic repulsion. When the pH was 5.0, a soluble OPI–HMP complex was formed by a combination of electrostatic attractions, hydrophobic interactions and hydrogen bonding. At pH values below 5.0, other aggregates of HMP and OPI were generated through electrostatic attraction and hydrogen bonding and formed insoluble aggregates. When the OPI to HMP mass ratio was 3:1 and the pH was 5.0, the particle sizes of the emulsion were the smallest at 8.75 μm due to the strong electrostatic interactions. The emulsification activity and stability were much higher than those of the OPI, and they formed dense osmotic networks, which protected the curcumin. The rates for curcumin encapsulation and retention reached 83.87 ± 1.50% and 88.70 ± 2.50%, respectively, and the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging rate was maintained at a high level (approximately 72%). These results confirmed the possibility of using the OPI-HMP complex emulsions as excellent and stable nutraceutical delivery systems for lipid soluble bioactive compounds in food and biomedical applications.