*3.5. Rheological Properties of Native and Partially Gelatinized Starch* 3.5.1. Apparent Viscosity

The flow curves of starch solutions/pastes are presented in Figure 3A,B. The apparent viscosity curves decreased as the shear rate increased, and the steady shear curves were almost parallel to one another, except for the native starch solution and starch sample pre-heated at 59 ◦C for 1 min. This result indicated that these samples exhibited shearthinning behavior, albeit at different levels. The shear-thinning behavior of starch samples is attributed to the destroying of the molecular network in starch pastes due to applied shear strain [9]. Similarly, shear-thinning behavior was also observed in pre-heated Peruvian carrot starch and cross-linked waxy maize starch dispersions [25,40]. Table 3 shows the power-law fitting parameters of the steady flows. Data for the native starch solution and starch sample pre-heated at 59 ◦C for 1 min (DSG = 39.41%) are not shown because these two samples failed to form uniform pastes, were unstable, and sedimentation occurred during the rheology measurements; thus, they do not fit the power law model. Partially gelatinized potato starch with a DSG of 56.11% can form a stable paste with a fine shearthinning property, as well as starch samples with a DSG larger than 56.11%. Equation (3) can nicely fit the rest of the potato starch flow curves with R2 between 0.997 and 0.999. The consistency index (K) of the starch pastes increased significantly with increased heating time and temperature. This indicated that starch pastes with higher DSGs showed higher

apparent viscosity. All flow behavior indexes (n) were less than 1.0, further indicating the pseudoplastic and shear-thinning behavior of the samples [41]. The n value increased with the heating time and temperature, indicating that starch samples with higher DSGs showed reduced pseudoplasticity. Pseudoplasticity can be attributed to the progressive orientation and alignment of the starch molecules, and the breaking of hydrogen bonds formed among amylose molecules under the influence of the shear field [42]. A high degree of macromolecular disorganization enhanced the solubility of gelatinized potato starches, leading to the formation of viscous pseudoplastics [43].

**Figure 2.** Scanning electron micrographs (upper case: **A**–**G**) and polarized light micrographs (lower case: **a**–**g**) of the native (**A**,**a**) and partially gelatinized potato starch granules (**B**–**G**,**b**–**g**) heated at 59 ◦C (**B**–**D**,**b**–**d**) and 60 ◦C (**E**–**G**,**e**–**g**) for different times (**B**,**b**,**E**, textbfe: 1 min; **C**,**c**,**F**,**f**: 6 min; **D**,**d**,**G**,**g**: 18 min).

**Figure 3.** The apparent viscosity flow (**A**,**B**), temperature dependence of G- (**C**,**D**), and frequency dependence of G- (solid symbols) and G-- (open symbols) (**E**,**F**) of native and partially gelatinized potato starch samples (**A**,**C**,**E**: pre-heated at 59 ◦C; **B**,**D**,**F**: pre-heated at 60 ◦C).

**Table 3.** Effect of shear rate and frequency on the consistency index (K, K- , and K--), flow behavior index (n, n- , and n--), and determination coefficient (R2, R-2, and R--2) of starch pastes and gels pre-heated at 59 ◦C and 60 ◦C.


Data are means ± SD. A, B, C, D, E represent the significant difference of starch samples in column by heating at 59 ◦C (*<sup>p</sup>* < 0.05); a, b, c, d, e represent the significant difference of starch samples in column by heating at 60 ◦C (*p* < 0.05).

#### 3.5.2. Temperature Sweep

Figure 3C,D shows the temperature dependence of G of starch samples. Apart from the native starch sample and partially gelatinized starch samples pre-heated for 1 min at 59 ◦C, the peak G of other starch samples decreased as the heating time increased. This indicated that the elasticity modulus of partially gelatinized starch decreased at the regelatinization process with its DSG. The hydrogen bonds between starch molecules of the partially gelatinized starch samples were broken to some extent in the pre-gelatinization process, thus decreasing its modulus in the reheating process. For the native potato starch samples, starch granules first gradually swelled and had a relatively high interaction between starch molecules, requiring more energy to break the bond. This may primarily explain why the peak G decreased with the increased DSG. Tanδ against temperature is used to identify the gelatinization onset in oscillatory tests by the temperature at the maximum point of Tanδ [44]. As the heating time increased, the gelatinization onset temperature of the samples pre-heated at 59 ◦C increased from 58.4 ◦C to 65.9 ◦C, and that of the samples pre-heated at 60 ◦C increased from 60.13 ◦C to 67.90 ◦C (Supplementary Data Figure S2 and Table S1). This phenomenon indicated that higher degrees of starch gelatinization delayed the re-gelatinization process. This might be due to the disruption of less stable crystallites in the first instance, followed by the melting of the remaining more stable crystallites at the higher temperature [15,32]. The retrogradation may also influence the gelatinization temperature of partially gelatinized starch by forming different crystalline structures. The results are consistent with the onset temperature (T0) obtained from the DSC measurements.
