Dual-functionality of aqueous plant extracts of Cosmos caudatus and Anredera cordifolia in Sardine Surimi: Enhancing gel network formation and retarding lipid oxidation via Protein–Polyphenol interactions

This study investigated the effects of aqueous binahong leaf extract (BLE) and aqueous kenikir leaf extract (KLE) powders on the formation of protein conformational changes, gel network formation, textural properties, and storage stability of sardine ( Sardinella spp.) surimi gels. BLE exhibited higher total phenolic content and antioxidant activity than KLE, indicating stronger radical-scavenging, reducing, and metal-chelating activities. Incorporation of BLE and KLE significantly improved gel strength at optimal concentrations, with BLE-treated gels showing the highest breaking force, deformation, hardness, gumminess, and chewiness, while reducing expressible moisture content without affecting whiteness. FTIR analysis revealed shifts in the Amide A region and increased C–O stretching intensity, suggesting enhanced protein–polyphenol interactions. Furthermore, amide I deconvolution demonstrated that BLE promoted conformational rearrangement of myofibrillar proteins, characterized by increased β-sheet content and decreased α-helix and β-turn structures. These structural changes indicate enhanced intermolecular interactions and the formation of a more organized protein network. Dynamic rheological analysis showed higher storage modulus (G′) values in BLE-treated gels during thermal gelation, while scanning electron microscopy revealed a denser and more homogeneous microstructure. The observed increase in β-sheet content was consistent with the improved gel strength and viscoelastic properties, suggesting that protein secondary-structure modification played an important role in network reinforcement. During refrigerated storage, BLE effectively retarded lipid oxidation, as evidenced by lower peroxide value and thiobarbituric acid reactive substances (TBARS) levels compared with the control. Overall, aqueous leaf extracts, particularly BLE, enhanced gel quality and oxidative stability by modulating protein secondary structure and strengthening protein–polyphenol interactions, supporting their potential application as sustainable clean-label additives for surimi processing.