High-oxygen pretreatment of whole fruit delays softening of fresh-cut apples: a metabolic perspective associated with cell wall homeostasis

Abstract Softening is one of the major obstacles for fresh-cut products, leading to quality deterioration and economic losses. Although individual physiological and biochemical aspects of softening have been studied, metabolomic mechanisms under high-oxygen (HO) conditions remain unclear. In this study, the textural properties (hardness, resilience, and chewiness), cell wall components [water-soluble pectin, chelator-soluble pectin, and sodium-carbonate-soluble pectin (SSP)), cell wall-degrading enzyme activities (polygalacturonase, pectin methylesterase, β-galactosidase, and α-ʟ-arabinofuranosidase), cellular structure, and corresponding metabolomics (carbohydrate and amino acid metabolism) were analyzed in fresh apple fruits treated with 90% HO during storage at 4 °C for 15 d. The results showed that HO treatment significantly delayed softening, as evidenced by enhanced hardness, resilience, and chewiness, along with suppressed activities of key cell wall-degrading enzymes, and increased levels of chelator-soluble pectin and SSP. Ultrastructural observations confirmed that HO treatment maintained cellular integrity. Metabolomic analysis revealed that the elevated soluble sugar enhanced the substrate for pectin biosynthesis, supporting cell wall stabilization. Additionally, modified methionine and phenylalanine were correlated with reduced respiration, ethylene production, and browning. These findings provide comprehensive insights into the metabolic mechanisms that alleviate softening in fresh-cut apples under HO conditions.