Design of sulfur dioxide automatic control fumigation system for table grape preservation

Abstract: Sulfur dioxide (SO2) is the most efficient preservative in commercial storage of table grapes and there is no other substitute so far. But several problems have not been solved yet, such as incomplete sterilization to the grapes, formation of bleached spots and accumulation of phytotoxicity due to the inhomogeneous release of SO2. Many studies had been done to decrease the negative effects caused by excessive SO2. In order to achieve the on-line real-time monitoring, the SO2 concentration precisely control as well as recover SO2 during fumigation process, a SO2 fumigation automatic control equipment was designed in this paper. This equipment was composed of three parts: SO2 concentration control device, grape fumigation device and lye absorption device. These parts were systematically controlled under programmable logic controller (PLC). The SO2 concentration, fumigation temperature, humidity and pressure were all monitored through the Kingview 6.55 software. Multiple functions could be implemented by this software, such as real-time animation display, trend curve drawing, database query, report printing and so forth. The fumigation system was applied to measure the SO2 concentration, pressure and recovery of SO2 gas during the short-time SO2 fumigation to the 'Munage' grapes at (0±1) ℃ (RH=90%±5 %). The results showed that the error range of SO2 concentration was low, the maximum value of pressure deviation was 10 % and the SO2 recovery was above 99%. The SO2 concentrations of 2 000, 3 000 and 4 000 μL/L were achieved after 42, 88, and 203 s respectively after they were set the device control precision was 0.09 μL/L. That meant this equipment and control system were able to meet actual demand. The high-concentration-short-time SO2 fumigation technology was also studied in this article. The effects of different concentrations of SO2 (0, 1 000, 2 000, 3 000, 4 000 and 5 000 μL/L), fumigation time (2, 4, 6, 8, 10 and 12 min) and pressure (normal pressure, 0.02, 0.03, 0.04, 0.05, 0.06 MPa) on the postharvest quality of the 'Munage' grapes were discussed. The optimal SO2 fumigation condition was: concentration of SO2 was 3 000 μL/L, fumigation time was 10 min and the pressure was 0.05 MPa. Under this condition, the decline of the firmness was retarded and the browning index, decay rate, weight loss rate, drop rate and bleaching index were all decreased. Therefore, the postharvest quality of 'Munage' grapes was better maintained. The results showed that SO2 fumigation automatic control equipment could accurately achieve the real-time online monitoring of SO2 concentration, fumigation temperature, humidity and pressure at the same time. It could precisely control and quickly achieve the required SO2 concentration. The gas recovery device could rapidly absorb the SO2 residual which increased the safety of this fumigation equipment and the edible safety of grape was also improved as well. Compared with the untreated grapes, the grapes after high-concentration-short-time SO2 fumigation kept better postharvest quality. In general, SO2 fumigation automatic control equipment and control system had a potential application on postharvest fumigation of vegetable and fruit, in the meantime this study would provide the technology and equipment support to the grape industry.