3.4 Microrheological characterization
Microrheological techniques could monitor the Brownian Motion of tracer particles and track interparticle interactions (Mengjie et al., 2021). The Brownian motion of the particles is described by the MSD versus time curve, which reflects the viscoelastic characteristics of the sample. The interaction forces between the model particles with viscoelasticity limit their mean square displacement. Since the particles are not free to move, they are trapped in a three-dimensional microstructural network. These interactions make the samples viscoelastic, which is manifested in the root mean square displacement versus de-correlation time curves as non-linear MSD for all samples. Fig. 5 shows the results for the root mean square displacement of walnut butter particles. The MSD curves of commercial walnut butter and all three groups of FLWB were nonlinear. It indicates that both commercial walnut butter and FLWB are typical viscoelastic products (Fernandes and Salas Mellado, 2018; Tisserand et al., 2012). MCT-WB and DG-WB had a long decorrelation time, while commercial and CLA-WB had a shorter decorrelation time. The samples with longer decorrelation times had higher viscoelasticity when moved the same distance (Yun et al., 2018).
Fig. 6 showed the results of elasticity index (EI), solid-liquid balance (SLB) and fluidity index (FI) of walnut butter. the EI value of FLWB tended to decrease with increasing amount of grease addition and the fluidity of walnut butter was enhanced, which was consistent with the results of frequency scan in rheological experiments. the FI value of FLWB increased with increasing amount of oil addition. This indicates that the particle migration rate and fluidity of FLWB were enhanced with the addition of oil. The FI of DG-WB was higher than that of commercial walnut butter, and this phenomenon may be due to the relatively low viscosity of DG, resulting in the higher fluidity of DG-WB. With the increase of oil addition, the FI of MCT-WB or CLA-WB gradually approached the FI of commercial walnut butter (Fig. 6C). Therefore, it was feasible to prepare samples of MCT-WB or CLA-WB by adjusting the amount of oil addition to make them closer to commercial walnut butter. Fig. 6B showed that SLB < 0.5 for all walnut butter except for the MCT-7 sample (which was mainly liquid behavior), indicating that the samples were mainly gel behavior. In summary, the microscopic rheological properties of CLA-WB are closer to those of commercial walnut butter.