Diffusivity of whey protein and gum arabic in their coacervates

Structural properties of whey protein (WP)/gum arabic (GA) coacervates were investigated by measuring the diffusivity of WP and GA in their coacervate phase as a function of pH by means of three different complementary techniques. The combination of these measurements revealed new insights into the structure of coacervates. Nuclear magnetic resonance (NMR) measured the self-diffusion coefficient of the GA in the coacervate phase prepared at various pH values. Fluorescence recovery after photobleaching (FRAP) was measured using a confocal scanning laser microscope. The WP and GA were covalently labeled with two different dyes. The time of fluorescence recovery, related to the inverse of the diffusion coefficient, was evaluated from the measurements, and the diffusivity of the WP and GA on a long time scale could be individually estimated at each pH value. Diffusing wave spectroscopy (DWS) combined with transmission measurement was carried out in the coacervate phase, and the diffusion coefficient, corresponding to the averaged diffusion of all particles that scattered in the system, was calculated as a function of pH. Independently of the technique used, the results showed that the diffusion of the WP and GA within the coacervate phase was reduced as compared to a diluted biopolymer mixture. NMR, DWS, and FRAP measurements gave similar results, indicating that the biopolymers moved the slowest in the coacervate matrix at pH 4.0-4.2. It is assumed that the diffusion of the WP and GA is reduced because of a higher electrostatic interaction between the biopolymers. Furthermore, FRAP results showed that in the coacervate phase WP molecules diffused 10 times faster than GA molecules. This result is very relevant because it shows that WP and GA move independently in the liquid coacervate phase. Finally, DWS measurements revealed that the coacervate phase rearranged with time, as evidenced by a decrease of the diffusion coefficient and a loss of the turbidity of the sample. A more homogeneous transparent coacervate phase was obtained after a few days/weeks. Faster rearrangement was obtained at pH 3.0 and 3.5 than at higher pH values.