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.