Phenolic compounds were extracted from lyophilized samples
(0.5 g) with 10 mL methanol:H2O (80:20, v/v). The solid was
re-suspended by shaking in a vortex for 30 s, and left for 2 h in
the dark at room temperature with periodic mixing. The mixture
was centrifuged at 4500 g for 15 min at 4 8C, and the
supernatant containing the extracted phenolic compounds was
collected. The extraction operation was repeated once again with
the pellet. The methanolic extracts were pooled and the volume
was reduced in a rotary evaporator to about 1 mL. The residue
was taken to 5 mL with deionizer water and stored under
nitrogen at 30 8C. Extractions were carried out in triplicate and
the total phenolics (TP) were determined also in triplicate for
each extract by an adapted micro-scale protocol for the Folin–
Ciocalteu colorimetric method (Waterhouse, 2001). Gallic acid, a
phenolic acid which has been reported to be a major phenolic
component in potato (Rodriguez de Sotillo et al. 2004), was used
as standard. Briefly, an aliquot of 20 mL of the extract solution
was taken into a cuvette, then 1.58 mL of distilled water and
100 mL of Folin–Ciocalteu reagent were added, and the cuvette
was shaken thoroughly for 1 min. Finally 300 mL of sodium
carbonate solution (20%, w/v) was added, and the mixture was
allowed to stand for 2 h in the dark with intermittent shaking.
Absorbance was measured at 765 nm using deionized water as
blank. TP were calculated by interpolating the absorbance data in
a calibration curve prepared with gallic acid (concentration
range: 50.0–450 mg/L). Data were given as mg of gallic acid
equivalents per 100 g dry sample (mg GAE 100 g1 dw).