Effects of calcium hypochlorite treatment on postharvest decay and defense enzyme activity of kumquat fruits

Abstract: 【Objective】The objective of this study was to investigate the effects of calcium hypochlorite treatment to the postharvest preservation of kumquat fruits. Because peel and pulp of kumquat are edible,general fungicides used for citrus postharvest preservation with low, middle or high toxic chemical are inappropriate for kumquat. Calcium hypochlorite can form hypochlorous acid in water, the free chlorine can attack endogenous enzymes in microbial cells and cause microbial death. Moreover, the calcium hypochlorite treatment has low residue, and is harmless to the human body comparing with the other general disinfectants and bactericides. Calcium hypochlorite had been used on a wild range of horticultural products such as peach, orange, apple, strawberry and mushrooms to maintain product quality and reduce the decay rate. It was presumed that calcium hypochlorite treatment should also have the corresponding preservation effects on kumquat. However, there was little experimental evidences, and the period of usage, concentration and supporting measures of treatment also were not clear.【Methods】According to the application of calcium hypochlorite on postharvest preservation of apple, the current test set two concentrations, 0.15 g·L^-1 and 0.6 g·L^-1, respectively. Fruits harvested at January 4 and March 3 were tested, respectively. For packaging storage test, the kumquat fruits were soaked with calcium hypochlorite for 2 minutes at 10-12 ℃,and then forced ventilation, dry storage at room temperature(10±2) ℃ with humidity at 85%- 90% for 30 d. For each treatment, 200 fruits were used. Rot rate of treated kumquats was investigated every 7-10 d.The packaging storage test was repeated twice. For Penicillium inoculation test, 45 fruits were used. Rot rate and lesion size were counted. Content of soluble solids, hydrogen peroxide, soluble protein, and activity of superoxide dismutase, peroxidase and catalase were measured at room temperature for every 5-7 d.Water treatment was used as control.【Results】The kumquat fruit collected in January 4 showed better storability, the fruit rot rate was low, after the 35 d of storage, the fruit rot rate increased significantly. In the late period of storage(49 d) the fruit rot rate of both the 0.15 g·L^-1and 0.6 g·L^-1calcium hypochlorite treatments were lower, compared with the control, and the fruit rot rates were 7.5%, 5.5% and 9.5%, respectively. The decay rate of the kumquat fruits harvested in March was significantly increased at the 17 th day. The decay rates at the 25 th day of both the 0.15 g·L^-1and 0.6 g·L^-1calcium hypochlorite treatments were lower, compared with the control, and the decay rates were 12%, 8% and 14%, respectively. The inoculation experiment showed that the fruit decay rate and lesion size of the calcium hypochlorite treatments were smaller as the control. After 4 d of the inoculation, the fruit incidence rates of the 0.15 g·L^-1and 0.6 g·L^-1calcium hypochlorite treatments and the control were 73.33%, 62.22% and 77.78%, respectively, and the lesion diameter were 0.99 cm, 0.93 cm and 1.04 cm, respectively. The weightlessness rate experiments showed that the fruit weight of the kumquat fruits treated by 0.6 g·L- 1calcium hypochlorite dropped fastest, at the 20 th day of storage, the fruit weight loss rate of the 0.15 g·L^-1, 0.6 g·L^-1calcium hypochlorite treatment and the control were 1.50% and 2.55% and 1.62%, respectively. The result showed that the total acid content of the calcium hypochlorite treatment was 0.3% lower than that of the control.The content of soluble solid content in each treatment fluctuated at 17% and the difference was not significant. The H2O2 content in the storage process fluctuated and gradually decreased, the H2O2 content of the calcium hypochlorite treatment was significantly lower than that of the control. The fruits CAT activity decreased and then raised during the storage, but showed an overall downward trend. The fruit CAT activity of the calcium hypochlorite treatment was lower than that of the control, and the fruit activity of the 0.15 g·L^-1calcium hypochlorite treatment was the lowest. The fruit POD activities of the 0.6 g·L^-1calcium hypochlorite treatment in storage increased until the 27 th day and then decreased slightly. However, during the whole storage period, the POD activity of the 0.6 g·L- 1calcium hypochlorite treatment was significantly higher than that of the 0.15 g·L^-1calcium hypochlorite treatment and the control. The fruit POD activity of the 0.15 g·L^-1calcium hypochlorite treatment was lower than that of control. During storage, the kumquat fruit SOD activity decreased gradually and the 0.6 g·L- 1calcium hypochlorite treatment can effectively slow the decrease of the SOD activity and increased the activity of SOD in the process of storage, and, the effect of the 0.15 g·L^-1calcium hypochlorite treatment was not obvious, compared with the control, the difference was not significant.【Conclusion】Calcium hypochlorite treatment could effectively delay the senescence of fruit, and did not affect the normal flavor of kumquat. Calcium hypochlorite treatment in kumquat postharvest storage had non-toxic side effects and prospects for practical application. However, according to the different harvest period, the concentration of calcium hypochlorite should be correspondingly adjusted. For the fruit harvested at January 4, soaking with 0.15 g·L^-1and 0.6 g·L^-1calcium hypochlorite for 2 minutes could effectively reduce the incidence of fruit decay, while for the fruit harvested at March 3, Soking with 0.6 g·L^-1calcium hypochlorite for 2 minutes was the best treatment in this study.