Elastic modulus and equilibrium swelling of stranded and particulate protein hydrogels at acid pH

Abstract

The effect of the pH and ionic strength on the gelation of protein systems, resulting in very different microstructures, has been extensively studied in the past, yet the swelling of such gels is poorly understood. A systematic study on the swelling of two protein systems, whey protein isolate and egg white, is presented here at acidic pH relevant to foods. The swelling behavior is critically determined by the general gel micro-structure, stranded or particulate, and not for example by the nature of the proteins. Transparent stranded gels swollen at pH > 6 > pI can be modeled considering the net charge of the proteins and the ionic strength, but at lower pH up to ∼4 the gels collapse to a highly shrunken state regardless of the pH or the salt concentration. At pH < 4 < pI stranded gels swell again, but much less than expected using models developed at higher pH, suggesting higher crosslinking degrees or lower net charges. Opaque particulate gels are only mildly affected by the pH or salts concentration, due to their rigid structure that cannot accommodate large volume changes meanwhile the forming aggregates are stable. The shear modulus of gels at different swelling degrees can be explained using power laws developed for polymeric networks, but no effect is seen by the swelling pH in the apparent crosslink density, implying that the chemical interactions in the gels are not substantially modified during swelling.