CHARACTERIZATION OF POLYPHENOL OXIDASE FROM WATER CALTROP (TRAPA ACORNIS NAKANO) FRUITS

Abstract

Polyphenol oxidase (PPO) isolated from water caltrop fruits showed metabolic activity toward catechol, caffeic acid, gallic acid, chlorogenic acid, tyrosine and pyrogallol. Catechol was the most suitable substrate. The optimum pH for PPO was found to be broad (5.0–8.0), and the optimal temperature was between 20 and 30C depending on different substrates used in the experiments. The PPO was heat-stable between 20–30C for 50 min and almost completely inactivated at 60C and 70C after 5 min. Activation energy (Ea) values were found to be −38.24 kJ/mol for catechol, −35.14 kJ/mol for caffeic acid, −17.03 kJ/mol for gallic acid, −14.49 kJ/mol for chlorogenic acid, −21.53 kJ/mol for tyrosine, and −23.33 kJ/mol for pyrogallol, respectively. Thiourea, L-cysteine, ascorbic acid, citric acid and reduced glutathione at 0.1–1.0 mmol/L markedly inhibited PPO activity, whereas SDS, ZnCl2 and MnSO4 at 1–10 mmol/L enhanced PPO activity. Two isoenzymes of the PPO were detected. PRACTICAL APPLICATION Water caltrop is a minor food and vegetable crop in the southern China. The fresh, tender, starchy fruits are consumed raw as they are sweet, delicious and thirst-quenching. However, the fresh water caltrop fruit has a short postharvest shelf life because it is susceptible to become brown or even black discoloration when stored at ambient temperatures. We investigated the effects of pH, temperature, substrate and inhibitors on the polyphenol oxidase (PPO) activity isolated from water caltrop fruits. These findings are important because they could help water caltrop producers to develop proper storage and processing methods to avoid the undesirable colour changes.