*2.10. Statistical Analysis*

Data are reported as averages of triplicate observations and expressed as the mean ± standard deviation. ANOVA followed by Tukey's test at the 0.05 significance level was used to evaluate the differences among sample means. ANOVA was conducted using SPSS 18.0 (SPSS Inc., Chicago, IL, USA). All graphs were generated using Excel and Origin 9.

#### **3. Results and Discussion**

#### *3.1. Thermal Properties*

The DSC thermograms in Figure 1A,B clearly show the phase transition peaks of starch samples, and as heating time increased, there was an obvious decrease in the areas under the thermal transition peak above the extrapolation, and the curves tend to be flat. The DSC parameters including T0, ΔH, R, and PHI are shown in Table 1. The TP and Tc of the starch samples showed no significant differences (data not shown). The T0 and ΔH of the native potato starch samples were 56.94 ◦C and 12.73 J/g, respectively, which were values close to previous reports [29,30]. The T0 of the partially gelatinized starch samples was higher than that of native potato starch, and it increased as the heating time and temperature increased. The gelatinization temperature of the partially gelatinized starch modified by spray drying or high hydrostatic pressure was also reported to be higher than that of the native starch [13,31]. The increasing DSC thermal transition temperature of the heat-gelatinized starch could be due to the colloidal molecular structure of the starch granules, the amylopectin chain length, and the reordering of the crystalline structure after hydrolysis [31]. Specifically, a previous study reported that the increasing gelatinization temperature of pre-heating starch was interpreted as being due to the disruption of less stable crystallites in the first instance, followed by the melting of the remaining and more stable crystallites at a higher temperature [15,32]. However, partially gelatinized starch modified by ball milling showed the opposite variation tendency [12]. The difference might be due to the different residual crystalline structure of partially gelatinized starch after various modifications. On the contrary, ΔH, R, and PHI decreased as the heating time and temperature increased. Table 1 also shows that the calculated DSG of partially gelatinized potato starch varied from 39.41% to 90.56%. As the heating time and temperature increased, the DSG of the starch samples increased. Although the DSG of the starch sample heated at 59 ◦C for 9 min was slightly higher than the sample heated for 12 min, no significant difference was observed between them. The thermal parameters are known to depend on the stability of the amorphous and crystalline regions of starch [33]. Specifically, thermal parameters depend on the thickness of crystals, their polymorphic structure, and the free energy of the surface of the face side [34]. Samples with higher heating temperatures and longer heating times decreased the crystalline regions and the crystal strength within a starch granule, thus requiring less energy for full gelatinization. This may explain the decrease of R, PHI, ΔH, and the increase of DSG of potato starch as the heating time and temperature increased.

**Figure 1.** DSC thermograms (**A**,**B**) and FTIR spectra (**C**,**D**) of native and partially gelatinized potato starch samples (**A**,**C**: pre-heated at 59 ◦C; **B**,**D**: pre-heated at 60 ◦C).

#### *3.2. Water-Binding Capacity (WBC)*

Table 2 shows the WBC of starch samples under 20 ◦C, 40 ◦C, 60 ◦C, and 80 ◦C. Results indicated that the WBC of all partially gelatinized starch samples was higher than that of the native potato starch. With increased pre-heating time, the WBC of starch samples significantly increased. Moreover, a significant increase in the WBC value can be observed when the reheating temperature increased from 20 ◦C to 60 ◦C. However, the WBC did not significantly increase when the reheating temperature further increased from 60 ◦C to 80 ◦C. Similarly, a previous study on pre-gelatinized rice starch showed that the water absorption index of modified samples was significantly higher than that of native starch due to the disruption of crystalline structure and the gelatinization of starch [17]. The differences in the WBC of potato starch samples may be attributed to variation in their granular structures [27]. The loose association of amylose and amylopectin molecules in starch granules is responsible for high WBC [35]. The formation of hydrogen and covalent bonds between starch chains engaged by hydroxyl groups could reduce the WBC, and differences in the availability of water-binding sites may also lead to changes in the WBC [36]. In this case, the crystalline structure of samples may be (partially) disrupted due to the breakage of intra- and intermolecular hydrogen bonds when the starch is preheated in excess water. The hydroxyl groups of amylose and amylopectin were exposed to a certain extent, potentially forming hydrogen bonds with water molecules and, thus, causing the increased WBC of starch samples with higher DSGs.


**Table 1.** DSC parameters of native and partially gelatinized starch samples.

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



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, f represent the significant difference of starch samples in column by heating at 60 ◦C (*p* < 0.05); <sup>a</sup>- , b- , c- , d- represent the significant difference of starch samples heated at a different temperature (*p* < 0.05).
