Depletion-induced phase separation of aggregated whey protein colloids by an exocellular polysaccharide

An attractive interaction, commonly referred to as depletion interaction, is induced between aggregated whey protein colloid (AWC) particles when they are mixed with exocellular polysaccharides (EPSs) from a lactic acid bacterium. This interaction originates from a loss of conformational entropy of the EPSs near the surfaces of neighboring AWC particles and leads to a phase separation at high enough EPS and AWC concentrations. The effect of the depletion interaction on the properties of the mixtures of EPS and AWC particles is first studied in the stable, that is, one-phase, region. Small-angle neutron scattering (SANS) and dynamic light scattering (DLS) were used to characterize the strength of attractions. The SANS results can be described quantitatively by a model for depletion interaction. From Omstein-Zernike plots, we derive the position of the spinodal. The DLS results can be described qualitatively quite well by using a recently derived expression for the wavevector (Q)-dependent diffusion coefficient as a function of the correlation length. Further, the experimental phase boundary is determined and compared with a mean-field theory, which evaluates the free energy of a mixture of colloids and large nonadsorbing polymers. The independently calculated spinodal was found to be consistent with the experimentally determined position of the phase boundary. Spinodal phase separation kinetics is investigated by small-angle light scattering (SALS). At low Q, a scattering peak was detected, which shifted to lower Qs with time, in agreement with other experimental data and theoretical predictions for spinodal decomposition. Both the scaling of the scattered intensity with Q and the scaling of the Q-position of the peak with time agree with theoretical predictions.