Shear-induced morphological changes in associative and segregative phase-separated biopolymer systems

Abstract

Shear-induced morphological changes of phase-separated whey protein isolate (WPI; 5%, w/v) droplets dispersed in high methoxyl pectin solutions (2%, w/v) subjected to pH changes (6.1 → 5.2 → 3.2) and addition of salt (200 mM NaCl) were investigated. Systems were sheared (0.1–50 s−1) on an optical rheometer and sizes and morphologies determined by microscopy and light scattering techniques. Apparent viscosities were calculated from shear stress versus strain rate measurements. The microstructures of phase separated WPI – pectin dispersions at pH 6.1 was altered at low to intermediate shear rates (0.1–10 s−1) from initially containing spherical particles to elongated ones. Shearing also led to increases in particle size due to shear-induced coalescence. At high shear rates (>10 s−1) particle size decreased due to droplets being disrupted. Morphologies of dispersions changed substantially when pH was decreased to 5.2 yielding core–shell like WPI droplets surrounded by aggregated pectin – whey protein isolate “shells”. Those structures were substantially less sensitive to shear-induced disruption, and high deformational forces were required to break them up. At pH 3.2 small pectin – WPI aggregates were formed that were resistant to superimposed shear stresses. Addition of salt increased shear-induced droplet coalescence at pH 6.1 but had little influence on morphology of structures at pH 5.2 and 3.2. Results were explained in terms of modulations of molecular interactions between the two biopolymers yielding either phase-separated WPI droplets suspended in pectin with ultralow interfacial tensions or electrostatically-networked biopolymer aggregates with high interfacial tensions.