CHARACTERIZATION OF POLYPHENOL OXIDASE AND ANTIOXIDANTS

Crude polyphenol oxidase (PPO) was extracted from pawpaw (Asimina triloba) fruit. The enzyme exhibited a maximum activity at pH 6.5–7.0 and 5–20 °C, and had a maximum catalysis rate (Vmax) of 0.1363 s-1 and a reaction constant (Km) of 0.3266 M. It was almost completely inactivated when incubated at 80 °C for 10 min. Two isoforms of PPO (MW 28.2 and 38.3 kDa) were identified by Sephadex gel filtration chromatography and polyacrylamide gel electrophoresis. Both the concentration and the total activity of the two isoforms differed (P < 0.05) between seven genotypes of pawpaw tested. Thermal stability (92 °C, 1–5 min) and colorimetry (L * a * b*) analyses showed significant variations between genotypes. Pawpaw fruit samples that were briefly heated (1–2 min) had improved color storage stability due to PPO inactivation. Moreover, antioxidants were extracted from the same seven genotypes of pawpaw fruit. All genotypes demonstrated significant (P < 0.05) radical-scavenging capability and inhibited lipid oxidation in a liposome system. Overall, these findings suggested that PPO-induced browning during pawpaw fruit processing may be minimized through controlling pH and temperature. All genotypes of the fruit als