EXTRACTION AND CHARACTERIZATION OF POLYPHENOL OXIDASE IN PAWPAW (ASIMINA TRILOBA) FRUIT

Abstract

ABSTRACT Crude polyphenol oxidase (PPO) was extracted from pawpaw (Asimina triloba) fruit pulp, partially purified by gel filtration and electrophoresed for molecular weight (MW) determination. The enzyme activity was assayed by monitoring the absorbance (420 nm) of the PPO extract-catechol mixtures over time. The PPO activity varied with pH and was the greatest at pH 6.5–7.0. The crude enzyme exhibited a maximum activity at 5–20C, but a brief exposure to 40–80C resulted in a rapid decline or complete loss of the enzyme activity. The enzyme kinetics based on the reaction rate in the linear region (0–60 s) showed a Vmax of 0.1363/s and a Km of 0.3266 M for the crude PPO. The PPO existed as two isoforms with MWs of 28.2 and 38.3 kDa, respectively. PRACTICAL APPLICATIONS Pawpaw fruit is a low-acid, sweet and aromatic fruit grown in North America. With its intense flavor resembling a combination of banana, mango and pineapple, pawpaw fruit is being used in value-added products such as ice creams, yogurts and baked foods. However, pawpaw fruit has a short postharvest shelf life because it is susceptible to brown or even black discoloration when stored at ambient temperatures. This study has identified that polyphenol oxidase, an enzyme that oxidizes phenols (an endogenous compound present in pawpaw fruit pulp), is the cause for the discoloration. We also found that this enzyme is most active at neutral pH and at room temperature. These findings are important because they could help pawpaw growers and food processors to develop proper storage and processing methods to avoid the undesirable color changes.