The supernatant (0.3 mL) was analyzed with
1 mL of Salkowski's reagent (180 mL concentrated H2SO4 dissolved
in 150 mL H2O, with addition of 9 mL of 1.5 M FeCl3$6H2O), after
incubation for 60 min at room temperature in the dark. Optical
densities were read at 530 nm.