Preparation, Characterization, and In Vitro Digestion Behavior of Alginate–Chitosan Microspheres Loaded with Ziziphus jujuba Pulp

In this study, sodium alginate–chitosan composite microspheres (S-C Ms) were prepared by ionic gelation to encapsulate Ziziphus jujuba pulp from wild jujube pulp. The effects of sodium alginate (SA) concentration, chitosan (CS) concentration, and core-to-wall ratio on encapsulation efficiency (EE%) and loading capacity (LC%) were systematically investigated. The results showed that both EE% and LC% were maximized when the SA concentration was 2.0% (w/v) and the CS concentration was 1.5% (w/v). The FTIR and XRD analyses confirmed the successful encapsulation of a phenolic-rich extract from Z. jujuba pulp (PRE) and its transformation into an amorphous state, while the SEM observations revealed that the composite microspheres possessed a well-defined morphology and a dense internal structure. Particle size analysis further indicated a narrow and uniform size distribution. Thermogravimetric analysis (TGA) and in vitro simulated digestion demonstrated that S-C Ms exhibited a pH-responsive release profile, characterized by slow, limited release in the gastric phase and markedly enhanced release in the intestinal phase. The release mechanism in simulated gastric fluid was dominated by Fickian diffusion, whereas it shifted to an erosion-controlled process in simulated intestinal fluid. Consistently, the swelling ratio of the microspheres was low at pH 1.2 but increased sharply at pH 7.0, reflecting a “gastric protection–intestinal release” behavior. Antibacterial assays showed that P-loaded microspheres exerted significant inhibitory effects against Staphylococcus aureus and other test strains, with the antibacterial activity possibly associated with the controlled release during the in vitro digestion of compounds with antimicrobial potential, such as phenolic compounds. Overall, SA-CS composite microspheres exhibited favorable encapsulation performance, structural stability, and controlled-release potential, making them a promising delivery and protection system for Ziziphus jujube pulp bioactive compounds.