3.3.3 Thixotropy of walnut butter
Thixotropy could reflect the phenomenon that the structure was disrupted and the viscosity of the system decreases when shear was applied to a system. After the removal of shear, the sample could gradually and reversibly return to the initial stress and viscosity, which was typical for non-Newtonian fluids (Hadjistamov, 2019). The thixotropic behavior was related to the “untangling-entanglement” process of protein molecules and the alignment of molecular protein chains in the shear direction (Benchabane and Bekkour, 2008).
When the shear rate applied to commercially available walnut butter SS1, SS2 and SS3 increased and then decreased, the viscosity curves of walnut butter did not overlap while formed a closed loop region surrounded by the upper and lower curves of viscosity, which could be called thixotropic loop (Fig. 4A). The thixotropic ring area was negatively correlated with the reversibility of viscosity, while the sample recovered gradually when the shear rate was reduced (Ashok et al., 2015). Firstly, the formation of the thixotropic ring could be due to the non-covalent interaction between proteins in walnut butter at the initial stage of shearing, which formed a three-dimensional network structure with more intermolecular entanglement points and higher shearing resistance. A linear increase in the applied shear rate during the shearing process resulted in a gradual deformation between the walnut butter particles and a disruption of the three-dimensional network structure. This increased the number of broken structures and decreasing the viscosity (Nikzade et al., 2012). During the shearing process, the linear increase of applied shear rate led to gradual deformation between walnut butter droplets. As a result, the protein three-dimensional network structure was disrupted and the viscosity of walnut butter was reduced (Reza et al., 2019). Secondly, the thixotropy size was related to the intermolecular forces. Some researches about the thixotropy of guar gum and papaya seed gum found that the stronger the intermolecular interact ions, the greater the structural strength and the longer the time required for structural rearrangement (Wang et al., 2019). DG-7, CLA-6 and CLA-7 formed distinct thixotropic loops (Fig. 4B - D). In addition to this, the initial viscosity values of CLA-6.5 and CLA-7 were closer to those of commercial walnut butter. In summary, the thixotropy of CLA-WB was more similar to that of commercial walnut butter.