It was determined according to Porter, Hrstich, and Chan (1985) using a reagent comprising 154 mg of FeSO4•7 H2O per liter of n-butanol:hydrochloric acid (3:2) was prepared. A total of 250 μL of each sample extract and 2.5 mL of the described reagent were incubated at 90 °C for 15 min. The blank consisted of 2.5 mL of the reagent and 250 μL of methanol:acetic acid (99.5:0.5). The absorbance was measured at 540 nm using a model U1100 UV/Visible spectrophotometer (Hitachi, Japan). The results were expressed as mg of quebracho tannin/g DW.