Characteristics of Pressure-induced Gels of β-Lactoglobulin at Various Times after Pressure Release

Abstract

Aggregation and gelation of aqueous solutions of a β-lactoglobulin (β-Lg) isolate (pH 7.0; 100 to 140 g/kg protein) were induced by pressure application and release (P-gels; 450 MPa, 25 °C, 15 min), or by heating (T-gels; 87 °C, 45 min). Pressure-induced aggregation led to porous gels prone to exudation in contrast to heat-induced gels which displayed a finely stranded network with high water retention. Pore size and strand thickness were greater for P-gels than for T-gels by one or two orders of magnitude. The matrix of the strands of P-gels consisted of highly packed particles 10 to 20 nm in diameter, as estimated by SEM, suggesting a random aggregation model with equally attractive sites per β-Lg particles (or primary aggregates). P-gels displayed a lower rigidity than T-gels. Moreover, P-gels could be totally dispersed and solubilized by homogenizing in water immediately after pressure release. Thus, pressure treatment at 450 MPa induced weaker intermolecular or interparticular forces than heating at 87 °C for 45 min. In contrast to T-gels, P-gels of β-Lg underwent mechanical and protein solubility changes when stored at 4 °C following pressure release, clearly indicating a time-dependent strengthening of protein–protein interactions. It appears that primary aggregates of β-Lg further aggregated during storage through hydrophobic interactions and disulfide bonds. Increasing the protein concentration of the initial solutions from 100 to 140 g/kg, and therefore the probability of protein–protein interactions, increased pore size and strand thickness of P-gels, with a marked trend to phase separation and protein microparticulation. Adding sucrose to the initial solutions decreased pore size and strand thickness and lessened the solid behaviour of P-gels, probably by reducing the number of protein–protein interactions induced by pressure.