Effects Of Chitosan And Heat Treatment On Quality And Physiology During Storage And Characterization Of PPO In Water Chestnut

Effects of chitosan coating and heat treatment on the appearance quality, nutritional quality and metabolism physiology during storage and the characterization of polyphenol oxidase (PPO) in water chestnut (Trapa acornis Nakai) fruits were studied. The main results obtained are as follows:1. Effect of chitosan coating on nutritional quality and physiology of water chestnut fruits during storageThe application of 1.0% and 2.0% chitosan coating reduced water loss, browning index, and delayed the decrease of the contents of vitamin c (Vc), soluble solid substance (SSC), titratable acids (TA), soluble protein, soluble sugar, sucrose, reducing sugar and phenolic compounds, inhibited the increase of peroxidase (POD), superoxide dismutase (SOD), ascorbate peroxidase (APX), PPO and phenylalanine ammonia-lyase (PAL) activity and reduced MDA accumulation in water chestnut fruits. Therefore, the storage time of the fruits was prolonged. However, the application of 0.5% chitosan coating had no significant efftect.2. Effect of heat treatment on nutritional quality and physiology of water chestnut fruits during storageThe heat treatment (45℃, 90s and 70℃, 60s) effectively decreased water loss, browning index, POD, SOD activety, enhanced SOD, CAT and PAL activity, and prevented the decrease in eating quality associated with prevention of Vc, SSC, TA, soluble protein, soluble sugar, sucrose, reducing sugar and phenolic compounds of peeled water chestnut fruits. However heat treatment at 90℃ for 30s had a harmful effect to peeled water chestnut fruits.3. Properties and characterization of PPO in water chestnut fruitsThe properties and charaterization of PPO of water chestnut fruits were investigated. The results showed that optimum pH and temperature were 5.0⁸.0 and 20-30 ℃ based on different substrates, respectively. The enzyme activity was heat-stable 20³0 ℃ for 50 min and nearly completely inactivated at 60℃ and 70℃ over 5 min. The best suitable substrate was catechol due to the lowest K_m (17.14 mM) and the biggest Vmax/K_m(83.3 min^-1) among various substrates used in the experiment. The most effective inhibitor was L-cysteine, sequently followed by ascorbic acid and citric acid. SDS, ZnCl₂ and MnSO₄ were effective activators for PPO activity. Activation energy (Ea) values were calculated from the Arrhenius equation and the values were in the range of -14.49 to -38.24 KJ mol^-1 when different substrates were used. Two isoenzymes of the PPO were identified by polyacrylamide gel electrophoresis.