| Longan (Dimocarpus longan Lour.) is a famous subtropical fruit species in South China. Since longan fruit mature during the hot and humid August/September in China, the fruit deteriorate rapidly after harvest due to pericarp browning, aril breakdown and decay, the fruit have a very short postharvest life of 3-4 days at ambient temperature. This short postharvest period limits its long-distance transportation, marketing and consumption. Therefore, postharvest research becomes a key problem in the development of longan production. Studies of pericarp ultrastructure relation to water loss, browning and decay of postharvest longan fruit, the mechanism of desiccation-induced browning of longan pericarp, the technological parameter and theoretical basis of key link in postharvest handling of longan fruit such as the differences of pericarp morphology and structure among various cultivars and its relationship to storability and transport of longan fruit, effect of harvesting date on quality and storability of fruit, vacuum precooling of fruit after harvest, technology of prolonging the shelf life of cold-stored longan fruit at ambient temperature were conducted, and commercial postharvest handling system for longan fruit, which was based on the international research results on postharvest technology of longan fruit, was set up in this dissertation. The results were as follows:Pericarp ultrastructure was closely related to water loss, browning and decay of longan fruit. Pericarp structure of longan fruit was organised with exocarp, mesocarp and endocarp by scanning electron microscope observation. Periderm layer in the exocarp was thin. Undeveloped subrin and incomplete subrin layer, a lot of micro-cracks with mutual connection, many lenticels with different shape and size appeared on the surface of exocarp. The lenticel channel connected with the interstices between cells in the mesocarp, which were large and presented in great numbers. Longan fruit stalk was organised vascular bundle, stone-cell and ventilating tissue, which were large cell interstices and intercellular cavity. These ultrastructures of longan pericarp were facilitated to postharvest fruit water loss, pericarp browning and pathogens infection, which caused poor storability and low resistant to .diseases of fruit.Water loss of longan fruit was mainly from the pericarp rather than the aril. Selective dehydration of the pericarp occurred with little movement of water between the aril and the pericarp. Development of pericarp browning increased with an increased rate of water loss from pericarp and storage time ( from 0 to 6 days ), and there was a storage time X rate of water loss interaction. Fruit weight loss and pericarp browning were significantly decreased(P<0.01) by polyethylene film bag (thick 0.015 mm) sealed packing.Water loss from pericarp caused reductions in activities of reactive-oxygen-scavenging enzymes ( SOD, CAT, APX, GR ) and contents of endogenous antioxidant substances ( AsA , GSH , carotenoid, flavonoid ), whereas increases in productive rate of O{ and content of MDA which was the terminal product of lipid peroxidation. Pericarp desiccation resulted in an increased relative leakage rate, which showed that membrane structure became more vulnerable. PPO and POD activities of longan pericarp tended to increase with an increased rate of water loss. Total phenolics of longan pericarp were initially high, and decreased 36.23% when fruit stored at 10癈 and 50% RH for 6 days. Pericarp pH value was low, and increased from 5.76 to 6.36. In addition, the observation with transmission electron microscope on the fine ultrastructure of pericarp cells demonstrated the injury of vacuole and plastid membrane, and disruption of organelle by water loss. Accordingly, the results suggest that desiccation-induced browning of longan pericarp may be due to a reduction of antioxidant capacities, an accumulation of active oxygen, an increase in membrane lipid peroxidant, a disruption of the integrity of cellular membrane structure, and the d...
|
PDF Full Text Request