Construction, Interfacial Characteristics, and Stability of External Gelation Soy Protein Isolate–Dextran/Sodium Alginate Water-in-Oil-in-Water Emulsions and Freeze-Dried Microcapsules Loaded with Beech Mushroom-Derived Spermidine

During thermal processing of the beech mushroom, water-soluble bitterness-related compounds migrate into the cooking liquor. Spermidine (SPD), one of the representative hydrophilic polyamines, has potential nutritional value, but its direct exposure may also contribute to bitterness. To improve its utilization while limiting the direct exposure of SPD, SPD recovered from beech mushroom cooking liquor was used as the core material to prepare soy protein isolate–dextran (SPI–Dex)/sodium alginate (SA) external gelation water-in-oil-in-water (W/O/W) emulsion and freeze-dried microcapsules. The study evaluated SPD recovery, emulsion stability, and the structural and encapsulation properties of the resulting microcapsules. The initial SPD concentration in the cooking liquor was 69.17 mg/L and increased to 520.10 mg/L after membrane filtration, low-temperature concentration, and food-grade enrichment, with an overall recovery of 72.16%. The emulsions showed a typical W/O/W multiple structure, with encapsulation efficiency (EE) and retention efficiency (RE) of 92.90–99.76% and 92.47–96.87%, respectively. SA improved emulsion structure, interfacial charge, and physical stability. After freeze-drying, the microcapsules showed a porous network structure, low water activity (0.2139–0.2279), and low moisture content (2.14–2.88%), with EE of 56.44–98.13% and RE of 70.21–89.12%. These results show that the SPI–Dex/SA system can effectively encapsulate and stabilize beech mushroom-derived SPD, and may provide a feasible strategy for limiting its direct exposure in food systems while improving the utilization of thermal processing by-products.