2.5. Determination of DPPH• Radical and ABT•+ Radical Cation Scavenging Activities

The DPPH assay was accomplished with the procedures described by Yamaguchi et al. with minor modification [21,23]. Briefly, 200 μL of the diluted free or bound extracts were added to 3 mL DPPH• radical solution (100 μM), which was prepared in methanol. The reaction was kept in the dark at room temperature for 30 min, and a spectrophotometer was used to measure the absorbance at 517 nm. The DPPH scavenging activity was expressed with inhibition (percent) of DPPH• absorbance [21].

The assay of ABTS•⁺ radical cation scavenging activity was conducted according to the procedure of Re et al. with slight modification [21,22,24]. First, 7 mM ABTS and 2.45 mM potassium per sulfate was mixed at room temperature in dark for 20 h to generate ABTS•⁺ radical cation. Then, methanol was used to dilute the ABTS•⁺ mixture to an absorbance around 0.700 at 734 nm. Last, 3.9 mL of ABTS+ solution was added to 0.1 mL of the extracts which were first diluted appropriately. The reaction mixture was kept at room temperature for 6 min, then the absorbance was recorded at 734 nm by a spectrophotometer.

Trolox (0.5 mM) was served as a reference antioxidant. The results of DPPH radical scavenging activities and ABTS•⁺ radical cation scavenging activities were expressed as µM of Trolox equivalents (TE) per 100 g of potato powder using a standard curve of Trolox.