Considerations when storing apples in converted storage space
Finding enough storage space for an abundant apple crop may be challenging but using converted storage space may be an option.
This year has been excellent for the Michigan apple crop. Minimal freeze damage this spring and mild weather conditions has led to more apples than estimated for the 2022 growing season. For best apple quality, cold storage and controlled atmosphere storage are still recommended. However, with so many apples, storage space may be difficult find for fruit destined for fresh and processing markets.
How can we store extra apples?
One option may be to convert general storage space to apple storage space. The key thing to remember is that fruit quality is highly affected by temperature and humidity. General temperature recommendations for mature apples range from 32-39 degrees Fahrenheit. While temperatures may vary depending on the type of space being converted, humidity should be maintained around 95% to prevent fruit dehydration.
Maintaining fruit quality will also differ by variety, so storage operators should consider which varieties are best suited for each storage situation. Pressure testing with a penetrometer is a useful tool to assess the status of fruit before and during storage. Handheld penetrometers are available from multiple sources. Apples are tested with a portion of the peel removed using an 11-millimeter diameter plunger. General guidelines for recommended pressures are available in the Michigan State University Extension (MSU Extension) article, “Checking apple maturity: What to look for.”
Ambient temperature storage can be used to store apples for a short time. Ambient storage refers to storage that is the same temperature as the environment, as opposed to climate-controlled storage, which uses meticulously controlled, forced-cooled air. Cool fall temperatures will help keep apples longer in ambient temperature storage.
An important consideration for ambient temperature storage will be to maintain temperature so fruit stay cool during the day but do not freeze if outside temperatures dip below freezing at night. Air circulation is also important to cool fruit and maintain temperature. Storage operators could use fans to circulate air through the storage space. In addition, fans could also be used to blow cool night air from outside into the storage space to keep fruit cool during the day.
According to a North Carolina State publication, fruit degradation increases with increasing temperature. Apples stored at 40 F will degrade twice as fast as apples stored at 32 F. Apples stored at 60 F will degrade six times faster than apples stored at 32 F.
Fruit best suited for ambient temperature storage may be later season processing varieties such as Idared, Red Delicious, Rome, Fuji and EverCrisp. Varieties such as Fuji, Red Delicious or Evercrisp that are prone to watercore may benefit from one-to-two-day storage at ambient temperature before refrigerated storage to reduce watercore tendencies.
The preharvest fungicide program should also be considered for fruit. If preharvest fungicide protection against the sooty blotch and flyspeck disease complex was weak, these diseases can continue to develop in storage. If fungal symptoms or signs of decay are present on fruit at harvest time, the storage duration should be short.
Ambient temperature storage with 1-MCP may also be an option for some storage operators. In 2001, Randy Beaudry, professor of horticulture at MSU, conducted research on ‘Redchief Delicious’ to test the effects of storage temperature and frequency of 1-MCP application on fruit firmness and quality. Fruit were stored at 32 F, 41 F, 50 F, 59 F and 68 F. The 1-MCP treatments were applied twice a week, once a week, once a month or once a year.
For ‘Redchief Delicious’, one application of 1-MCP generally maintained fruit firmness (16 – 16.5 pounds) for about 50 days after harvest at all storage temperatures. Fruit remained firmer with more frequent applications of 1-MCP, and fruit firmness was highest with weekly applications of 1-MCP (firmer than fruit stored in controlled atmosphere for six months).
- Delayed apple harvest can lead to faster fruit softening in storage even with 1-MCP treatment.
- 1-MCP treatment is a possibility, but the fruit will need to be confined to a relatively airtight environment to contain the released gas. MSU Extension recommends several options for 1-MCP sourcing.
- Using 1-MCP-releasing sachets in a non-sealed environment and/or in an environment in which the fruit occupies less than half of the volume of the room is not advised. Treatment of 1-MCP in trailers and pallet overwraps has been successful if no dedicated room is available.
- Managing air temperature in the cold room by circulating ambient air into the room should not be done during the 1-MCP treatment as the treatment will not be effective if the 1-MCP in the interior atmosphere air is diluted with outside air. Resume cooling after the 24-hour 1-MCP treatment if ambient air circulation is being used.
- Monitor fruit quality regularly for firmness and post-harvest rot. Samples for later quality analysis should be set aside at the time of room filling. Storage at higher temperatures leads to more fruit rot, although 1-MCP treatment slows fruit senescence and senescence-related decay.
- Any space that stores fruit should comply with food safety requirements.