State-of-the-art CA storage facilities that maintain the most appropriate storage conditions exist in most apple-producing countries. Johnson (1999a) recently reviewed the development of CA storage for apples in the UK. He felt that the recommended temperatures and atmospheres of carbon dioxide and oxygen were approaching the limits tolerated by stored apples and that it was unlikely that there would be benefit from further adjustment of conditions without risking damage to the fruit. However, it was recognised that future recommendations would be sensitive to particular demands by consumers and retailers.
In the current situation of a world over-supply of apples there is a greater emphasis on eating quality, particular texture, than on the duration of storage. Future recommendations may need to promote other quality attributes, for example flavour or health properties such as antioxidant content. Recognising the desire to reduce chemical inputs such as post-harvest treatment with fungicides or diphenylamine (DPA), it may be necessary to adopt the most appropriate conditions to control fungal rotting and superficial scald respectively.
There may be further development of ‘adaptive’ or ‘dynamic’ CA systems where the lowest possible oxygen concentration is maintained using a feedback system which ventilates the store with air in response to the detection of fermentation products (ethanol) in the atmosphere or changes in the calculated respiration rate or other physiological indicators such as chlorophyll fluorescence.
Recent work in Holland has demonstrated improved quality in Elstar apples using a hand-operated dynamic control system based on ethanol detection (Schouten et al., 1997). The oxygen concentrations in some commercial apple stores in the South Tyrol region of Italy are being controlled using a chlorophyll fluorescence sensor to detect low-oxygen stress (Zanella et al., 2008). Similar systems based on respiratory responses improved the firmness of Cox apples compared with conventionally controlled ultra-low oxygen (ULO) conditions (Jameson, 1997).
Although dynamic or adaptive control offers the prospect of tailoring the store atmosphere to suit particular consignments of fruit it cannot be assumed that optimum quality will be achieved in all cultivars using these methods.
- Cox apples are susceptible to late storage corking and McIntosh apples may develop corky flesh browning when kept under ULO conditions for prolonged periods.
- Dynamic or adaptive CA is an exciting prospect for the control of scald on Bramley apples without the use of DPA. Bramley can tolerate oxygen levels as low as 0.4% but not for the entire storage period. With dynamic or adaptive control, oxygen concentration could be maintained below the current recommended minimum (1.0% O2) and varied according to the oxygen demand of the fruit. Significant extension of the scald-free period for fruit not treated with DPA is anticipated using this approach.
Further research requirements
1. There will be a continued need to provide storage recommendations for existing and new cultivars of apple that enable growers to achieve the quality and period of supply demanded by consumers. Work is currently underway to refine the storage recommendations for Braeburn apples with the objective of overcoming problems with core flush that currently prevents storage until late April. There is a need to fully evaluate the storage potential of ‘club’ varieties being introduced into the UK. Robust recommendations are likely to be generated more effectively through controlled factorial experiments as opposed to ‘ad hoc’ treatments applied using of commercial stores. Existing recommendations may need to be modified in line with expected changes in quality requirements that currently are directed more towards health benefits in addition to sensory fulfilment.
2. Further work is required to test the robustness of dynamic or adaptive control techniques in UK apples from a range of orchards produced in different years. Work should focus on Bramley since there is major opportunity to control scald without the use of DPA or SmartFreshTM.