Complex coacervation between β-lactoglobulin and Acacia gum: A nucleation and growth mechanism

Complex coacervation between proteins and polysaccharides is a demixing process mainly driven by electrostatic interactions. During this process many structural transitions occur, involving the formation of soluble complexes, aggregated complexes, and coacervates. The dynamic mechanism of complexation/coacervation was studied on β-lactoglobulin (BLG)/Acacia gum (AG) mixed dispersions (0.1 wt% total concentration; BLG:AG ratio of 2:1) using small angle static light scattering (SALS). Acidification of BLG/AG dispersions was induced by dissolution of 0.11 wt% glucono-δ  -lactone, allowing in situ SALS measurements. Time evolution of turbidity, scattered light intensity at 46° scattering angle (I46I46) or slope of scattering functions at high q   range revealed the existence of six pH-induced structural transitions. During BLG/AG complexation and before coacervation took place, scattering profiles displayed a monotonic decrease of I(q)I(q) as a function of q  . A correlation peak in the scattering functions was only observed when coacervates appeared in the system. The wave vector qmaxqmax corresponding to the maximum in scattered intensity first shifted toward larger q values, indicating an increasing number of coacervates, then shifted toward smaller q   values, as a consequence of the system coarsening. The power laws qmax∼t−αqmax∼t−α and Imax∼t−βImax∼t−β gave values of 1.9 and 9.2, respectively, values much larger than those expected for intermediate and late stages of spinodal decomposition. From these results, it was concluded that complex coacervation between BLG and AG was a nucleation and growth type process. In addition, the temporal evolution of I46I46 followed power laws with two different exponents. First exponent corresponding to BLG/AG complexation was 3.0±0.33.0±0.3 and indicated a diffusion-controlled growth mechanism. Second exponent corresponding to the initiation of phase separation to the coacervation process was 6.5±0.36.5±0.3 and revealed an interfacially-controlled growth mechanism.

Exponent γ   of the power law I(q)∼q−γI(q)∼q−γ in the high q region obtained by small angle static light scattering on acidified BLG/AG dispersions at 0.1 wt% total biopolymer concentration and BLG:AG weight ratio of 2:1. Main structural transitions and corresponding pH are indicated in the figure. The shape of γ vs pH curve is the signature of complexation/coacervation between proteins and polysaccharides.Figure optionsDownload as PowerPoint slide