Identification of cold resistance of squash and cucumber fruits by electrical conductivity and Logistic equation

Abstract: The present study was undertaken to investigate cold-sensitive fruits' cold resistance at low temperatures. Squash and cucumber were two typical kinds of cold-sensitive fruits, and they were very sensitive to low temperatures and easily suffered from chilling injury for inappropriate cold storage. Therefore, the research took squash, Jinyou cucumber and Shenqing cucumber as samples, and analyzed the changes of electrolyte leakage rate at different low temperatures (2, 8, 12 and 20℃) for different days (2, 4, 6, 8, 10 and 12 d). Because the changes in cell membrane permeability were significantly earlier than other chilling injury symptoms, such as the index of internal browning, but the changes between different varieties of cold-sensitive fruits were not always the same. That was the reason why the research took two different kinds of cucumbers as samples. The electrolyte leakage rate was often taken to identify cold-sensitive fruit's cold resistance. In addition, it has achieved some progress and breakthroughs at home and abroad in resent years. The electrolyte leakage rate was tested and documented every two days. Storage temperature and time, maturity, penetration time, measurement unit were considered as major influential factors on the research for electrolyte leakage rate. Among all the factors, penetration time and measurement unit were the most innovative and unique ones. Semi-lethal temperatures were determined by the logistic equation and they were taken as the important basis to judge whether cold-sensitive fruits suffered from chilling injury or not and the order of cold resistance. The result showed that the electrolyte leakage rate achieved stable when penetration time reached 90 minutes. At the same storage temperature, for the growth of the electrolyte leakage rate, there was little difference between the two varieties. With the extension of the storage time, the growth of the electrolyte leakage rate increased and showed significant difference (P<0.05). At the end of the storage time, the growth of the electrolyte leakage rate became stabilized, indicating that the cell membrane permeability had been completely destroyed at this moment. That was to say, the lower the storage temperature, the longer the storage time, the higher the electrolyte leakage rate, the worse the fruits' quality. Meanwhile, semi-lethal temperature of squash was between 5.37-6.56℃, Jinyou cucumber was between 6.69-7.86℃ and Shenqing cucumber was between 8.04-8.96℃. That was to say, these cold-sensitive fruits would have maintained a high quality when they were stored at this temperature range. They were confirmed individually and accurately. The tendency of the electrolyte leakage rate followed the Logistic equation and had a linear relationship with semi-lethal temperature. The result of semi-lethal temperature showed significant difference between different varieties, with the extension of duration time at low temperatures, the value of semi-lethal temperature increased and had a positive correlation with the duration time (P<0.05). The order of cold-sensitive fruit's cold resistance from high to low was squash, Jinyou cucumber, Shenqing cucumber and it had no significant relationship with fruits' maturity.