*3.3. FTIR Spectra of Native and Partially Gelatinized Starch*

The deconvoluted FTIR spectra of the native and partially gelatinized potato starch samples are shown in Figure 1C,D. FTIR spectroscopy was suggested to be sensitive to the so-called short-range order, defined as the double helical order, rather than the longrange order which is related to the packing of double helices [37]. Bands in the spectral region of 800–1200 cm−<sup>1</sup> reflected changes in polymer conformation and the hydration of processed starches [38]. Bands at 1047 and 1022 cm−<sup>1</sup> were associated with the ordered and amorphous structures of starch, respectively. The band at 995 cm−<sup>1</sup> was mainly caused by the bending vibration of C-OH, corresponding to the hydrogen bond structure formed between the hydroxyl groups of starch macromolecules. The FTIR spectra of potato starch samples clearly show that, as pre-heating time increased, the intensities of the peak around 1022 cm−<sup>1</sup> gradually increased, the peak around 1047 cm−<sup>1</sup> became flatter, and the peak around 995 cm−<sup>1</sup> significantly decreased. The ratio 1022/995 cm−<sup>1</sup> significantly

increased, while the ratio 1047/1022 cm−<sup>1</sup> slightly decreased as heating temperature and time increased (Supplementary Data Figure S1). These data clearly show that higher DSGs of starch lead to the loss of the short-range ordered structure. Sevenou et al. [37] found that the ratio 1047/1022 cm−<sup>1</sup> of native and fully gelatinized potato starch decreased from 1.37 cm−<sup>1</sup> to 0.30 cm<sup>−</sup>1, and the ratio 1022/995 cm−<sup>1</sup> increased from 0.40 cm−<sup>1</sup> to 2.50 cm<sup>−</sup>1. Moreover, the variation of maximum absorbance at 995 cm−<sup>1</sup> changed more significantly as a function of heating time than at 1047 cm<sup>−</sup>1, indicating that the hydrothermal treatment has a more significant effect on the amount of exposed hydroxyl groups than the ordered and amorphous structures of starch. The significant decrease of the peak around 995 cm−<sup>1</sup> also explains the increase of the WBC of samples as the pre-heating time increased, since the band at 995 cm−<sup>1</sup> mainly represents the hydrogen bond structure formed between the hydroxyl groups of starch macromolecules.

#### *3.4. Morphological Properties of Native and Partially Gelatinized Starch*

The microstructure of the native and partially gelatinized potato starch granules is presented in Figure 2. The image clearly shows that the native potato starch granules are characterized by an oval or elliptical shape with a smooth surface (Figure 2A). Starch samples heated at 59 ◦C and 60 ◦C for 1 min showed most starch granules retaining their original shape. However, with increased heating time (Figure 2B–G), the starch granules started to break and collapsed into flakes or blocks, and gradually lost their typical oval shape. The oval or elliptical shape of starch granules is largely not observed after extensive heating. The structure changes of the granules under hydrothermal treatment are mainly caused by starch gelatinization. In this process, water penetrates the starch granules and initiates irreversible swelling and microcrystal melt [9]. The polarized light micrographs showed that native potato starch granules have strong birefringence patterns (Figure 2a), reflecting a high degree of ordered molecular orientation and average radial orientation of helical structures in native potato starch [39]. The partial gelatinization treatment resulted in the disappearance of birefringence in potato starch granules to varying degrees. Compared with native potato starch sample, the partially gelatinized starch samples showed larger voids at the granule center. The loss of radial orientation in partially gelatinized potato starch granules is mainly caused by increased mobility of starch chains at the granule center following hydrothermal treatment. More thermal energy was imparted to starch chains during heating and resulted in the destruction of more molecules double helix structure; thus, less birefringence was observed.
