Less for more : Lower initial protein concentrations enhance the linear viscoelasticity of heteroprotein complex coacervates

Heteroprotein complex coacervation is exploited in biopolymer-based systems for food or pharmaceuticals. While the effects of pH, ionic strength, and charge stoichiometry have been widely investigated, the role of the initial total protein concentration in governing the properties of the resulting dense phase remains insufficiently understood. Here, we systematically examine coacervates formed by β -lactoglobulin and lactoferrin over a broad range of initial total protein concentrations (3.75–30 g/L). Surprisingly, rheological measurements show that decreasing the initial protein concentration leads to a marked increase in viscoelastic properties, with the complex modulus | G ∗ | increasing from 38.5 ± 5.1 Pa to 1563.9 ± 78.7 Pa. Time–temperature superposition is valid for all mixtures, and comparable activation energies are obtained ( ∼ 50–100 kJ/mol), indicating that decreasing the initial total protein concentration does not modify the fundamental stress relaxation mechanism but instead affects the structural organization of the coacervate phase. Gravimetric analysis reveals a small yet systematic reduction in water content, from ∼ 70% to ∼ 65%, as the initial total protein concentration decreases (30 to 3.75 g/L). Consistently, small-angle X-ray scattering shows a decrease in the correlation length from 95 to 86 Å (30 to 7.5 g/L), demonstrating that coacervates formed at lower initial concentrations adopt a more compact structure. These results demonstrate that small changes in hydration and intermolecular spacing induce large variations in viscoelasticity. The initial total protein concentration emerges as a key parameter controlling the structure and viscoelasticity of heteroprotein complex coacervates, providing new insight for the rational design of food-grade soft materials. • Lower initial protein concentration enhances HPCC viscoelasticity. • Eightfold decrease in initial concentration reduces water content (70%–65%). • Time–temperature superposition applies to all mixtures. • Reduced correlation length correlates with enhanced viscoelasticity.