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].