Controlled Atmosphere Storage Guidelines and Recommendations for Apples
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Harvesting Apples at Optimum Maturity
For successful controlled atmosphere (CA) storage, harvest apples when they are physiologically mature but not ripe. Harvest each cultivar at the proper maturity to achieve maximum storage life and marketing season. Apples harvested too early are of poor colour and small size and have little flavour. They may fail to ripen or ripen abnormally, and the overall quality will be poor. Characteristics of immature apples that contribute to inadequate flavour development include high water loss, low sugar content, high acidity, low aroma volatile production and high starch content. Immature apples are also more likely to develop storage disorders such as 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-flavours and are more susceptible to watercore and internal breakdown.
For these reasons, determining optimum apple maturity for harvest is essential for maximizing 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 for a given cultivar provides an approximate date of harvest maturity. Confirm the date 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.
Complete harvest for long-term storage before 20% of the apples have an IEC greater 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). It is important to note that not all apples mature and ripen in the same manner each year. 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
Segregate apples into lots at harvest by their storage potential. The following types of apples are not suitable for long-term storage because of their potential for internal breakdown (or developing bitter pit):
After harvest, cool the apples as rapidly as possible. Fruit off the tree mature much faster; with warmer temperatures, fruit begin to ripen sooner. Try to get the harvest from each day into the cooler by nightfall without straining the capacity of your cooling system to the detriment of apples already pre-cooled and in storage.
When using CA storage, the quicker the apples are cooled and the desired atmosphere is achieved, 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 will be. The objective should be to cool the apples and achieve the desired atmosphere within 5 days of initial harvest.
CA storage will not improve fruit quality - place only the best fruit 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 with harvesting apples at the proper maturity, followed by rapid cooling and establishment of the CA, then proper maintenance of the desired temperature and atmosphere. In general, the standard CA recommendations range between 2.5%-3% O2 and 2.5%-4.5% CO2 at 0°C-3°C. Due to recent research using new storage technologies and strategies, cultivar-specific CA recommendations have been reviewed.
Table 1 lists CA storage recommendations for commercial apple cultivars in 2012, including storage atmospheres, temperature and approximate storage-life.
= 1-methylcyclopropene (1-MCP), an ethylene inhibitor.
Note: Not all low-oxygen regimes have been tested in combination with SmartFresh treatment.
This Factsheet was authored by Dr. Jennifer R. DeEll, Fresh Market Quality Program Lead, OMAFRA, Simcoe. It was reviewed by Leslie Huffman, Apple Specialist, OMAFRA, Harrow.
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