Interfacial Assembly and pH-Driven Structural Transitions in Sodium Caseinate–Pectin Complexes at Oil–Water Interfaces

The interfacial and colloidal behavior of sodium caseinate (CAS) interacting with low-methoxyl pectin (LMP) was investigated to elucidate the pH-dependent assembly of protein–polysaccharide complexes at oil-water interfaces. Turbidity measurements revealed pH-driven CAS-LMP complexation, with soluble assemblies forming near the casein isoelectric point (pI ≈ 4.6) and insoluble coacervates appearing under stronger acidification. Interfacial tension and dilatational rheology showed that equilibrium interfacial tension was only weakly affected by pH or LMP concentration, whereas adsorption kinetics and interfacial elasticity were markedly modified. At pH 5.0, CAS/LMP assemblies exhibited a pronounced increase in dilatational modulus, indicating cooperative interactions within the interfacial layer. The colloidal stability of oil-water dispersions reflected these interfacial changes. At pH 7.0, high LMP contents (》0.25 wt.%) induced reversible depletion flocculation, while at pH 5.0, moderate LMP levels (≥ 0.1 wt.%) suppressed aggregation through combined electrostatic and steric effects. Under acidic conditions (pH 3.0), charge neutralization and bridging flocculation dominated, leading to irreversible aggregation. Ultrasonication efficiently redispersed transient aggregates at low LMP content (≤ 0.1 wt.%), but not in strongly bridged systems. Overall, this study demonstrates that the interfacial assembly and stability of caseinate-pectin systems can be systematically tuned by pH and biopolymer ratio, providing insight into the structure-property relationships governing protein-polysaccharide biointerfaces. • pH-controlled CAS-LMP complex formulation and modulated their interfacial adsorption dynamics. • LMP notably increased the interfacial elasticity of CAS layers near its isoelectric point. • Moderate LMP concentrations mitigated droplet aggregation and improved emulsion stability at pH 5. • Elevated LMP levels induced reversible depletion flocculation under neutral pH. • Strong bridging interactions dominated at acidic pH and produced aggregates resistant to sonication.