Plasma Affects Sugar Conversion during Storage of Citrus sinensis Osbeck: Melatonin Is a Key Factor

Background: Atmospheric pressure plasma as a green preservation technology for fruits and vegetables can effectively reduce the number of microorganisms on the surface of fruits and vegetables to achieve the effect of reducing the decay rate and prolonging the freshness period. In addition, plasma can affect the quality of fruits and vegetables and delay the aging of fruits and vegetables. However, the intrinsic mechanism by which plasma affects the quality of fruits and vegetables is unclear. Objective: The aim of this work is to observe the quality changes of Citrus sinensis Osbecks led by plasma treatment during storage period and to analyze the intrinsic mechanism behind it, for the purpose of providing a theoretical reference or the application of plasma in fruit preservation and quality enhancement. Methods: Postharvest Citrus sinensis Osbecks were treated with dielectric barrier discharge (DBD) for 5min, and then the total soluble solids, titratable acids and solid-acid ratios of fruit pulp during storage before and after treatment were measured using a hand-held digital refractometer, and an acid-base burette. Changes in melatonin, sucrose converting enzyme, and simple sugar content of fruit were measured with an ultraviolet-visible spectrophotometer. Results: It was found that the soluble solids and solid-acid ratio of Citrus sinensis Osbecks by DBD treated were significantly increased during storage, while titratable acid did not change significantly. Melatonin and sucrose converting enzyme measurements revealed that plasma treatment significantly increased melatonin content and sucrose converting enzyme activity in the pulp. Measurements of monosaccharides showed that the glucose and fructose contents of the treated Citrus sinensis Osbeck pulp increased, while the sucrose content decreased. Conclusion: The reactive oxygen and nitrogen species generated with DBD plasma can effectively increase the melatonin content in the pulp of Citrus sinensis Osbecks, thereby improving citrus quality including promoting sugar conversion and enhancing flavor.