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.