[Changes in cell wall components and cell wall-degrading enzyme activities of postharvest longan fruit during aril breakdown].

Longan (Dimocarpus longan Lour.) fruits are very susceptible to pericarp browning and aril breakdown, and postharvest aril breakdown is one of the most important factors degrading the quality and shorting storage life of longan fruit. Changes in aril breakdown index, cell wall components and cell wall-degrading enzyme activities in aril of longan cv. Fuyan fruits using sealed polyethylene film bags (0.015 mm thick) at (10+/-1) degrees C were investigated. The main results were as follows. Development of aril breakdown was higher with storage time (from day 0 to day 36). Aril breakdown index was positively and significantly correlated with storage time (P<0.01). During development of aril breakdown, the total dry weight of the cell wall materials, protopectin, cellulose, semicellulose and cell wall protein contents of aril decreased progressively. The total dry weight of the cell wall materials, contents of protopectin, cellulose, semicellulose and cell wall protein of aril were all negatively correlated with aril breakdown index. There were low beta-galactosidase activity, and high activities of pectinesterase (PE), polygalacturonase (PG) and cellulase in aril of harvested fruit. PE activity in aril gradually decreased during development of aril breakdown. The activities of PG and cellulase in aril increased significantly during storage from day 6 to day 12 and from day 0 to day 12, respectively. The peaks enzyme activities of both PG and cellulase appeared on the 12th day after harvest, then the enzyme activity decreased; whereas, the activities of PE, PG and cellulase changed little from day 0 to day 24, and then rapidly decreased. The beta-galactosidase activity in aril decreased slightly during storage from day 0 to day 24. However, the beta-galactosidase activity increased significantly after day 24. Especially, the beta-galactosidase activity increased rapidly after 30 d of storage, in the meantime, the activities of PE, PG and cellulase almost disappeared. From the results it can be seen that the development of aril breakdown was due to the degradation of cell wall components such as protopectin, cellulose, semicellulose and cell wall protein. The early and middle phases of development of aril breakdown were mainly brought about by the action of PE, PG and cellulase, whereas, beta-galactosidase played the key role at the late phase of aril breakdown.