Stability and pH-Dependent Mechanism of Astaxanthin-Loaded Nanoemulsions Stabilized by Almond Protein Isolate

Pickering emulsions (PEs) of natural plant proteins enriched in fat-soluble components are gaining consumer interest for healthier and sustainable products. The aim of this study is to prepare PEs for stabilizing almond protein isolated (API) particles loaded with astaxanthin using ultrasound technology. The loose structure of the API at pH levels of 3 and 12, with contact angles of 68.92° and 72.56°, respectively, facilitated its transfer from the aqueous to the oil phase. The adsorption of the API at the oil-water interface was 71.56% and 74.69% at pH levels of 3 and 12, respectively, which was significantly higher than that of the emulsions at other pH levels (5, 7, and 9). After 14 days of storage at 4 °C, PEs at pH levels of 3 and 12 did not undergo phase separation, with small and homogeneous droplets. CLSM revealed a monolayer arrangement of the API at the oil-water interface. These results indicate that PE is more stable at pH levels of 3 and 12 than at other pH levels (5, 7, and 9). In addition, the stabilized astaxanthin PE showed the largest astaxanthin encapsulation (91.43%) at a pH of 3. The emulsions had significantly lower a* values and higher L* values at a pH of 3 compared to a pH of 12, indicating better astaxanthin stability in the PEs. These results will help to expand the application of API-PE loaded with astaxanthin at different pH values.