Composition and Structure of Whey Protein/Gum Arabic Coacervates

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Abstract
Complex coacervation in whey protein/gum arabic (WP/GA) mixtures was studied as a function of three main key parameters: pH, initial protein to polysaccharide mixing ratio (Pr:Ps)ini, and ionic strength. Previous studies had already revealed under which conditions a coacervate phase was obtained. This study is aimed at understanding how these parameters influence the phase separation kinetics, the coacervate composition, and the internal coacervate structure. At a defined (Pr:Ps)ini, an optimum pH of complex coacervation was found (pHopt), at which the strength of electrostatic interaction was maximum. For (Pr:Ps)ini = 2:1, the phase separation occurred the fastest and the final coacervate volume was the largest at pHopt = 4.0. The composition of the coacervate phase was determined after 48 h of phase separation and revealed that, at pHopt, the coacervate phase was the most concentrated. Varying the (Pr:Ps)ini shifted the pHopt to higher values when (Pr:Ps)ini was increased and to lower values when (Pr:Ps)ini was decreased. This phenomenon was due to the level of charge compensation of the WP/GA complexes. Finally, the structure of the coacervate phase was studied with small-angle X-ray scattering (SAXS). SAXS data confirmed that at pHopt the coacervate phase was dense and structured. Model calculations revealed that the structure factor of WP induced a peak at Q = 0.7 nm-1, illustrating that the coacervate phase was more structured, inducing the stronger correlation length of WP molecules. When the pH was changed to more acidic values, the correlation peak faded away, due to a more open structure of the coacervate. A shoulder in the scattering pattern of the coacervates was visible at small Q. This peak was attributed to the presence of residual charges on the GA. The peak intensity was reduced when the strength of interaction was increased, highlighting a greater charge compensation of the polyelectrolyte. Finally, increasing the ionic strength led to a less concentrated, a more heterogeneous, and a less structured coacervate phase, induced by the screening of the electrostatic interactions.