Polyphenol Oxidase in Pawpaw (Asimina triloba [L.] Dunal) Fruit Pulp from Different Varieties

Pawpaw (Asimina triloba L. Dunal) is a tree fruit from the tropical Annonaceae family. Pawpaw currently has very limited commercial production because of its high perishability. Polyphenol oxidase (PPO) is responsible for enzymatic browning in pawpaw pulp. The objective of this research is to characterize PPO extracted from different pawpaw varieties. With respect to PPO activity, six of the varieties (Taytwo, Rebecca’s Gold, NC-1, Overleese, Rappahannock, and Green River Belle) exhibited PPO activity that was statistically higher than Quaker’s Delight and Lynn’s Favorite. The other four varieties (SAA Zimmerman, Shenendoah, KSU-Atwood, IXL) exhibited PPO activity that was not significantly different from each other or Quaker’s Delight and Lynn’s Favorite, but were significantly lower than Taytwo, Rebecca’s Gold, and NC-1. Kinetic parameters (Vmax, Km) and their ratio can be used to relate enzyme velocity with substrate affinity. Varieties that exhibited a high ratio, i.e. a very active enzyme due to a high Vmax and/or a low Km, are Rebecca’s Gold, Taytwo, NC-1, and KSU-Atwood). The results presented indicate that certain varieties exhibit conditions that suggest PPO could have a lower inherent impact on tissue browning, especially Lynn’s Favorite, Green River Belle, IXL, SAA Zimmerman, and Overleese. On the other hand, certain varieties (Taytwo, Rebecca’s Gold, NC-1, and perhaps KSU Atwood) exhibit PPO activity, Vmax, and Km values that suggest inherently high PPO activity and thus increased potential for browning. Overall, understanding PPO activity may help to explain post-harvest discoloration of pawpaw pulp and aid the commercial selection of more shelf-stable varieties.