The two critical environmental factors involved in properly storing potatoes and preventing common potato diseases are temperature and humidity. Adequate, unrestricted air movement is also necessary to maintain constant temperature and humidity throughout the storage pile—and to prevent excessive shrinkage from moisture loss and decay.
With some help from our friends at Industrial Ventilation, Inc. (IVI) and utilizing information from studies performed by University of California Cooperative Extension, we’ve put together these tips for making sure your storage is a happy place for your potatoes. More helpful storage information can be found at www.ivi-air.com/ivi-blog.
1. Dial in on temperature control
Several factors influence temperature management for stored potatoes:
- Whether the potatoes are to be marketed for processing, table stock or seed
- Tuber temperature at the time they are placed in storage
- The length of time tubers are to be stored
- The extent of tuber damage at harvest
- The presence of disease organisms
Low temperatures enhance sugar formation. If the storage temperature is maintained below 45 degrees for a long time, accumulated sugars in the tuber do not readily reconvert to starch (recondition). High-reducing sugar concentrations result in undesirably darkened chips and fries. Some varieties recondition better from low temperatures than others.
In general, the optimum long-term storage temperature for processing potatoes is approximately 45 degrees. For fresh market potatoes, a storage temperature of 40 degrees is recommended. Seed potatoes may be stored at slightly lower temperatures (38 to 40 degrees) for better weight loss and sprout control.
Raise the temperature just prior to storage removal. Cold potatoes are brittle and may easily bruise or shatter during handling. Before removing the tubers from storage, raise the temperature to 50 to 55 degrees. This can sometimes be accomplished by removing ventilation, which allows the heat from tuber respiration to accumulate. More frequently, however, heaters and/or forced ventilation of warm air are necessary.
2. Wound healing
Bruising and cuts can occur during harvest and handling. Wound healing is critical in order to minimize the entry areas for ever-threatening organisms of potato diseases. Temperature is critical at this phase of initial storage, as healing proceeds most rapidly at temperatures of 60 to 65 degrees Fahrenheit. In order to achieve the healing of any wounds, this temperature, plus high humidity, facilitates the forming of a suberin layer in three to five days. Cell regeneration, or complete wound healing, can take place beneath this suberin layer within 10 to 20 days, depending on the condition of the potatoes.
When potatoes are placed in storage, rapid cooling can result in more rot-forming organisms in the tuber because entry points remain unhealed. Rapid cooling, particularly in the absence of adequate air, can also result in blackspot and other potato damage.
3. Stop disease organism growth
Most common potato disease organisms increase their population growth at temperatures ranging from 40 to 80 degrees. Lower temperatures lower the possibility of disease incidence during storage. Since many of the common potato storage diseases naturally occur in the soil, they are transported into storage with the tubers. It is essential, therefore, to not only heal the exposed wound areas to minimize pathogen penetration, but also to lower the temperature as soon as possible after storing to minimize disease spread within infected tubers as well as from infected to sound tubers.
This fact emphasizes the importance of careful harvesting and handling techniques to minimize bruising, skinning and cutting. If diseases or field frost damage are evident in tubers, rapid lowering of temperatures, even without the healing and curing period, may be necessary to prevent breakdown. However, do not store these damaged potatoes for long periods of time.
4. Avoid respiration and sprouting
The potato tuber reacts like any living organism; respiration increases with increasing temperatures. When respiration increases, so does tuber weight loss. Since lower temperatures also maintain dormancy, temperatures need to be as low as possible without otherwise decreasing quality (e.g., sugar buildup in processing potatoes). For long storage periods of three to five months, depending on the variety, a sprout inhibitor is practical for potatoes not meant to be used for seed.
5. Humidity and air movement
Maintain a 95 percent relative humidity at all times. High humidity is essential for optimum wound healing during the curing period. It is also essential during the entire storage period to minimize tuber weight loss. At relative humidity levels below 90 percent, weight loss rapidly increases.
Air movement may not be necessary during the curing period because the heat and moisture generated by the potatoes may provide an favorable environment for wound healing. Do not allow condensation to form during storage. If it does occur, air circulation may be necessary. Thereafter, air movement may be required to maintain the desired temperature and humidity throughout the storage pile. This may also require passing humidified air and/or heated or refrigerated air through the pile.
Between 10 and 20 cfm (cubic feet per minute) of air per ton of potatoes is usually optimum, but the exact amount depends on the temperature of the outside air being brought in—warmer temperatures make more air necessary. The primary function of air movement is to maintain a uniform temperature and humidity. Intermittent air movement can also accomplish this goal. The key is continual, accurate monitoring.
Dirt and clods are a problem that can seriously affect air movement through the storage pile. They tend to form a solid barrier that slows down air movement and prevents air from reaching some areas of the pile. Remove as many as possible before potatoes go into storage.