Potato or apple PPO activity was determined by measuring the increase in absorbance at 420 nm with a spectrophotometer (Cecil instrument model CE 292, Cambridge, UK) at 25 C. 0.1 mL of the enzyme was added to the mixture containing 0.9 mL of 0.05 M sodium phosphate buffer at pH 7.0, 1.0 mL of 0.2 M catechol substrate solution in 0.05 M sodium phosphate buffer at pH 7.0, and 1.0 mL of RBE or CDRBE as an inhibitor. The change in absorbance at 420 nm was recorded for 1 min. As a control, 1.0 mL of distilled water was mixed with the same substrate solution before adding the enzyme. One unit of enzyme activity was defined as the amount of enzyme responsible for a change of 1 absorbance unit at 420 nm/min at 25 C and pH 7.0. The percent inhibition was calculated as follows