Phase Behavior of Whey Protein Aggregates/κ-Carrageenan Mixtures: Experiment and Theory

Abstract

The phase separation behavior of whey protein isolate (WPI) aggregates and κ-carrageenan (κ-car) mixtures was studied using the Vrij's theory and image analysis method. The intrinsic parameter (molecular mass and radius of gyration) for κ-car and the WPI aggregates was determined using intrinsic viscosity and reduced viscosity of each biopolymer. Confocal microscopy observations revealed the appearance of protein aggregate domains when phase separation occurred, with microgel droplets of WPI included in a continuous κ-car phase. The occurrence of aggregate droplet has not been reported before for the phase-separating WPI/κ-car mixtures. So far, network emulsion-like microstructures have been observed with WPI in a network structure. By using different WPI concentrations (4% or 6%), the microstructure of the systems changes while increasing the κ-car concentration. The size of the microgels (1–2.5 μm) depends on both κ-car and WPI concentration. Confocal microscopy combined with image analysis (method of the variance) was used effectively as objective means to determine the phase boundary of the phase-separating systems. Additional information on the depletion layer thickness, Δ, was obtained using self-consistent field theory. The results show that Δ has a constant value of 80.5 nm for $$ {{\hbox{c}}_{\kappa {\rm{ - car}}}} \prec {\hbox{2 g}}/{l} $$ , in agreement with ∆ ≈ R g (radius of gyration). Above this concentration, Δ decreases as a function of κ-car concentration. The experimental phase boundary was well predicted using Vrij's theory. This work showed a new approach to generate phase diagrams (e.g., under shear) of phase-separating systems.