Storage Cycle

Return to the Guide to Commercial Potato Production on the Canadian Prairies.

.Several distinct storage phases exist. The best management practices of each stage depend on tuber conditions, weather and the intended use of the crop.
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Pre-harvest period

Management over the entire growing season affects the storability of potatoes. Most storage problems usually start in the field before harvest begins. Growers should aim to place a mature, disease and bruise free crop into storage. Research has demonstrated that potatoes from healthy vines are much more resistant to storage decay than potatoes from vines that have been weakened from physiological stress or from foliar diseases, e.g. early or late blight. Good storage design and management practices help to lessen the effects of problem tubers, but storage will never improve a poor-quality crop. The storage should be prepared well in advance of harvest. Check all mechanical systems and clean and repair ducts. Use an accurate thermometer to check ventilation controls and check operation of the humidification system.

Thoroughly clean then disinfect the storage, handling and harvesting equipment with a quaternary ammonium compound such as Ag-Services Incorporated General Storage Disinfectant, Bardak 2210 Disinfectant Sanitizer, or DMR-23 Disinfectant. Since most disinfectants are inactivated by soil and plant debris, it is essential that this material be removed by thoroughly cleaning equipment and storage with a pressure washer or steam cleaner before the disinfectant is applied. Surfaces must remain wet for at least 10 minutes for the disinfectant to destroy disease organisms.

Harvest Period

Vine killing is not employed in all types of potato production (see section on Vine Killing) When vine killing, desiccate or flail far enough in advance of harvest to allow potato vines to dry as much as possible, and for the skin to set. Tubers with mature skin are more resistant to mechanical damage, bruising, shrink and pathogens (e.g. late blight, leak or pink rot), which cause tuber decay in storage.

Storage management is easier and the quality of potatoes coming out of storage will be better if steps are taken to reduce bruising, mechanical injury and infection from diseases at harvest. The ideal harvest temperature is between 50 and 59°F (10° and 15°C). To avoid shatter bruises, do not harvest when the tuber pulp temperature is less than 41°F (5°C). Tubers warmer than 64-68°F (18-20°C) and under drought stress are susceptible to black spot bruising. Harvesting when tuber pulp temperature exceeds 68°F (20°C) increases the risk of leak and pink rot diseases, which can result in extensive storage decay. Don’t bring severely frosted, chilled or diseased potatoes into storage. For more information see Reducing Harvest Damage in the section on Bruise Prevention and section 3.9.4 Special Storage Problems.

Post-harvest Curing Period

The greatest amount of shrink occurs after harvest and before curing is complete. Harvested potatoes are skinned and there is no barrier to moisture loss until suberin is formed over the wounds. This initial storage period promotes wound healing (suberization) and skin set, which are critical for long-term storage quality of potatoes. The temperature, relative humidity and length of the curing period are determined by the condition of the harvested potatoes. High humidity (95%) during the curing period is critical to prevent excessive shrinkage and to promote wound healing. Mature, healthy potatoes should be cured for about two weeks at 50-60°F (10-15°C) and 95% relative humidity.

Immature potatoes with high sugars should be cured for an extended period of time at 60°F (15°C) and 95% relative humidity until fry or chip colour is acceptable (see section Storing Chemically Immature Potatoes).

Frozen or rotting tubers should be cured at a lower temperature and relative humidity (see section 3.9.4 Special Storage Problems) and marketed as soon as possible.

The ventilation regime used during curing is determined by the cooling requirement and the need to provide fresh air (oxygen) to the tubers. Ventilate to maintain the pile at the desired suberization temperature. You may have to ventilate more frequently if there is condensation on the surface of the tubers. Sweating or condensation on surface tubers occurs when the upper tubers are cooler than those inside the pile. A small amount of free water is usually harmless, but any excess surface moisture will encourage soft rot. Continuous ventilation is recommended when condensation is present.

Cool tubers as quickly as possible from field temperature to curing temperature. Never cool with outside air that is significantly colder than the desired storage temperature, as tubers near the air ducts will be chilled. Plenum temperature should be no lower than 3-5°F (1.5-3°C) below tuber temperature.

Tubers that are harvested cold should be warmed to the desired curing temperature at a rate of about 2°F (1°C) per day, then suberized (see section Special Storage Problems). Warm air will cause condensation on the cool tubers, making them more susceptible to soft rot infection.

Following the curing period, the potatoes should be cooled to the long-term storage temperature at a rate of 4-5°F (2-3°C) per week. Rapid cooling can cause colour problems in processing potatoes.

Mid and Long-Term Storage

The objective of long-term storage is to maintain a consistent, ideal environment for the duration of the storage period. Long-term storage demands more critical control than short-term storage. Recommended storage temperatures depend upon crop condition, variety and intended end use. A 2°F (1°C) difference between pile top and bottom.

General recommendations for storage temperatures.

Seed potatoes		36-39°F	2-4°C
Table stock		38-41°F	3-5°C
Processing
	French fries	45-48F	7-9°C
	Shepody	48-50°F	9-10°C
	Chips	45-50°F	7-10°C

Ventilation can be either intermittent or continuous. When storing healthy mature tubers, ventilation at intervals of 8 - 12 hours per day at full airflow rates is sufficient to maintain pile temperature. However, continuous ventilation maintains more uniform storage temperatures, particularly when outdoor temperatures are extremely low. Reducing airflow through the pile can minimise weight loss in storage. One way of accomplishing this is to operate fewer fans.

Maintain a relative humidity of 95% or higher unless special storage disorders are present.

Marketing Period

Reconditioning is a storage procedure that improves chip or fry color. Reconditioning is accomplished by increasing storage temperatures to 50-64°F (10-18°C) for two to four weeks before marketing. The higher temperatures increases tuber respiration rate, which reduces sugar levels and improves chip or fry colour. Growers are advised to consult the processor prior to reconditioning as problems can develop when tubers are exposed to reconditioning temperatures. Higher storage temperatures increase shrinkage and rot, reduce specific gravity and may break dormancy of sprout-inhibited tubers. Potatoes may require immediate processing if serious storage decay develops. Reconditioned tubers should be processed within one month; otherwise a condition called irreversible senescence sweetening may occur. A storage facility containing several smaller (30,000 cwt or 1,360 t) bins is far more flexible with respect to reconditioning than a storage facility with only one or two 60,000 cwt (2,720 t) bins. Small bins can be individually reconditioned (warmed) prior to opening and moving.

Seed potatoes, marketed late in the season, should also be pre-warmed to break dormancy prior to planting and to reduce injury during handling.

Written by J. Holley