The roles of metabolism of membrane lipids and phenolics in hydrogen peroxide-induced pericarp browning of harvested longan fruit

The effects of hydrogen peroxide (H2O2), as exogenous reactive oxygen, on browning, membrane lipids metabolism, and phenolic metabolism in a pericarp of harvested ‘Fuyan’ longan (Dimocarpus longan Lour. cv. Fuyan) fruits were investigated. The results showed that as compared to the control fruit, there was a higher browning index, a higher lipoxygenase (LOX) activity and a higher cellular membrane permeability. In addition, there were lower relative contents of unsaturated fatty acids and higher relative contents of saturated fatty acids, and thereby lower index of unsaturated fatty acids (IUFA) and lower unsaturated degree of fatty acids in the pericarp of H2O2-treated longan fruit. Furthermore, the fruit treated with H2O2 displayed higher activities of polyphenol oxidase (PPO) and peroxidase (POD), and lower content of total phenolic. These results indicated that H2O2-induced browning in the pericarp of harvested longan fruit might be due to an increment of LOX activity and acceleration of unsaturated fatty acids degradation in membrane lipids, which damaged cellular membrane structural integrity, and the consequent loss of cellular compartmentalization that, in turn, resulted in the contact of PPO and POD with phenolic substrates and subsequent oxidation phenolic to form brown polymers.