Rheological Properties
The rheological properties of walnut kernel flours by roasting and
steaming treatment with different time were accessed as shown in Fig. 2.
The storage and loss modulus were G′ and G′′, respectively, which could
reflect the elasticity and viscosity of samples [40]. Fig. 2 A and B
exhibit the G′ and G′′ changes of untreated and heating processing
samples. It could be observed that the G′ of all samples was greater
than the G′′, which reflected the dominant viscoelastic solid-like
behavior in all samples [41]. As shown in Fig.2 (A and B), the G′
and G′′ values of other samples except for the untreated and R15
increased slightly with increasing angular frequency, suggesting if the
samples formed a gel, it could be stable and strong [42]. However,
the G′ and G′′ of untreated and R15 samples gradually with increasing
frequency increased, which suggested that the gel network structure of
two samples was more fragile [43]. Compared with untreated walnut
kernel, heating treatment resulted in the increase of G′ and G′′ values,
which was explained that the protein molecular interaction was improved
due to the denaturation and exposure of hydrophobic groups during heat
processing [44]. It was worthy of remark that G′ and G″ values of
steaming treatment were higher than that of roasting processing, which
was attributed that the steam contained water during heating. de Vries
et al. reported that the addition of a small amount of water in protein
could promoted the hydrated protein aggregates during heating [45],
which would cause the increase of G′ and G′′ values. These results
indicated that the rheological properties of walnut kernel were
significantly altered by different heat treatments [43].
Fig. 2 C describes the flow behavior of the walnut kernel flours with
different heating methods and time. As can be seen in Fig. 2 C, with the
increase of shear rate, all sample viscosity gradually reduced, which
showed the shear thinning behavior (pseudoplastic) [21]. It was
explained that the partial destruction of the protein network structure
in walnut kernel was caused by the increase of shear rate, which would
decrease the fluidity of the protein and the viscosity [21].
Compared to the untreated walnut kernel, the initial viscosity of
R15-R30 and S15-S30 was increased, the order of viscosity values from
high to low was steaming > roasting >
untreated sample. In addition, with the extension of steaming and
roasting time, the viscosity of the samples increased, this might be
related with the changes in the protein structure after heating. It was
supposed that the heating treatment for 15-30 min could cause the
crosslinking of internal molecules [43], namely, the formation of
three-dimensional network structure occurred due to the changes of
protein conformation in samples. on the other hand, the heat treatment
could increase the number of active molecules per unit volume, which
enhanced the effective collisions and interactions between molecules
[43]. It was concluded that the appropriate heat treatment could
lead to the unfolding of protein molecules and enhance the interaction
of molecules [43]. These findings further proved that the heat
processing could be helpful to result in the more interaction behavior
and improve the elastic properties of samples [43].