Walnut oil microcapsules prepared with different emulsification methods: Structure, stability, and release mechanism

In the present study, whey protein isolate (WPI), octenyl succinic anhydride (OSA), and resveratrol (Res) were used as wall materials for the preparation of walnut oil microcapsules via the direct mixing method (DM), the layer-by-layer method (LbL), and the Maillard combined mixing method (MCM), respectively. The structural characteristics, release characteristics, and molecular mechanism of microcapsules prepared by different emulsification methods were studied. The results showed that microcapsules prepared by DM (DM-M) exhibit excellent encapsulation performance and oxidation stability compared with microcapsules prepared by LbL (LbL-M) and MCM (MCM-M). The embedding rate of DM-M was 98.88 ± 0.09 %, and the particle size was the smallest (571.33 ± 24.65 nm), showing a smooth and uniform spherical structure. At the end of the intestinal digestion stage, the walnut oil release ratio of DM-M was as high as 80.92 ± 0.86 %, and the free fatty acid release amount was 37.90 ± 2.47 μmol/g, indicating that the shell of DM-M was completely broken down during intestinal digestion. Besides, the order of the interaction forces of DM-M was hydrophobic interaction (S3-S2: 9.99 ± 0.15 mg/mL) > hydrogen bond (S2-S1: 9.89 ± 0.09 mg/mL) > ionic bond (S1: 8.49 ± 0.19 mg/mL) > disulfide bond (S4-S3: 2.60 ± 0.07 mg/mL). The strength of the hydrophobic interaction, hydrogen bond, and ionic bond of LbL-M and MCM-M is weaker than that of DM-M, except for the disulfide bond. DM-M exhibited the strongest non-covalent force of the three microcapsules, which could form a dense structure and effectively prevent walnut oil droplet aggregation. This study provides a theoretical basis for developing high-quality walnut oil microcapsules, extending the shelf life of walnut oil, and embedding fat-soluble active substances.