Rheological properties of Lactoferrin/β-Lactoglobulin Coacervates

Heteroprotein complex coacervation has great potential in many food applications. However, understanding the rheology-coacervate structure relationship, as well as their sensitivity to slight changes in the physicochemical environment, is still an active research topic. Herein, heteroprotein complex coacervation between positively charged lactoferrin (LF) and negatively charged β-lactoglobulin (βLG) was investigated. The influence of ionic strength and temperature on the rheological properties of LF/βLG coacervates was examined using oscillatory shear rheology and microrheology from dynamic light scattering. The results indicated a liquid-like character with G’(ω) < G’’(ω) and an increase of both moduli with decreased temperature but a softening effect with increased ionic strength. The dependency of G’ and G’’ on angular frequency (ω) demonstrated a scaling of G” ∝ ω1 and a lack of terminal behavior with G’ ∝ ω1.4. The application of time-temperature superposition (TTS) and time-salt superposition (TSS) allowed the prediction of the rheological properties over a wide range of timescales and temperatures below the denaturation temperature of βLG and LF. The TTS and TSS suggested that increasing temperature or ionic strength accelerates coacervates dynamics but does not affect larger-scale physics. Microrheology experiments using polystyrene-coated microspheres as tracers, allowed access to a frequency range up to (ω ~ 106 rad/s) and revealed a scaling of G’= G” ∝ ω3/4 at the high-frequency terminal regime. By combining rheology and microrheology, we provided a comprehensive rheological study that underscores the influence of ionic strength and temperature on LF/βLG coacervates. This study highlights the similarities and differences between protein coacervates and polymer systems and offers new insights into the microstructure of coacervates relevant to various applications.