Browning Inhibition and Decolorization of Fresh Walnut Kernels and Their Effect on Browning-Related Enzyme Activity, Antioxidant Properties and Quality Attributes during Cold Storage

Fresh walnut kernels are prone to browning, making it difficult to produce and market it as a finished product. In this study, fresh walnut kernels (cv. ‘Xiangling’) were subjected to four browning inhibition methods and browned kernels were decolorized by two methods. According to the changes of color parameters and sensory quality during storage at 20 ℃, dipping in 1.0% ascorbic acid (AsA) + 0.05% citric acid (CA) for 5 min and dipping in 1.0% AsA at 20 ℃ for 1 h were found to be the best browning inhibition and decolorization conditions, respectively. To evaluate their effect on the quality maintenance of walnut kernels, the two treatments were performed individually or in combination with 1 kJ/m² ultraviolet radiation C (UVC), and the treated samples were subsequently packed in polythene (PE) bags (30 µm thick) and stored at 5 ℃. Those treated with deionized water under the same conditions served as a control group. Results showed that both individual and combined treatments inhibited the increase in the activities of polyphenol oxidase (PPO) and peroxidase (POD) during storage. The activity of phenylalanine ammonia-lyase (PAL) was significantly enhanced during 14-21 d in comparison to controls (P < 0.05). On 56 d, acid value (AV) of both individual and combined treatments in browning inhibition group as well as the combined treatment in decolorization group were significantly lower than their controls (P < 0.05). Combined treatments in two experiments simultaneously delayed the breeding of bacteria and mould, partially promoted the ferric reducing antioxidant power (FRAP), and the peroxide value (POV) was increased, while the oil content was little changed. The 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging capacity was not significantly different from that of the control group. AsA competitively inhibited PPO activity in walnut kernel skins and skinless walnut kernels. After 56 days of storage at 5 ℃, the brightness (L*) value of decolorized walnut kernels was similar to that of the browning inhibition group, and no mildew was observed. The results from this study could provide a theoretical and technical basis for the commercial production of fresh walnut kernel.