The impact of model rigid fillers in acid-induced sodium caseinate/xanthan gum cooperative protein gels

Abstract

This work evaluates the impact of model rigid fillers on the deformation and fracture behavior of acid-set sodium caseinate/xanthan gum (NaCas/XG) mixed biopolymer gels. Glass microspheres of varying size ranges were used as idealized filler particles. The large deformation elastic modulus showed a power law scaling behavior with increasing filler volume fraction (φf) for all fillers. The magnitude of this response was strongly dependent on filler size, as decreasing filler diameter produced a higher scaling exponent. For the smallest fillers tested (8 μm), the presence of amino functional groups on the filler surface produced a further increase in the scaling behavior. The stark increase in elastic modulus indicates the microspheres behaved as active fillers which bound to the electrostatically-induced cooperative gel network. The fracture strain (εr*) of both the neat and amino-coated 8 μm microspheres showed an abrupt decrease at low φf, which roughly followed the theoretically expected response proposed by Nielsen. In contrast, the larger fillers (90 μm and 180 μm) produced a roughly linear decrease in εr* with increasing filler loading. Above a threshold values of φf ~0.15, the fracture stress (σr*) displayed a power law scaling behavior, with the effect of filler size and surface chemistry analogous to that seen for the elastic modulus. Water loss results correlated well with the increase in mechanical strength, and T2 relaxometry suggested the hydrophilic fillers reduce water mobility within the gel network. These results indicate that the effect of filler size and surface chemistry can be used to modulate the large deformation and fracture mechanics of particle-filled food gels. This work adds to the growing body of literature on developing a more fundamental understanding of foods as composite soft materials.