Effects of Storage Temperature on Sugar Metabolism and Related Enzyme Activities of Postharvest Longan Fruits

To study the effects of different storage temperatures (25, 15, 4 ℃) after harvest on the sugar components and related enzyme activities of "Shixia" longan fruits. The longan pulp soluble solid content (SSC), the content of three main sugars and the changes in the activities of sugar metabolism-related enzymes were measured. Principal component analysis and load factor analysis were performed on samples at different storage temperatures and various periods, and the correlation analysis of each index was performed. The results showed that the SSC of longan stored at 25 ℃ decreased the fastest, from 23.5% to 22.0% after 8 days of storage, and the sucrose content decreased the fastest, but the glucose and fructose content showed an upward trend; both 15℃ and 4 ℃ could effectively delay SSC. The decrease in SSC of longan stored at 4 ℃ was slower and the sucrose and glucose content decreased the slowest, but the decrease in fructose content was faster than 15 ℃. The acid invertase (AI), neutral invertase (NI) and sucrose synthase (SS) activities of longan pulp stored at different temperatures all increase with the storage time, the amplitude and speed of the increase was the fastest at 25 °C, at 15 °C was secondly, and the 4 ℃ rise was the most gentle, indicating that low temperature significantly inhibited the increase in the activity of sucrose metabolizing enzymes. Correlation analysis showed that the sucrose content of longan stored at different temperatures had a very significant positive correlation with SSC, indicating that SSC could reflect the sucrose content to a certain extent; the sucrose content had a significant positive correlation with the glucose content, and it also had significant negative correlation with AI, NI, and SS. It was inferred that glucose and fructose were degraded from sucrose; glucose content and fructose content were extremely significantly positively correlated, but the two decline patterns in 15 ℃ and 4 ℃ storage fruits were inconsistent, suggesting that the consumption of the two was different. The above results indicate that low temperature could significantly inhibit the increase of sugar metabolism and related enzyme activities in longan fruit, thereby slowing down the decomposition of sugar during storage and prolonging the storage time. Among them, the storage effect at 4 ℃ was the best.