2.5.2. PPO Assay

4-Methylcatechol 80 mM prepared in Mcllvaine's buffer solution at pH 7.5 was used as substrate, and 12.5 mL of cold Mcllvaine's buffer solution at pH 7.5 was added to 6.25 mL of enzymatic extract. PPO activity for apple juice was determined by reading the absorbance at 420 nm and 25 ◦C and calculated on the basis of the slope of the linear portion of the curve (ΔA/min). An enzyme unit is defined as the enzyme activity that catalyses the conversion of 1 μmol of substrate into product in one minute.
