2.3. Polyphenol oxidase (PPO) assay and protein determination
Peel (3.0 g) from six fruits was homogenized in 10 ml of 0.02 M phosphate buffer (pH 6.8) at 4 ᵒC. The homogenate was centrifuged at 19 000 g (Beckman J20- 2) for 20 min and then supernatant was collected to assay PPO activity according to the method of Jiang (1999a), by measuring the oxidation of 4-methylca- techol. The increase in absorbance at 410 nm at 25ᵒC. was automatically recorded for 3 min, using a spectro- photometer (Beckman, DU-7), One unit of enzyme activity was defined as the amount which caused a change of 0.001 in absorbance per minute. The protein content was determined according to the dye-binding method of Bradford (1976) with albumin bovine serum as the standard.