Complex Coacervation of Whey Proteins and Gum Arabic

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Abstract
Mixtures of gum arabic and whey protein (whey protein isolate, WP) form an electrostatic complex in a specific pH range. Three phase boundaries (pHc, pHφ1, pHφ2) have been determined using an original titration method, newly applied to complex coacervation. It consists of monitoring the turbidity and light scattering intensity under slow acidification in situ with glucono-δ-lactone. Furthermore, the particle size could also be measured in parallel by dynamic light scattering. When the pH is lowered, whey proteins and gum arabic first form soluble complexes. This boundary is designated as pHc. When the interaction is stronger (at lower pH), phase separation takes place (at pHφ1). Finally, at pHφ2 complexation was suppressed by the charge reduction of the gum arabic. The major constituent of the whey protein preparation used was β-lactoglobulin (β-lg), and it was shown that β-lg was indeed the main complex-forming protein. Moreover, an increase of the ionic strength shifted the pH boundaries to lower pH values, which was summarized in a state diagram. The experimental pHc values were compared to a newly developed theory for polyelectrolyte adsorption on heterogeneous surfaces. Finally, the influence of the total biopolymer concentration (0−20% w/w) was represented in a phase diagram. For concentrations below 12%, the results are consistent with the theory on complex coacervation developed by Overbeek and Voorn. However, for concentrations above 12%, phase diagrams surprisingly revealed a “metastable” region delimited by a percolation line. Overall, a strong similarity is seen between the behavior of this system and a colloidal gas−liquid phase separation.