Sodium alginate/soybean protein–epigallocatechin-3-gallate conjugate hydrogel beads: evaluation of structural, physical, and functional properties

Sodium alginate (SA) hydrogel beads have been extensively studied as delivery systems for bioactive compounds. Key challenges include overcoming the highly porous and poor emulsifying properties of SA hydrogels. Herein, soy protein isolate (SPI) was modified by covalent and noncovalent conjugation with epigallocatechin-3-gallate (EGCG), followed by complexation with SA to change the SA structure and fabricate hydrogel beads with low porosities. Microencapsulation beads were fabricated from SA-, SA/SPI-, and SA/SPI-modified EGCG complexes with a corn oil/quercetin mixture core. After the covalent and noncovalent SPI-modified EGCG complexes were combined with SA, the OH stretching vibration shifted, indicating that hydrogen bonds formed between the protein and SA, and the crystal structure of SA was destroyed. To achieve crosslinking, the beads were injected into a CaCl2 solution, whereby Ca2+ ions replaced the Na+ ions in SA. Meanwhile, the addition of covalent and noncovalent SPI-modified EGCG complexes promoted the binding capacity of Ca2+ and SA. All hydrogel beads possessed open-cell microstructures with interconnecting pores. The SA/SPI-modified EGCG hydrogel beads exhibited smoother surfaces, thicker shells, and lower porosity than the SA hydrogel beads. Moreover, they exhibited significantly higher antioxidant activities. During digestion, all types of hydrogel bead maintained their structure, and only a small part of the encapsulated oil and quercetin was digested in the upper part of the gastrointestinal tract. In short, the formation mechanism of hydrogel beads was clarified, and hydrogel beads with low porosity and high antioxidation activities were successfully fabricated.