Abstract:
Thompson seedless grapes are prone to browning during dehydration, with their green grade rate decreasing as a result. In this study, laser confocal scanning electron microscopy, fluorescence value comparison, malondialdehyde, relative conductivity, and a browning index were used to determine the differences in the membrane lipid phase transition and integrity between browning and non-browning Thompson seedless grape cells under conditions of dehydration at 30 ℃ (1.5 m/s), 30 ℃ (0 m/s), 25 ℃ (1.5 m/s), and 25 ℃ (0 m/s). The results showed that when the water loss of Thompson seedless grapes reached 60%, the cell membrane was broken due to oxidative degradation, the contents of the cytoplasm and vacuoles were dissolved, and FM4-64 combined with the damaged cell membrane to produce a large number of red fluorescence substances. As a result, the red fluorescence intensity value was increased. The green fluorescent substances bound by FDA and esterase did not aggregate with cytoplasmic dissolution, the green fluorescence intensity decreased, the integrity of the cell membrane was destroyed, and the Thompson seedless grape showed varying degrees of browning. Simultaneously, the fluorescence ratio, malondialdehyde content, and relative conductivity increased greatly. Among these, the lipid phase transition temperature of the non-browning grape cell membrane was lower than the dehydration temperature, and the cell membrane was in the liquid crystal phase, maintaining the structure of the cell membrane and showing excellent stability. Browning grape samples, on the contrary, were characterized by cell membranes in the gel phase, which is not conducive to a stable cell membrane structure. Moreover, the fluorescence ratio, malondialdehyde content, and relative conductivity of browning grapes were higher than those of non-browning grapes, as well as the degree of cell membrane destruction. During the dehydration process, the cell membrane is always in the liquid crystal phase, which allows for a better maintenance of the cell membrane structure. An increase in the dehydration rate can also reduce the generation of browning. These findings provide a certain theoretical basis for reducing browning in the dehydration process of Thompson seedless grapes.
