Biochemical factors associated with a CO2-induced flesh browning disorder of Pink Lady apples

The underlying biochemical factors associated with a CO2-induced internal flesh browning (FB) disorder of Pink Lady apples (Malus domestica Borkh 'Cripps Pink') are poorly understood. To investigate this disorder, Pink Lady apples were stored in air or controlled atmosphere (CA) with 1.5 kPa O2 and 5 kPa CO2 at 0.5 °C for 2 and 4 months in 2004 and 2005. Following CA storage, fruit were separated into two categories, damaged (FB) and undamaged tissue from each of the categories was studied separately. Cell viability studies revealed that the cells were dead in the brown tissue of damaged apples. All healthy tissue in the same apples contained viable cells. Both brown and surrounding healthy tissues in apples with FB showed a decrease in ascorbic acid and an increase in dehydroascorbic acid during the first 2 months of storage in CA, the time period when FB developed. Undamaged, CA-stored apples retained a higher concentration of ascorbic acid after 2 months in storage. The level of hydrogen peroxide (H2O2) increased more in the flesh of CA-stored apples than in air-stored apples, an indication of tissue stress. In addition, concentrations of H2O2 were significantly lower in diphenylamine (DPA)-treated apples. Treatment with DPA also inhibited FB completely compared to untreated apples. Polyphenol oxidase (PPO) activity was similar for apples kept in air or CA storage and between undamaged and damaged fruit. The results showed a closer association between FB and the oxidant-antioxidant mechanisms such as ascorbic acid, H2O2 and DPA, than to the activity of specific browning enzymes like PPO. Further investigation of the protective effect of ascorbic acid is warranted as is further research on the underlying causes of apple fruit susceptibility to FB.