*2.2. Sample Preparation*

Deionized water was stirred in beakers at 300 rpm using a thermostatic water bath maintained at 59 ± 1 ◦C and 60 ± 1 ◦C, separately. Native starch powder was added after the deionized water reached the fixed temperature. Each batch of dispersion at a concentration of 10% (*w/w*) was thoroughly stirred for 1, 3, 6, 9, 12, 15, or 18 min, separately. Heating conditions were determined based on pre-experiments and the onset gelatinization temperature of native potato starch. The selected heating temperature provides starch samples with different DSGs in a convenient time period. Starch samples with a higher DSG are difficult to obtain when treated with a lower temperature in a relatively short time period, while starch is gelatinized too quickly when treated with a higher temperature. We therefore chose 59 ◦C and 60 ◦C as the temperatures and obtained a series of starch samples

with different DSGs. After hydrothermal treatment, the beakers were quickly moved into ice water and cooled to room temperature before being put in the freezer. These partially gelatinized starch dispersions were freeze-dried for around 48 h using a vacuum freeze dryer (GenesisTM SQ, Virtis, Warminster, PA, USA) [15,17]. Starch samples were ground into powders with a mortar after drying and the resulting powder samples were sealed in plastic bags and stored in a silica-gel desiccator at 25 ◦C.

#### *2.3. Differential Scanning Calorimetry (DSC)*

The thermal properties of starch samples were investigated using a differential scanning calorimeter (DSC8000, PerkinElmer, Waltham, MA, USA). Powder samples (3 mg) were weighed into aluminum pans, and 10 μL of distilled water was added before the pans were hermetically sealed. The pans were equilibrated at room temperature for 2 h before heating from 20 ◦C to 100 ◦C at a rate of 5 ◦C/min [18,19]. A sealed empty aluminum pan was used as a reference. Onset temperature (T0), peak temperature (Tp), conclusion temperature (Tc), and endothermic enthalpy (ΔH) were calculated from the DSC curves using the equipment software. Gelatinization range (R) was computed as (Tc – T0), and the peak height index (PHI) was calculated by the ratio ΔH/(Tp – T0), as previously described [20]. The degree of starch gelatinization (DSG) was determined using Equation (1) given below [21,22].

$$\text{DSG (\%)}=(1 - \Delta \text{H}\_{\text{sample}}/\Delta \text{H}\_{\text{native}}) \times 100\% \tag{1}$$

where ΔHnative and ΔHsample are the enthalpy change due to gelatinization of native and modified starches, respectively.

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

The WBC of starch samples was measured as previously described [23] with slight modifications. A suspension of 5 g of starch (dry wt. basis) in 75 mL of distilled water was agitated for 1 h at 150 rpm at 20 ◦C, 40 ◦C, 60 ◦C, and 80 ◦C, separately, and then centrifuged (3000× *g*) for 10 min. Free water was removed from the wet starch which was then continuously drained for 10 min at room temperature (20 ◦C). The wet starch was calculated using the difference between wet starch with tubes and empty tubes. The WBC of starch granules was calculated using the following formula:

$$\text{WBC } \text{(\%)} = (\text{W}\_2 - \text{W}\_1) / \text{W}\_1 \times 100\text{\%} \tag{2}$$

where W1 and W2 are the dry weight of original starch and wet weight of starch, respectively.

#### *2.5. Fourier Transform Infrared (FTIR) Spectroscopy*

The short-range molecular order of starch samples was determined using a FTIR spectrometer (Tensor 27, Bruker Opticals Company, Rheinstetten, Germany). FTIR spectra were recorded from 4000 to 600 cm−<sup>1</sup> at a resolution of 4 cm−<sup>1</sup> with an accumulation of 64 scans using the attenuated total reflectance (ATR) accessory. All spectra were baseline corrected automatically by OMNIC 8.2 and deconvoluted from 1200 to 800 cm−<sup>1</sup> with a half-bandwidth of 19 cm−<sup>1</sup> and an enhancement factor of 1.9. Intensity measurements were performed on the deconvoluted spectra by recording the peak height of absorbance bands from the baseline. The ratio of absorbance at 1047/1022 cm−<sup>1</sup> and 1022/995 cm−<sup>1</sup> was used to estimate the short-range ordered structure of starch [6,24].

#### *2.6. Scanning Electron Microscopy (SEM)*

The microstructures of the starch samples were observed using field-emission environmental scanning electron microscopy (SU8010, Hitachi, Tokyo, Japan) with a 10-KV acceleration voltage. Powder samples were added to double-sided adhesive tape mounted on an aluminum stub and sprayed with gold. Samples were photographed at 500× and 1000× magnification.
