Drying is the main processing procedure that is different for DBN and FBN; it changes the water status and distribution of noodles. It is well known that moisture is one of the reasons for noodles’e different eating qualities. However, the moisture content of cooked DBN and FBN were not significantly different (
Table 4), although the OCT was different for DBN and FBN in this study (
Table 1). On the other side, longer cooking time may result in a higher CL and CBR, as more degraded starch molecules due to extrusion may dissolve in the cooking water. Besides, processing can also induce varied water mobility and food structure, which can also affect the eating qualities. The water channels due to water migration and denser network due to moisture loss may influence the structure of the noodles [
19]. For fresh buckwheat noodles with different contents of extruded buckwheat flour, the viscous extruded buckwheat flour together with the wheat gluten may create varied network structure, contributing to buckwheat noodles with a higher proportion of buckwheat flour. To verify the above inference, the correlation between the cooking properties and eating properties of buckwheat noodles with the features of water of different statuses was performed (
Table 5).
4.1. Roles of Water Status/Distribution and Internal Structure on the Cooking Properties of FBN and DBN
CBR was correlated well with the
T22 of uncooked noodles and the moisture content after cooking (
p < 0.05) (
Table 5).
T22 showed a negative correlation with the CBR of FBN, but a positive correlation with the CBR of DBN. The higher
T22 of FBN was correlated with a lower CBR. A possible reason for this is that the SBW (the water that existed inside the gluten network [
20]) tended to be more mobile, and highly mobile water can lead to a more uniform and developed protein network, leading to a lower breakage ratio. This uniform inner structure was confirmed by the SEM images (
Figure 3a–c). In comparison, SBW was not the main form of water within DBN, and the longer
T22 implied that the SBW could not be bound tightly in the structure of dried noodles (
Table 4). DBN with a higher ratio of extruded buckwheat flour showed a longer
T22 and a less uniform but denser internal structure, leading to a larger CBR. Thus, FBN-20% with a relatively more homogenous structure (
Figure 3m) showed a higher resistance to breakage during cooking. The moisture content was negatively correlated with the CBR of FBN (
Table 5), which was mainly due to the loose structure of FBN-80% (with the lowest moisture content).
The CL of FBN showed a positive relationship with the
A23 (
Figure 1 and
Figure 2B). However, this is more likely to be due to the dissolution of the degraded amylopectin molecules [
21] from extruded buckwheat flour during noodle cooking. For FBN with a higher ratio of extruded buckwheat flour, the small molecules from the extruded buckwheat flour could be released more easily into the cooking soup [
22], causing the greater CL of FBN. Moreover, a weak or discontinuous protein matrix results in a protein network that is too loose and permits a large amount of extrudate to dissolve in the water during cooking [
23]. DBN-80% and FBN-80% showed denser but less uniform internal structures (more holes), which explained the higher cooking loss (
Figure 1 and
Figure 3). Chillo, Laverse, Falcone, Protopapa, and Del Nobile [
13] also found that an internal structure with more or fewer holes inhibited or improved the absorption of water, which contributed to the breakage susceptibility and cooking resistance of the noodles. However, further evidence is required to justify if it is a coincidence that the CL was correlated to the
A23 or if the CL could be attributed to other parameters relating to the water status.
4.2. Roles of Water Status/Distribution and Internal Structure on the Textural Properties of FBN and DBN
Water mobility (
Table 5) and inner structure shall be combined to explain the changes in textural properties [
19]. In this study,
T21 showed a positive correlation with the hardness of FBN, whereas
T23 showed a negative correlation with the hardness of DBN. Both the FBN with a lower ratio of extruded buckwheat flour (longer
T21) and the DBN with a lower percentage of extruded buckwheat flour (shorter
T23) exhibited a more uniform but less dense protein network (
Figure 3g,j,m,p), contributing to the higher hardness. The more compact internal structure led to a higher hardness, which was consistent with the study [
4]. Additionally, a more continuous and denser internal structure was found for DBN-20%, with more tightly bound water and free water.
The elasticity of DBN was negatively correlated with
A21 but positively correlated with
A23. The reason for the changes in the elasticity of DBN and FBN is different. Firstly, the elasticity of FBN was not significantly different, and there was no correlation found between the elasticity and the parameters reflecting water status changes for FBN. Secondly, as suggested by the improvement in rubber elasticity using linear or branched polymers [
24], a possible presumption is that a more degraded branched/bulk structure was maintained in DBN-20% and DBN-80%, and thus a higher proportion of free water (higher
A23) was retained in the bulk structure. Meanwhile, the polymers formed from a higher content of extruded buckwheat flour may entangle with each other and/or with the proteins within the buckwheat noodles during drying, which caused increased elasticity. Without the drying treatment, FBN delivered a relatively stable elasticity (about 1.0) compared to DBN (about 0.7–1.0).
The cohesiveness of the noodles is of particular interest to us, and it can reflect the integrity of the soft food’s structure. For FBN, the cohesiveness was negatively correlated with the
T21, A21, and
A22, but positively correlated with the
T23 and
A23. Interestingly, a shorter
T21, smaller
A21 and
A22, longer
T23, and larger
A23 were observed in FBN with a higher ratio of extruded buckwheat flour. The higher addition of extruded buckwheat flour could stick different components, including starch granules and protein, together, as indicated previously [
3], leading to an improved internal structure. Besides this, the moisture content was negatively correlated with the resilience of cooked DBN (
Table 5), which explained the lower resilience of DBN-80% (with a higher moisture content) after cooking.
From this study, the water distribution/status cannot solely explain the changes in the cooking or eating properties without understanding the changes in the inner structure of noodles. The relationship between the water and the texture extensively relied on the specific food system, as the form of water taking the dominant proportion might be different. Since the water distribution/status correlated well with the texture of seafood [
25], if a similar relationship can be found between the water distribution/status and the internal structure, then the texture of other starch-based foods shall be further confirmed. Besides this, a joint modification of water mobility and inner structure shall be considered for the improvement of food qualities. The water absorption ability of extruded buckwheat flour and gluten still merits further investigation to help explain the water mobility within the buckwheat noodle system.