Madeline Wimmer, UMN Extension educator and Cindy Tong, UMN Professor & Extension postharvest horticulturist
The Minnesota apple industry primarily sells apples through direct sales at farm stands, farmers markets, community supported agriculture (CSA), and agritourism. For example, it’s not uncommon for an apple orchard visit in the fall to be routine for elementary school classes, or for families to make a trip to an orchard on a Saturday afternoon to enjoy apples, pumpkins, and festivities. Thus, most apples grown in Minnesota aren’t stored for very long; but even short term storage requires some consideration in regards to how quickly apples are cooled, as well as choosing the optimum final storage temperature to prevent storage disorders.
Storage disorders are physiological issues that result from interactions between the storage environment and metabolic changes within the apples. Some apple varieties are more susceptible to storage disorders than others, and the quality of an apple going into storage matters, too. For example, we mentioned in a recent fruit update that there has been a larger than normal amount of apple russeting this year, which makes apples less resilient in storage because the skin (i.e., exocarp) has been disrupted.
Below is a table showing four popular University of Minnesota apple varieties, which storage disorders they are susceptible to, and how long they can be stored at 32° and 40° F.
Data from Cindy Tong, UMN Professor & Extension postharvest horticulturist.
Diffuse flesh browning: A condition in which a brown discoloration spreads throughout the apple flesh. It results from apples being stored in temperatures too cold for what the apple can tolerate, and is more likely to occur for larger apple varieties. Some research has indicated that large swings in orchard temperatures can increase the risk of diffuse flesh browning (1).
Lenticel breakdown: Lenticels are small openings that allow for gas exchange on the surface of apples. They are similar to stomata on leaves and can be thought of as somewhat similar to pores on our skin. Lenticel breakdown can have a similar appearance to bitterpit when looking at the outside of an apple, although lenticel breakdown does not extend as much into the apple flesh. It is also more likely to show up on the shaded side of apples. This issue tends to be more problematic for long-term storage.
Additional reading:
Plain talk about lenticel breakdown (Good Fruit Grower)
Soft scald: Similar to diffuse flesh browning, soft scald occurs when apples are stored in a temperature that is colder than they can tolerate. Unlike diffuse flesh browning, soft scald begins on the apple skin, but can also extend into the flesh. Honeycrisp and McIntosh are two varieties that are susceptible to soft scald.
Soggy breakdown: This cold-induced issue is very signature to Honeycrisp in particular and looks similar to diffuse flesh browning, but it has more defined borders between healthy and affected parts of the apple flesh. Symptoms can look like spongy, small brown spots within the apple flesh.
Images: A collection of apple disorders caused by or worsened by storage conditions. This includes 1) soft scald and 2) internal flesh browning, which result from storage temperatures that are colder than an apple can tolerate, as well as 2) bitter pit—a physiologically disorder related to calcium use in the apple fruit—which can start in the field, and worsen in storage.
The Minnesota apple industry primarily sells apples through direct sales at farm stands, farmers markets, community supported agriculture (CSA), and agritourism. For example, it’s not uncommon for an apple orchard visit in the fall to be routine for elementary school classes, or for families to make a trip to an orchard on a Saturday afternoon to enjoy apples, pumpkins, and festivities. Thus, most apples grown in Minnesota aren’t stored for very long; but even short term storage requires some consideration in regards to how quickly apples are cooled, as well as choosing the optimum final storage temperature to prevent storage disorders.
Storage disorders are physiological issues that result from interactions between the storage environment and metabolic changes within the apples. Some apple varieties are more susceptible to storage disorders than others, and the quality of an apple going into storage matters, too. For example, we mentioned in a recent fruit update that there has been a larger than normal amount of apple russeting this year, which makes apples less resilient in storage because the skin (i.e., exocarp) has been disrupted.
Below is a table showing four popular University of Minnesota apple varieties, which storage disorders they are susceptible to, and how long they can be stored at 32° and 40° F.
Data from Cindy Tong, UMN Professor & Extension postharvest horticulturist.
Refer to the listings below to understand more about each storage disorder:
Bitter pit: Bitter pit is a condition related to cell wall breakdown due to issues with inadequate calcium getting into the fruit. Symptoms can show up when apples are still growing on the tree and worsen in storage. There are a few ways that growers can try to manage bitter pit prone apple varieties like Honeycrisp, most of which was covered in our final 2024 fruit update, for reference.Diffuse flesh browning: A condition in which a brown discoloration spreads throughout the apple flesh. It results from apples being stored in temperatures too cold for what the apple can tolerate, and is more likely to occur for larger apple varieties. Some research has indicated that large swings in orchard temperatures can increase the risk of diffuse flesh browning (1).
Lenticel breakdown: Lenticels are small openings that allow for gas exchange on the surface of apples. They are similar to stomata on leaves and can be thought of as somewhat similar to pores on our skin. Lenticel breakdown can have a similar appearance to bitterpit when looking at the outside of an apple, although lenticel breakdown does not extend as much into the apple flesh. It is also more likely to show up on the shaded side of apples. This issue tends to be more problematic for long-term storage.
Additional reading:
Plain talk about lenticel breakdown (Good Fruit Grower)
Soft scald: Similar to diffuse flesh browning, soft scald occurs when apples are stored in a temperature that is colder than they can tolerate. Unlike diffuse flesh browning, soft scald begins on the apple skin, but can also extend into the flesh. Honeycrisp and McIntosh are two varieties that are susceptible to soft scald.
Soggy breakdown: This cold-induced issue is very signature to Honeycrisp in particular and looks similar to diffuse flesh browning, but it has more defined borders between healthy and affected parts of the apple flesh. Symptoms can look like spongy, small brown spots within the apple flesh.
Other storage considerations:
- Humidity: Having adequate moisture levels (i.e., relative humidity levels between 90-95%) can help prevent apples from dehydrating over time. This is especially important for growers investing in longer storage periods.
- Excluding diseases: Be sure to routinely clean storage bins and environments to prevent the development of diseases like blue mold (Penicillium sp.) in storage.
- Monitor stored apples and equipment: Check in on stored apples regularly and check the temperatures and relative humidity to make sure everything is working properly.
- Harvest at apples for the anticipated storage duration: Apples being stored for longer periods of time will do better harvested slightly earlier than those planned for short-term storage. This should be based on the iodine-starch test, where apples can be harvested between 3-4 on the Cornell scale when anticipating longer storage periods.
- Stepwise cooling: Some apple varieties that are very prone to cold damage can benefit from first conditioning apples to 50° F and gradually cooling the apples down to the final storage temperature (37-39° F).
- Tong, C. B., Chang, H., Boldt, J. K., Ma, Y. B., DeEll, J. R., Moran, R. E., Bourgeois, G., & Plouffe, D. (2016). Diffuse Flesh Browning in ‘Honeycrisp’ Apple Fruit is Associated with Low Temperatures during Fruit Growth. HortScience, 51(10), 1256-1264. Retrieved Sep 18, 2024, from https://doi.org/10.21273/HORTSCI11179-16
Photo credit: Photos 1 and 3 under images are credited to Cindy Tong.
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