Characteristics of Radish Seed Protein and Its In Vitro Digested Products After Ultrasound-Heat Co-Treatment

Radish seed, a traditional Chinese herbal medicine rich in oil, generates a significant byproduct called radish seed meal after oil extraction, which contains high levels of protein. To develop novel plant-based proteins, this study investigated the effects of heat treatment (37-95°C, 30 min) with or without ultrasonication (20 kHz, 40% amplitude, 10 min) on the characteristics of radish seed protein (RSP) extracted from radish seed meal. Results showed that RSP particle size decreased after heating at 37-65°C (with or without sonication) but increased at 75-95°C. The combination of heat treatment and ultrasonication yielded the highest RSP solubility (93.35% ± 0.33%). Heating at 65°C improved RSP emulsifying capacity (from 10.90 ± 0.28 to 20.15 ± 0.12 m²/g), which was further enhanced by sonication (22.33 ± 0.10 m²/g). Similarly, emulsion stability, foaming capacity, and digestibility were significantly improved by sonication combined with 65°C heating. Notably, the antioxidant activity of digested RSP was boosted by sonication and 65°C heating, with peak ABTS radical scavenging and lipid peroxidation inhibition observed at 8 h of digestion, while other activities (reducing power, Fe²⁺-chelation, and DPPH/·OH scavenging) peaked at 10 h. Additionally, 8-h-digested RSP treated with sonication and 65°C heating exhibited the highest α-amylase (61.70% ± 1.10% at 1 mg/mL) and α-glucosidase (55.35% ± 0.53% at 1 mg/mL) inhibitory activities. These findings demonstrate that ultrasonication combined with mild heating effectively enhances RSP functionality and bioactivity, providing a foundation for its application as a source of bioactive peptides in functional foods. PRACTICAL APPLICATIONS: Ultrasonication combined with mild heating could be used to enhance RSP's functionality. The digested RSP under simulated intestinal condition at 8 h may serve as a source of bioactive peptides to help control blood glucose levels.