Natural preservatives for superficial scald reduction and enhancement of protective phenolic-linked antioxidant responses in apple during post-harvest storage

Superficial scald during post-harvest storage is a serious problem for long-term preservation and shelf-life of some apple and pear cultivars. Development of superficial scald and related physiological disorders such as enzymatic and non-enzymatic browning are associated in part with oxidative breakdown and redox imbalance. Therefore, targeting natural antioxidants from food-grade sources as post-harvest treatment to reduce superficial scald has merit. Such natural antioxidants can potentially counter oxidation-linked damages associated with superficial scald through stimulation of antioxidant enzyme responses and biosynthesis of less-oxidized phenolics involving protective redox-linked pathway such as proline-associated pentose phosphate pathway. Based on this rationale, bioprocessed food-grade oregano extract (OX) and soluble chitosan oligosaccharide (COS) were targeted as post-harvest treatment (2 and 4 g L^-1) and were compared with diphenylamine (DPA) (1 and 2 g L^-1) to reduce superficial scald and to improve protective phenolic-linked antioxidant responses in "Cortland" cultivar stored at 4 °C for 15 weeks. Overall, significant reduction of superficial scald and conjugated triene was observed with DPA and OX (2 g L^-1) post-harvest treatments. Furthermore, stimulation of antioxidant enzyme responses such as increases in superoxide dismutase and guaiacol peroxidase activity was also observed, but was more evident with DPA and COS treatment. Overall, results of this study indicated that critical balance of less-oxidized phenolics and antioxidant enzymes and associated anabolic PPP-linked redox regulation is essential for improving post-harvest preservation and reduction of superficial scald in apple.