Large deformation rheology of gelatin gels

Abstract

We studied the linear and the non-linear elastic behaviour, the breaking stress and breaking strain of gelatin gels as a function of a number of experimental conditions: gelatin concentration, gelatin bloom value, ageing time, ageing temperature, pH, NaCl and CaCl2 concentration, whey protein concentration, the amount of pre-shearing, strain rate or compression speed, using both shear deformation and compression. We analyzed the stress-strain curves using the BST-equation (Blatz et al., Trans. Soc. Rheol. 18, (1974) 145) and extracted a parameter that characterizes the linear elastic behaviour at small deformations (the moduli E or G) and one that characterizes the non-linear elastic behaviour at large deformations (the elasticity parameter n). The phenomenological BST equation describes rheological experiments adequately both in shear deformation and in compression. We found that the modulus correlates with the breaking stress. For the non-linear elastic properties of gelatin, we found that the elasticity parameter n correlates with the breaking strain Qualitatively, the non-linear elastic properties can be explained by assuming that the gelatin chains are partially in a crystalline triple helix state (the cross-links) and partially in a random coil state (the network bonds): the more extensive the rigid cross-link regions, the shorter and more stretched the network bonds become as a result of an externally applied deformation. The network bonds behave as anharmonic springs under extreme extension. Manipulation of the breaking strain was attempted in two ways: (i) by changing the (non-linear) elasticity parameter of the gel: this is possible by using a gel that has been further aged; (ii) by adding defects to the gel structure: this is possible by either pre-shearing the gel or by adding whey protein particles. The pre-shearing gives rise to a temporary effect, the addition of whey protein particles to a permanent effect.