Study of β-lactoglobulin/acacia gum complex coacervation by diffusing-wave spectroscopy and confocal scanning laser microscopy

Abstract

Complex coacervation has been investigated on mixtures of β-lactoglobulin (β-lg) and acacia gum (AG) at pH 4.2 where these two macromolecules interact electrostatically. Changes in β-lg/AG complex coacervation induced by the presence of β-lg aggregates were considered. The nature and structure of particles resulting from complex coacervation were determined by using confocal scanning laser microscopy (CSLM). CSLM revealed fundamental differences in the structure of each of the studied dispersions (at 1 wt.% total concentration). Spherical vesicular coacervates and precipitates (based on β-lg aggregates) were the hallmark of BLG/AG dispersions (β-lg dispersion containing insoluble aggregates). Only coacervates were visible in AF-BLG/AG dispersions (β-lg dispersion free of insoluble aggregates). The latter were characterised by the presence of large foam-like coacervates induced by partial coalescence of single coacervates, especially at the 2:1 protein to polysaccharide (Pr:Ps) ratio. Diffusing wave spectroscopy (DWS) was used to study the stability of dispersions as a function of time. Depending on the Pr:Ps ratio and the presence of β-lg aggregates, the intensity correlation function ( g 2( t)) shifted to lower correlation times rapidly after mixing of both macromolecules. This revealed the formation of a large number of small particles, characterised by faster Brownian motion. At 1 and 5 wt.% total concentration, the 8:1 Pr:Ps ratio exhibited a rapid decrease of the backscattered intensity in time, both for BLG/AG and AF-BLG/AG mixtures, revealing rapid sedimentation/coalescence of particles. This precluded the achievement of a stable correlation function. For the 2:1 Pr:Ps ratio, mixtures exhibited both coalescence and sedimentation phenomena as confirmed by shifts in the g 2( t) towards larger correlation times and the decrease of the initial value of g 2( t) with time. Mixtures obtained for the 1:1 Pr:Ps ratio were characterised by small variations in the DWS signal, emphasising the stability of produced particles. The increase of the total biopolymer concentration reduced the effect of both Pr:Ps ratio and presence of protein aggregates. From CSLM and DWS observations, possible differences in the complex coacervation mechanism in both types of mixtures were highlighted. The use of protein aggregates to control complex coacervation was underlined.