The total phenolic content was evaluated using a modified colorimetric method described previously by Singleton and Rossi [29].
The method involves the reduction of Folin–Ciocalteau reagent by
phenolic compounds with a concomitant formation of a blue complex. In this study, 0.50 ml of natural juice were separately mixed
with 3.0 ml of distilled water and 0.25 ml of Folin–Ciocalteau
reagent. Immediately, 0.75 ml of saturated sodium carbonate and
0.95 ml of distilled water were added. The mixture was then incubated for 30 min at 37 C, and the absorbance was read at 735 nm
using an UV–Vis spectrophotometer (UV-1800 Shimadzu). The
measurement was compared to a calibration curve prepared using
the standard gallic acid solution. The total phenolic content was
expressed as milligrams of gallic acid equivalents (GAE) per
100 ml of juice.