Phase separation driven by aggregation can be reversed by elasticity in gelling mixtures of polysaccharides and proteins

Aqueous solutions of aggregates of the globular protein beta-lactoglobulin (beta-lg) were mixed with the polysaccharide kappa-carrageenan (kappa-car). So-called cold gelation of the proteins was induced at 60 degrees C by adding salt. In the presence of kappa-car, the growth of the protein aggregates induced phase separation that was arrested by gelation of the proteins. Different structures were observed with confocal laser scanning microscopy (CLSM) depending on the beta-lg concentration. Reducing the temperature to 20 degrees C led to aggregation and gelation of kappa-car. The effect of kappa-car on the shear modulus of the mixed gels both at 60 degrees C and during subsequent cooling and heating ramps was investigated. If the system was cooled before beta-lg gelled a transient micro-phase separation could be observed induced by the conformational transition and subsequent aggregation of kappa-car. However, as the elasticity of the kappa-car network increased the phase separation was reversed. kappa-car aggregation without network formation induced by the presence of NaI led to stable micro-phase separation. The complex process of cluster formation and (transient) micro-phase separation was quantified by measuring the turbidity. The features of this biopolymer mixture are particularly well suited to demonstrate the importance of aggregation and elasticity on the texturation of mixed gels in general.