Comparative analysis of antioxidant activities between dried and fresh walnut kernels by metabolomic approaches

Fresh walnut kernel (FW) and dry walnut kernel (DW) are significantly different in antioxidant activity. However, the metabolomic mechanism underlying these differences remains to be explored. Here, we evaluated the antioxidant activities of FW and DW of four walnut varieties, and characterized their metabolites by GC-MS and LC-MS/MS. The influence of different metabolites on antioxidant activity of walnut kernel was investigated through a multivariate analysis. DW showed significantly higher antioxidant activities than FW. A total of 144 metabolites were detected in all samples. Pearson correlation analysis demonstrated that phenolic metabolites and polyunsaturated fatty acids significantly affect the antioxidant activity. A further network analysis of 21 target differential metabolites indicated that the higher antioxidant activity of DW may be mainly attributed to the higher content of phenolic acid polymers. This study reveals the potential mechanism for improving of antioxidant activity of walnut kernels by drying.