Evidence for Oxidative, Free Radical-Mediated Metabolism of 1-Methylcyclopropene In Fresh-Cut Tissue and Cell-Free Homogenates of Apple Fruit

Previous studies have demonstrated significant and variable non-target sorption of 1-MCP by fruit and vegetables. In many cases, sorption is markedly enhanced upon tissue processing (slices). In this study, non-specific sorption of 1-MCP was examined in fresh-cut tissues and cell-free homogenates (CFH) of five apple cultivars. Fruit were processed into cubes of approximately 10 g, enclosed in sealed jars, and provided with 20 µL L^-1 gaseous 1-MCP. There were significant differences in the sorption of 1-MCP by fresh-cut tissues from different apple cultivars. Fresh-cut ‘Jonagold’ and ‘McIntosh’ absorbed 1-MCP over 6 h with the highest capacity showing 97.5 ± 0.46 and 97.6 ± 0.40%, respectively, followed by ‘Cameo’ at 93.9 ± 0.08%. ‘Honeycrisp’ had the lowest capacity with 88.5 ± 0.63%. Sorption rates showed a similar trend among cultivars. 1-MCP sorption rates for fresh-cut ‘Honeycrisp’, ‘Cameo’, ‘Jonagold’ and ‘McIntosh’ were 100.8 ± 0.4, 111.9 ± 3.6, 127.9 ± 3.1, and 130.9 ± 2.6 ng kg^-1 s^-1, respectively. Sorption of 1-MCP by CFH from the different cultivars paralleled the sorption by fresh-cut tissues. The similar patterns of 1-MCP sorption of fresh-cut tissue and CFH of different apple cultivars indicated that 1-MCP sorption was influenced by 1-MCP metabolism. The capacity to sorb 1-MCP was markedly influenced by tissue aging, with tissue of ‘Delicious’ aged for 6 h showing a 60% reduction in 1-MCP sorption compared with freshly prepared tissue., Removal of cut-surface tissue following tissue aging resulted in nearly 80% recovery in 1-MCP sorption rate and capacity. Therefore, wounding and aging operate in opposing fashion in controlling 1-MCP sorption in fresh-cut apple. The similarity of changes in oxygen consumption rate and 1-MCP sorption rate in fresh-cut tissue provided further evidence that oxidation contributes to 1-MCP metabolism in fresh-cut tissue. 1-MCP sorption rates over initial 2 h for fresh-cut tissue and CFH under 0.02 kPaO₂ was reduced by 74 and 68%, respectively, compared with 20 kPaO₂. Ascorbate inhibited 1-MCP sorption 82 and 65% in fresh-cut tissue and CFH, respectively. Hypotaurine, a quencher of the hydroxyl radical, suppressed 1-MCP sorption by fresh-cut slices and CFH by 88 and 68%, respectively. The data suggest that the oxidative burst associated with fresh-cut processing enhances sorption of 1-MCP through oxidative destruction. The data are consistent with free radical-mediated stress models for the surface of fresh-cut tissues (Acta Hort. 682:1819) and provide information useful in understanding the responses of fresh-cut fruit and vegetables tissues to 1-MCP.