Oxidation Mechanisms of Walnut Kernels Stored in Different Packages

Objective: To investigate the oxidation mechanism of walnut kernels. Methods: Walnut kernels were subjected to vacuum or non-vacuum packaging before accelerated oxidation at 60 ℃. Samples were taken periodically for ultrastructural and morphological observation by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) and for the measurement of oxidation indicators such as peroxide value, carbonyl value, and cell membrane permeability. Changes in cellular ultrastructure during the oxidation process of walnut kernels were analyzed and compared under vacuum and non-vacuum packaging conditions. The oxidation process was assessed from a multi-dimensional perspective and the oxidation mechanism was explored as well. Results: Under non-vacuum conditions, walnut kernel oil began to become sticky on the 14th day of accelerated oxidation and then gradually became stickier. With oxidation time, intracellular protein bodies (PB) exhibited sequential changes including an increase in surface roughness, aggregation, movement toward the cell periphery, decomposition and reassembly. The oil body membrane (OBM) was ruptured and the intracellular reticulum was disrupted, thus causing oil leakage into the intercellular space and resulting in a reduction in the cell volume. Cellular structural changes were much smaller under vacuum conditions than under non-vacuum conditions, the oil started to become sticky on the 28th day, and the oxidation indicators were significantly lower than those under non-vacuum conditions (P < 0.01). The trend of changes in three oxidation indicators was consistent with that of cell ultrastructure. Conclusion: TEM and SEM combined with oxidation indicators can provide clearer insights into cellular ultrastructural changes in walnut kernels after oxidation. The oxidation mechanisms of vacuum and non-vacuum packaged walnut kernels are the same, but their oxidation degrees are different. Notably, vacuum packaging can significantly reduce the oxidation of walnut kernels.