Effect of Frozen Storage on Polyphenol Oxidase, Antioxidant Content, and Color of Pawpaw (Asimina Triloba [L.] Dunal) Fruit Pulp

Polyphenol oxidase (PPO) activity values for pawpaw pulp during frozen storage were measured for the main effect of month of storage at three levels (0, 4, 8 months) and treatment at four levels (vacuum, air, ascorbic acid or n-acetylcysteine). A significant effect of treatment was observed in PPO activity (p<0.001). Post hoc analysis revealed no significant difference between samples that were vacuum packaged and those for which no attempt to exclude air was made. The addition of the two chemical browning inhibitors significantly lowered PPO activity. Ascorbic acid exhibited a significant 69% reduction in PPO activity compared to vacuum and air samples and n-acetylcysteine was significantly more effective than ascorbic acid and almost completely inhibited PPO activity compared to the vacuum and air samples. CIELAB tristimulous color values (L*, a*, b*) were used to generate the applied color values total color difference (DE), browning index, hue and chroma in pawpaw pulp for the two main effects. Analysis of variance for the main effects showed significance for all seven color attributes at p<0.001. For the main effect of storage time, ANOVA showed significance during storage for all seven color attributes at p<0.001, indicating that there were color changes during storage. Pawpaw pulp samples at 8 months of storage were significantly darker (lower L*), more yellow (higher b*), more vivid (higher chroma), and had a higher browning index than the samples at 0 or 4 months of storage. For the main effect of treatment, ascorbic acid and n-acetylcysteine treatment produced pawpaw pulp that was significantly different than samples to which air was not excluded for all seven dependent color variables. Specifically, n-acetylcysteine and ascorbic acid produced pulp that was lighter (higher L*), less red (lower a*), and more yellow (higher b* and hue), more vivid (higher chroma), and exhibited more color difference (higher DE). A strategy to inhibit enzymatic browning during frozen storage would be useful for the nascent pawpaw industry.