91A-9 |
Inhibition mechanism of oxalic acid on apple PPO |
R. YORUK1, M. R. Marshall, and M. O. Balaban. (1) Food Science and Human Nutrition Department, University of Florida, PO BOX 110720, SW 23rd Drive, Gainesville, FL 32611-0720 Browning on food products arise largely because of the oxidation by PPO on phenolic compounds to quinones and their polymerization. Oxalic acid is a common component of spinach, rhubarb, etc., and is a naturally occurring inhibitor of PPO causing irreversible and competitive inhibition of mushroom PPO. The objective of this study was to demonstrate the potential mode of inhibition for oxalic acid on apple PPO. Apple PPO was extracted from an apple acetone powder in a pH 7.2 phosphate buffer. Oxalic acid solutions were prepared using oxalate-two-hydrate. Enzyme activity toward catechol was assayed spectrophotometrically with and without added inhibitor at 25°C, and after incubation at 25 and 0°C up to 30 min. Scanning studies were performed at 220 through 700 nm wavelengths to seek the possible effect of oxalic acid on quinone product formation and its reducing ability, if any. Oxalic acid incubated with different concentrations of cupric acetate was assayed for its inhibitory activity on PPO. Oxalic acid’s inhibition on apple PPO depended on both its concentration and incubation time. Inhibition increased from 50% to 90% upon incubation of the enzyme with 8 mM oxalic acid for 15 min at 25ºC. In addition, it effectively inhibited the enzyme at refrigerated temperatures. Scanning studies showed that the ability of oxalic acid to diminish the quinone product formation was concentration dependent. However, it did not reduce quinone to diphenol. It was also demonstrated that after incubation with cupric acetate, oxalic acid could not inhibit PPO. Oxalic acid was shown to inhibit apple PPO by 90 % at low concentration (8mM), similar to previous research on mushroom PPO (10 mM) and apple PPO (0.05%). Oxalic acid was very likely a good chelator for copper since copper saturated oxalic acid lost its inhibitory ability on apple PPO.
Session 91A, Food Chemistry: Enzymes, vitamins and plant pigments
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