Complex coacervation of an arabinogalactan-protein extracted from the Meryta sinclarii tree (puka gum) and whey protein isolate

Puka gum (PG), a high molecular weight arabinogalactan-protein (AGP) polysaccharide extracted from the native New Zealand Meryta sinclarii tree formed electrostatic complexes with whey protein isolate (WPI) under specific conditions. The PG and WPI mixtures were slowly acidified using glucono-delta-lactone (GDL), and the progressive change from soluble polymer solution to soluble complexes and to the formation of insoluble complexes (through the association of soluble complexes) were monitored using visual examination, turbidity, particle size (dynamic light scattering) and zeta-potential measurements at total biopolymer concentration Cp = 0.3% w/w, protein:polysaccharide ratio Pr:Ps = 2:1, [GDL] = 0.14% w/w, and T = 20 °C. This process was characterised by identifying the phase boundaries, pHc (pH = 5.7 at which soluble complexes of WP and PG were formed) and pHΦ (pH = 4.7 at which complexes associated to form coacervates). The coacervates formed under these conditions appeared as a sticky, viscoelastic and highly extensible material. The effect of ionic strength (0–100 mM NaCl) on complex formation was also studied and a pHc/pHΦ-ionic strength phase diagram was constructed. Coacervation was fully suppressed at ionic strengths above 60 mM NaCl. Rheological measurements of the insoluble coacervate phase at different final coacervation pH (Pr:Ps = 2:1 and 4:1; Cp = 3.0% w/w) showed highest complex modulus G* at approx. pH 3.6 for both Pr:Ps = 2:1 and 4:1, i.e. electrical equivalence pH (EEP), where electrostatic interactions are strongest and the overall charge on the insoluble complex is neutral.