Influence On Fruit Quality And Antioxidant Activity From Walnut Different Harvest Period

Early harvest is a common phenomenon in walnut production in China.In order to find out the optimal harvest time of different walnut varieties and further understand the difference in nut quality and antioxidant activity of different walnut varieties during the harvest period.In this study,nine walnut varieties,including ’Xilin3’,’Xiangling’,’Xiluo2’,’Qingxiang’,’Liaohe4’,’Luguang’,’Zhonglin5’,’Chico’ and ’Vina’,from Yangling,Shaanxi province,were selected as the research object.From July 31 to September 10,2020,walnut ripening period was sampled to determine the economic properties,fruit quality and antioxidant activity of nuts,study the relationship between various indexes,and comprehensively evaluate the quality and suitable harvest time of different varieties of walnut nuts by principal component analysis,so as to provide basis for timely harvest,improving yield and quality and increasing economic benefits.The main results are as follows:(1)Economic character indexes of walnut fruit: during the harvesting period,the growth indexes of the mean of three diameters,single fruit weight,kernel weight and kernel rate of 9different varieties of walnut were relatively consistent,and showed an increasing trend on the whole.The variation rules of different indexes show certain differences.The plumpness of nuts increased significantly with the postponement of harvest time,and the kernels were basically fully developed around August 20.There are great differences among varieties of nut flavor.The whiteness index of kernel color was higher and the Browning index was lower in the early harvest stage.On August 20,the whiteness index decreased gradually,and the browning index increased significantly after August 30.(2)Nutritional quality of walnut fruit: crude fat content showed an increasing trend during harvesting,and reached the peak value in most varieties around August 20,with the content ranging from 66.42% to 73.02%.Fatty acids were mainly unsaturated fatty acids,accounting for 90.33%～91.41%.The soluble sugar content and fruit moisture content decreased gradually.The soluble protein content showed an increasing trend in general,and the variation trend of different varieties differed greatly.The content of amino acids increased first and then decreased,and the variation was not large.(3)Functional components and antioxidant activity of walnut fruit: During the harvest period,the vitamin E content of ’Xilin3’,’Xiangling’,’Xiluo2’,’Liaohe4’ and ’Luguang’increased firstly and then decreased,while the other varieties increased gradually.Considering the contents of total phenols and flavonoids,free radical scavenging ability and reducing ability,the five varieties with strong antioxidant ability were ’Xiangling’,’Qingxiang’,’Liaohe4’,’Zhonglin5’ and ’Vina’.The antioxidant activity of ’Qingxiang’ and ’Vina’ was the highest from August 30 to September 10,and the other varieties reached the highest antioxidant activity around August 20.(4)Correlation analysis among the indexes: plumpness,mean of three diameter,single fruit weight,kernel weight and kernel percentage were positively correlated with each other.Browning index of kernel color was negatively correlated with whiteness index.Crude fat was negatively correlated with soluble sugar,water content,oleic acid and linoleic acid contents.The soluble sugar content and water content,soluble protein and amino acid content were positively correlated.There was a significant positive correlation between the contents of total phenols and flavonoids and the reducing ability of FRAP.There was a significant positive correlation between the content of vitamin E and the content of total flavonoids,and a significant negative correlation between the content of vitamin E and the scavenging ability of ABTS free radicals.DPPH radical scavenging ability was positively correlated with ABTS radical scavenging ability,and FRAP reducing ability was negatively correlated with DPPH radical scavenging ability.(5)Four principal components were selected by principal component analysis to reflect the effect of 21 indexes on walnut harvest time.Crude fat content,production index and sensory properties as the first principal component,saturated fatty acids,amino acids,soluble protein and soluble sugar nutrition indicators for the second principal components,such as vitamin E,DPPH radical scavenging ability,ABTS radical scavenging,flavonoids and other functional ingredients is the third principal component,unsaturated fatty acids for the fourth principal component.The results of principal component analysis and comprehensive evaluation mathematical model showed that in Yangling area of Shaanxi Province,’Xiangxiang’ and ’Xiluo2’ were the best harvest date on August 10,’Xilin3’,’Xiangling’,’Liaohe4’,’Luguang’ and ’Zhonglin5’ were the best harvest date on August 20,and ’Chico’ and’Vina’ were the best harvest date on September 10.