Study On The Relationship Between Antioxidant Metabolism And Browning Of Walnut Fruits During Cold Storage

Fresh walnut undried with a unique taste and flavor and high nutritional and health value,has been favored by consumers.Walnut fruit with green husk is a main consumption form of fresh walnut,but the green husk is prone to external browning and soft browning of during storage,which seriously affects its storage life.The previous research found that cold storage can effectively prolong the storage life of walnut fruit,but the browning is still obvious.At present,there are few reports on the browning changes and the relationship between antioxidant metabolism and the browning of walnut fruit during low-temperature storage.‘Qingxiang’and‘Xifu1’walnut fruits were stored at 0℃and 2℃,and then the fruits browning and kernels quality were monitored during storage,as well as the relationship between browning and antioxidant metabolism was further explored by analyzing and comparing the reactive oxygen metabolism and As A-GSH cycle-related indicators.The aim is to find out the reasons of the walnut fruits browning during cold storage and provide a theoretical basis for solving the postharvest browning in the fresh walnut industry.The main results are as follows:（1）Under low-temperature storage,the browning degreeof‘Qingxiang’and‘Xifu1’walnut fruits increased with the storage time,and‘Qingxiang’was more prone to soft brown than‘Xifu1’.Compared with 2℃condition,the 0℃condition effectively reduced the browning degree and delayed the soft browning process and maintained the color of the two-varieties walnut fruits with the lower browning index,a*andΔE values,and the higher L*and b*values on 60d.（2）Compared with 2℃,0℃maintained the lower PPO activity and higher total phenolic content in green husk of the two-varieties walnut fruit.Compared with‘Xifu1’,the‘Qingxiang’walnut fruit has the higher PPO activity and lower total phenolic content in green husk.According to the correlation analysis,the lower total phenolic content resulted from the higher PPO activity may be the important reason leading to the severe soft browning of‘Qingxiang’fruit.（3）Compared with 2℃,0℃maintained the higher O₂^-·and H₂O₂ contents in green husk of the two-varieties walnut fruits during storage,and significantly increased the SOD,CAT and POD activities in‘Qingxiang’green huskwhich effectively reduced the browning of‘Qingxiang’fruits.Also the 0℃condition increased the POD activity but decreased the SOD and CAT activities of‘Xifu1’green husk on 60 days,implying that the high POD activity may play a major role in reducing the browning of‘Xifu1’fruit.（4）Compared with 2℃,0℃maintained the higher As A and GSH contents and higher APX,MDHAR and DHAR activities but lower As A/DHA ratio,GSH/GSSG ratio and GR activities in green husk of‘Qingxiang’fruit,as well asthe higher As A and GSH contents,As A/GSH ratio,and the activities of APX,GR and MDHARof‘Xifu1’green husk.（5）At 0℃and 2℃,there were no significant differences in the contents of crude fat,soluble protein and soluble sugar,as well as AV and PV of‘Qingxiang’and‘Xifu1’walnut kernel.In summary,compared with 2℃,0℃stimulates the increase of reactive oxygen level to a certain extent,thereby increasing the levels of reactive oxygen metabolism and As A-GSH circulation,and then effectively reducing the browning of walnut fruit.Theperformance between the two varieties is different.