Effects of gaseous chlorine dioxide fumigation on oxidative damage and antioxidant defense system involved in pericarp browning during storage of Longan fruit cv. Daw

Abstract

Oxidative damage caused by an imbalance between the production of free radicals and the capacity of the antioxidant defense mechanism, is a cause of browning problem in many postharvested fruits that reduce in shelf life. Chlorine dioxide (ClO2) is a new chemical used to reduce pericarp browning of longan fruit. However, its ability to counteract the disorder through neutralization by antioxidants has not been well studied. Therefore, the changes in free radicals (reactive oxygen species, ROS), oxidative membrane damage, antioxidant defense system involved in pericarp browning of longan fruit during storage as well as the effects of gaseous ClO2 fumigation in reducing the free radicals (ROS) and oxidative damage and stimulating the antioxidant defense system in longan fruit were investigated. The research was divided into 2 experiments to study the effects of gaseous ClO2 on 1) ROS free radical contents and oxidative membrane damage and 2) antioxidant defense system during pericarp browning of ‘Daw’ longan fruit stored at 251 °C. Longan fruit was fumigated with 10 mg l1 ClO2 for 10 minutes and stored at 251 °C with 82% relative humidity for 7 days. The fruit was randomly sampled every day to determine ROS free radical contents, oxidative membrane damage, antioxidant defense system and pericarp browning. The production and accumulation of reactive oxygen species (ROS) such as superoxide radical (O2•−), hydrogen peroxide (H2O2) and hydroxyl radical (OH•) increased during storage of longan fruit. These increases coincided with oxidative membrane damage as measured by lipoxygenase (LOX) activity, contents of conjugated diene, malondialdehyde (MDA), protein carbonyl and electrolyte leakage rate (EL rate) which increased with storage time in the pericarp of longan. Fumigation with ClO2 reduced ROS accumulation and oxidative membrane damage of longan pericarp during storage. The contents of O2•−, H2O2, OH•, conjugated diene, MDA and protein carbonyl, activity of LOX and EL rate in the pericarp of ClO2 treated fruits were significantly lower than those in the control fruits throughout the storage period. Pericarp browning of longan was positively correlated with the levels of ROS and oxidative membrane damage during storage. The increases in activities of polyphenol oxidase (PPO) and peroxidase (POD) correlated with browning index (BI). Fumigation with ClO2 reduced the activities of PPO and POD and retarded browning index. During storage, the antioxidant defense system of longan fruit tended to decrease and was consistent with pericarp browning, ROS content and oxidative membrane damage during storage. Superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activities of longan fruit increased and reached the peak on Day 2 of storage and then decreased gradually afterwards. The total phenolic content (TPC), ascorbic acid (ASA), -tocopherol, total glutathione contents and total antioxidant capacity (TAC) analyzed by 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging, 2,2’-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging and ferric ion reducing antioxidant power (FRAP) methods gradually decreased throughout storage time. ClO2 fumigation enhanced the antioxidant defense system which was associated with less pericarp browning of longan during storage. The activities of SOD, CAT and APX and contents of TPC, ascorbic acid, -tocopherol and total glutathione and TAC in the fruits treated with ClO2 were significantly higher than those in the control fruits throughout the storage period. In addition, ClO2 also promoted gene expressions of the three antioxidant enzymes that had a close relationship with the increased activities than the control. The decline in antioxidant defense system leads to an increase in ROS accumulation and oxidative membrane damage as well as subsequent pericarp browning during storage of longan fruit. Fumigation with ClO2 can enhance the efficiency of antioxidant defense system and reduce ROS accumulation and oxidative membrane damage which consequently alleviate pericarp browning of longan fruit during storage.