*2.4. Total Phenolics, Individual Phenolic Acid Content, and Antioxidant Capacity*

Sample extracts were prepared as described by Wang et al. [15] with some modifications. Briefly, 3.0 g of sample was ground in 15 mL of 80% methanol in a mortar on ice. The homogenate was then sonicated for 20 min and centrifuged at 12,000× *g* for 15 min at 4 ◦C. The residue was reextracted twice with 2 × 15 mL of extracting solvent. The supernatants were then pooled, and vacuum evaporated at 40 ◦C to remove all methanol residues. The remaining extract was adjusted to pH 1.5 with 6 mol L <sup>−</sup><sup>1</sup> HCl and centrifuged at 12,000× *<sup>g</sup>* for 15 min at 4 ◦C. Afterward, the sample was extracted five additional times with an equal volume of ethyl acetate-ether (*v*/*v*; 1:1), then pooled. The supernatant was dried with anhydrous sodium sulfate, filtered with filter paper, and vacuum evaporated at 35 ◦C until dry. The residue was dissolved in 5 mL methanol, and the sample was filtered through a 0.45 μm membrane prior to analysis.

A total phenolic standard curve was constructed as described by Kalt et al. [16]. The total phenolic content (TPC) of the sweet potato tuberous roots extracts were determined spectrophotometrically using Folin–Ciocalteu's reagent, and the results were expressed as gallic acid equivalents per gram of dry mass (mg GAE kg−<sup>1</sup> DW).

The levels of individual phenolic acids in tuberous roots were determined via HPLC analyses. Phenolic acid standards were prepared in methanol (*m*/*v*; 0.01:10) and stored in brown reagent bottles before use. Afterward, 10 μL of the filtrate was injected into an HPLC system fitted with a refractive index detector (RID-1260, Agilent) and an Agilent ZORBAX Eclipse Plus C18 column (4.6 mm × 250 mm, Agilent). The mobile phase was composed of (A) 2% acetic acid (aqueous) and (B) acetonitrile, and gradient elution was performed as follows: 0 min, 97:3; 20 min, 95:5; 35 min, 85:15; 65 min, 70:30; and 65 min, 0:100. The mobile phase was vacuum filtered through a 0.45 μm membrane filter before use. All HPLC analyses were conducted at a 1 mL min−<sup>1</sup> flow rate at room temperature, and the results were expressed as mg kg-1 DW.

DPPH radical-scavenging activity was determined as described by Oliveira et al. [17] with some modifications. Scavenging activity was calculated using the following Equation (4):

Scavenging activity = [(A517 of control − A517 of sample)/A517 of control] × 100 (4)

DPPH scavenging activity values were expressed as mg Trolox equivalent antioxidant capacity·kg−<sup>1</sup> (mg TEAC kg−<sup>1</sup> DW).

Ferric reducing antioxidant power (FRAP) assays were conducted as described by Apati et al. [18] with some modifications. Briefly, 1 mL sample solution was mixed with 2.5 mL of phosphate buffer (0.2 mol L<sup>−</sup>1, pH 6.6) and 2.5 mL of K3Fe(CN)6 (*v*/*v*; 1:100).The mixture was incubated at 50 ◦C for 20 min and added 2.5 mL of trichloroacetic acid (*v*/*v*; 1:10), which was then centrifugated at 3000 rpm for 10 min. The supernatants were sucked 2.5 mL and mixed with 2.5 mL of bidistilled water and 0.5 mL of FeCl3 (*v*/*v*; 0.1:100). The absorbance was measured at 700 nm. The results were expressed as mg TEAC kg−<sup>1</sup> DW.
