Mechanisms Responsible for Non-linear and Fracture Properties of Gel-based Foods

Abstract

The well-defined chemical, physical, and especially elastic properties possessed by polyacrylamide gels facilitate the characterization of the gel network through rheological measurements. The results produced fracture properties associated with gel network crosslink density and chain length. The linear stress-strain relationship up to fracture for the gels, as well as the constant fracture strain regardless of network chain length, indicated there was no direct relationship between fracture strain and network chain length. Therefore, gel fracture did not simply result from the limitation of network chain stretching, and a new perspective is required to explain strain-hardening phenomena frequently observed for biopolymers.

Alginate gels showed strain-hardening behavior at large deformation. Considering the large size of junction zones, serving as the crosslinks in the gels, a hypothesis was proposed to explain the strain-hardening mechanism: the behavior was attributed to the deformation of rod-like junction zones. This hypothesis was substantiated by a relationship between the gel network and formulation. A mathematical model, based on the hypothesis, was developed and supported this claim.

The fracture stress of alginate gels correlated with gel formulation, similar to polyacrylamide gels. While association of fracture strain with the gel formulation was complicated. The fracture strain was not directly correlated with a single parameter, but rather, the fracture strain was predicted based on fracture stress, small-strain shear modulus, and a fitted parameter describing non-linearity of the gel.

For elastic gels, no viscous flow is experienced during large deformation; but for viscoelastic gels, viscous flow was observed. These observations revealed the fracture characteristics of elastic and viscoelastic gels. For elastic gels, the fracture occurred when the chemical bond between chemical crosslinks failed on a macroscopic level. However, for viscoelastic gels, the viscous flow resulted from the micro-structural change within junction zones, which was a rate or time-dependent process. The failure of viscoelastic gels was caused by the disruption of junction zones.

Additives influenced the gel fracture properties for elastic and viscoelastic gels. For elastic gels, the effect of dextran and glycerol on fracture properties was caused by a transition in the fracture mode. For alginate gels, the effect of additive on the gel fracture properties was not related to fracture mode, but depended on the additive molecular size. The liquid phase viscosity conveyed by additives did not account for the fracture property change.