Influence of pH on gel properties of Meretrix meretrix clam protein with κ-carrageenan: Phase behaviors, nonlinear rheological properties and gelation mechanism

In the present study, the complex coacervation of Meretrix meretrix clam protein (MMCP) and κ -carrageenan (KC) was investigated based on turbidimetry at various pH (12–1) and blending ratio (9:1–1:9). The pH c and pH φ1 showed a ratio-independent behavior around pH 9.5 and pH 6.0, respectively. The MMCP/KC complex were divided into three phases by crucial pH: mixed polymers, soluble complexes and insoluble coacervates. The storage modulus ( G′ ) of MMCP-KC binary hydrogels at pH 4.5 was 165 % and 71.4 % higher than that at pH 10.5 and 7.5, respectively, exhibiting the best viscoelasticity. According to nonlinear rheological analysis, the MMCP/KC binary hydrogels had weak strain overshoot properties, as well as intracycle shear thinning and strain-stiffening properties, which exhibited the best structural stability at pH 4.5. Correspondingly, the MMCP/KC binary hydrogels at pH 4.5 exhibited a well-organized and dense microstructure with the lowest lacunarity about 0.17 accompanied by microphase separation and protein self-aggregation, which allowed more water to be entangled in the gel network and enhanced the thermal stability of the binary hydrogels. Furthermore, the electrostatic and hydrophobic interactions maintained the main intermolecular forces of MMCP/KC binary hydrogels under acidic conditions, with the total proportions of 42.8 % and 34.7 %. The blue shift of amide I bands, decreased free -SH groups and increased surface hydrophobicity of MMCP/KC binary hydrogels further confirmed the main contribution of electrostatic and hydrophobic interactions in the gel network. These results provided theoretical and methodological basis for designing novel marine protein-polysaccharide binary hydrogels with excellent gel properties.