Inhibition of membrane lipid super oxidization and browning by rapid dehydration technique during drying of grapes

Abstract: Xinjiang is abundant in grapes. According to statistics, the production of grape accounted for 28.4% of the whole fruit production in Xinjiang in 2012. Since the mature period of grapes is very short, dehydrated drying after grape harvest is an important method of processing. Thompson seedless grape (Vitis vinifera L. var Wuhebai) is the main species of green raisins. As green raisins produced from traditional dry room are likely to browning, it's well acknowledged as a key issue for the development of Xinjiang raisins. It was reported that the browning of actinidia arguta, longan, litchi and peach was caused by the polyphenol oxidase (PPO) contacting with the phenolic substrates, which resulted in the strengthen of the membrane lipid peroxidation and the increase of cell membrane permeability in fruits. These researches were focused on the relationship between browning caused by dehydration treatment before storage or nature water loss during storage and the membrane lipid peroxidation. But the dehydration was different from dehydration drying process. In order to investigate the mechanism that rapid dehydration inhibited the membrane lipid peroxidation, PPO, browning of Thompson seedless grapes during the drying, the changes of physiological and biochemical properties caused by grape browning in dehydration drying process were investigated in this study, and subsequently obtained the theoretical basis for preventing raisins browning and maintaining the quality of Thompson seedless grapes. Xinjiang Thompson seedless grapes were used as experiment materials. After being operated by the drying agent, two treatments, i.e. rapid dehydration and slow dehydration were conducted. Samples were taken every 10% loss in weight of the samples and stored for further use. The effects of different drying methods on dry basis moisture content, drying rate, browning, membrane permeability (relative conductivity), malonaldehyde (MDA) content, lipoxygenase (LOX) activity, superoxide dismutase (SOD) activity, catalase (CAT) activity, peroxidase (POD) activity, polyphenol oxidase (PPO) activity and total phenol content were studied. The results showed that there were two stages in the drying of Thompson seedless grapes, i.e. constant rate drying and falling rate drying, and the significant browning of Thompson seedless grape during two drying methods both occured in the falling rate drying stage. The browning of Thompson seedless grape in the slow dehydration was in positive correlation with LOX activity, and the browning of Thompson Seedless grape in the rapid and slow dehydrations was in positive correlation with membrane permeability and MDA content, and negatively correlated to SOD activity, CAT activity, PPO activity, and total phenols content. It was found that the browning in the dehydration process of Thompson seedless grapes was closely related to the membrane lipid peroxidation. Rapid dehydration treatment significantly slowed down the development of the browning, as well as MDA content and membrane permeability; besides, LOX activity was also inhibited. The relatively high activity of active oxygen scavenging enzymes including SOD, CAT and POD and the more content of total phenols were maintained, and PPO was kept at a low level of activity. In conclusion, rapid dehydration can effectively inhibit the membrane lipid peroxidation of Thompson seedless grapes from cell membrane damage, maintain the integrity of the cells, and retard the browning of Thompson seedless grapes during drying and heating processes. The results provide a reference for the application of rapid dehydration in the drying of Thompson seedless grapes.