Inhibitory effects of heat water and potassium sorbate on gray mold in postharvest kiwifruit

Abstract: The present study investigated the effects of the heat shock treatment and potassium sorbate solution on the suppression of gray mold (Botrytis cinerea) of 'xuxiang' kiwifruits under postharvest conditions. The treatment in the test included potassium sorbate at 5 g/L(PS), hot water at 48°C (HT) for 8 min, and a combining of HT with PS (HT+PS), while the sterile water was applied in the control (CK) group. The test was first conducted on the inhibition effects of PS and HT on the mycelium growth of B. cinerea. The 10 μL of spore suspension was treated with or without HT before the inoculation on potato dextrose agar medium with or without PS treatment. After each treatment, the spore suspensions were further cultured at 24°C for evaluation. After 4 days of in vitro culture, the PS treatment was found to significantly inhibit the growth of mycelium compared with CK (P<0.05). The optimal result was obtained from the HT+PS treatment, after which the observed colony diameter was only 13% that of the CK. In the injury inoculation test, the fruits that inoculated with B. cinerea spore suspension were treated with HT, PS, HT+PS, or clean water for 8 min. The fruits were then dried, and stored at 0°C for 30 days and 24°C for 7 days. The lesion diameter, disease index and the activity of resistance-related enzymes were measured in the inoculated kiwifruits after different treatments. Under both storage conditions (0°C and 24°C), PS, HT and PS+HT treatments significantly reduced the decay of gray mold in kiwifruits compared with CK (P<0.05). The activities of defense-related enzymes including polyphenol oxidase, peroxidase, chitinase and β-1,3-glucanase were remarkably enhanced in the treatment groups compared with the CK. However, no significant difference in the lesion diameter was detected among the PS, HT, and CK treatments after storage (P>0.05). The optimal result was again obtained from the HT+PS treatment, after which the colony diameters were significantly reduced compard with the CK for the fruits treated at 24°C and 0°C, respectively. The incidence index of the PS+HT treatment at 24°C and 0°C was significantly reduced compard with the CK, respectively. The influence of each treatment on the quality of kiwifruit was further investigated. The fresh fruits were air-dried after different treatments, then stored at 0°C±0.5°C for 90 days. The indicators of the fruit quality were tested every 10 days during storage, and the level of soluble solid content in the fruits was maintained in all treatments. However, the decrease of the titratable acid and ascorbic acid was delayed following all treatments compared with the CK. Similarly, at the end of 90-day storage period, the weight loss and decay were significantly reduced in all treatment groups compared with the CK (P<0.05). The optimal results were again achieved with the HT+PS treatment, after which the rates of weight loss and decay were only 2.11% and 0%, respectively, after 90 days of storage, compared with 4.61% and 3.08%, respectively, for the CK group. These findings indicate an important role for heat water and potassium sorbate treatment in delaying the decay and senescence of postharvest kiwifruit.