CA Storage Guidelines and Recommendations for Apples - 2003

Harvesting Apples at Optimum Maturity

For successful controlled atmosphere (CA) storage, apples must be harvested when they are physiologically mature but not ripe. Each cultivar must be harvested at the proper maturity in order to achieve maximum storage life and marketing season. If apples are harvested too early, they are of poor color, small size and have little flavor, they will fail to ripen or may ripen abnormally, and the overall quality will be poor. High water loss, low sugar content, high acidity, low aroma volatile production, and high starch content are characteristics of immature apples that contribute to inadequate flavor development. Immature apples are also more likely to develop storage disorders like superficial scald and bitter pit. Harvesting apples too late can result in a short storage life. Such apples are too soft for long-term CA storage, and are more susceptible to mechanical injury and disease infection. Over-mature apples may develop poor eating quality and off-flavors, and are more susceptible to watercore and internal breakdown.

For the above reasons, the determination of optimum apple maturity for harvest is essential for maximum storage life and quality, while minimizing postharvest losses. Numerous methods have been suggested for determining harvest date, but no single test is completely satisfactory, and some are too unpredictable, complicated or expensive. Days after full bloom is generally fairly constant, but can vary in any one year. Therefore, days after full bloom should be used as a general reference to indicate the approximate date when apples might reach harvest maturity, which is then confirmed using tests such as internal ethylene concentration (IEC), starch-iodine staining, flesh firmness, and soluble solids content (sugars). In general, an IEC of 1 ppm is considered to be the ultimate threshold above which fruit ripening and flesh softening are initiated and progress rapidly. Harvest for long-term storage should be completed before 20% of the apples have an IEC higher than 0.2 ppm. Using the starch-iodine test, apples destined for long-term storage should have 100% of the core tissue starch degraded (no stain) with greater than 60% of the flesh tissue still having starch present (stain). However, it is important to note that not all apples mature and ripen in the same manner each year, and often there will be a need to compromise between correct maturity and the required firmness and sugar levels for market.

Guidelines for Placing Apples into CA Storage

Apples can be segregated into lots at harvest by their storage potential. The following types of apples should not be designated for long-term storage because of their potential for internal breakdown (or to develop bitter pit): (1) large fruit from lightly cropped trees, (2) fruit from excessively vigorous trees, (3) fruit from young trees just coming into bearing, (4) fruit from interior portions of trees that are heavily shaded, (5) early-picked fruit high in starch, and (6) fruit with a low number of seeds (< 5 per fruit).

After harvest, cool the apples as rapidly as possible. Fruit off the tree mature much faster and begin to ripen sooner the warmer the temperature. Try to get the harvest from each day into the cooler by nightfall. However, do not strain the capacity of your cooling system to the detriment of apples already pre-cooled and in storage, or the entire lot. When using CA storage, the quicker you cool the apples and achieve the desired atmosphere, the longer the apples will store and be of good quality upon removal. The longer it takes to adjust the oxygen (O₂) and carbon dioxide (CO₂) levels, the less effective the length of storage. The objective should be to cool the apples and achieve the desired atmosphere within 5-7 days from initial harvest.

CA storage will not improve fruit quality, and thus only the best fruit should be placed in CA storage. If over- or under-mature or poor-quality apples are put into CA, the result will be poor-quality apples upon removal. Successful CA storage begins by harvesting apples at the proper maturity, followed by rapid cooling and establishment of the CA, and then proper maintenance of the desired temperature and atmosphere thereafter. In general, the current standard CA recommendations range between 2.5-3% O₂ and 2.5-4.5% CO₂ at 0-3^oC. However, due to recent research using new storage technologies and strategies, these current CA recommendations needed to be reviewed. The following table lists CA storage recommendations for commercial apple cultivars in 2003, including storage atmospheres, temperature, and approximate storage-life.

1 SmartFresh^™ (1-methylcyclopropene; 1-MCP) is currently not registered in Canada; it significantly improves firmness retention and extends storage life; it may alter the requirement for current O2/CO₂ levels in CA

2 DPA (diphenylamine) drench applied cosmetically to help control CO₂ injury.

3 Not ‘Marshall’ McIntosh, this strain is low O₂ sensitive and may develop low O₂ injury.