Postharvest drying of walnuts: Effect of drying methods on walnut quality

It has been demonstrated that drying can significantly promote the antioxidant active substances of fresh walnut, as well as enhance lipid content and conversion of glycerol phospholipids to lysophospholipids in walnut. Currently, oven drying (OD) and natural air drying (NAD) are the two most commonly used drying methods for walnut. However, the differences in their effects on the quality of the final product remain unclear. This study investigated the effect of OD and NAD on the phenotypic and physiological properties and lipid oxidation of four main walnut varieties W10, W179, W185, X2 in Xinjiang, China, and determined the correlations among these properties. The results showed that OD could maintain plump and yellowish white kernels, whereas NAD led to noticeably darker and more wrinkled kernels at the end of drying. In addition, an analysis of quality-related metabolites revealed that two representative lipids PC (18:2_18:2) and PC (18:1_16:0) had completely different patterns under OD and NAD, and the metabolites highly relevant to these two lipids were mainly enriched in amino acid and ascorbate metabolism pathways. Comprehensive physiological and metabolite analysis indicated that OD outperforms NAD in maintaining walnut quality, and may be applied to industrial walnut drying.