State-of-the-art CA storage facilities that maintain the most appropriate storage conditions exist in most apple-producing countries. Johnson (1999a) 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.
With the loss of DPA, the commercial development of Dynamic controlled atmosphere (DCA) systems has allowed establishment of lower oxygen concentrations below 1% and where the lowest oxygen limits (LOL) of fruit consignments are measured using a feedback system that ventilates the store with air in response to detection of either fermentation products (ethanol) in the atmosphere or changes in the ratio of CO2 produced/O2 consumed, referred to as the respiration quotient (RQ). Moreover, other physical indicators such as chlorophyll fluorescence, which measures the magnitude of fruit stress, have also been used to detect apple’s lowest oxygen limit of fruit.
In the Netherlands, a system to measure the concentration of alcohol in a representative sample of apples (Schouten et al. , 1997) has been developed and is marketed under the brand Storex, While HarvestWatch (Isocell); a fluorescence based sensor system is used extensively across Europe and South America to store apples at low oxygen (<1.0% O2) and to measure fruits’ response to decreasing oxygen concentrations. Commercialisation of technologies that set out to check the Respiratory Quotient (RQ) of apples have been marketed by SCS as SafePod in the UK and USA, measuring the RQ and changes in fruit respiration on a 70 kg sub-sample of fruit, by lowering the O2 concentration within the SafePod, allowing a safe oxygen limit above the LOL to be set for the whole store, without endangering the whole store. RQ measurements are also used on a whole store basis in the Van Amerongen DCA system.
Maintaining the oxygen below 1% may not be suitable for all apple cultivars. So far the best results have been achieved in reducing the severity and delaying the onset of superficial scald, in the cultivars Bramley’s Seedling, Golden Delicious and Granny Smith and reducing the incidence of internal browning in cultivars such as Braeburn. The concentration of CO2 used in DCA or Intelligent Controlled Systems will need to be determined empiricially, but often is reduced in proportion to the decrease in O2.
When considering the use of systems which lower oxygen concentrations to near the threshold of anaerobic respiration, it is imporant to ensure that the sensing and CA equipment is calibrated and tested regularly to prevent accidental damage to fruit.
In the UK, earlier experimental work with reducing superficial scald on Bramley for 8-10 months by storing in 5% CO2, 0.4-0.6% O2 has been replicated in commercial stores. An important consideration is the length of time apples remain in refrigerated storage after the CA atmosphere has been broken. If a delay of 2-3 weeks occurs before sufficient orders have been received before the store is emptied, then there is a chance that scald may start to develop. This needs to be factored in when choosing the type of technology.
Work in the Netherlands 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.