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In the northern production areas a large percentage of the potato crop is stored for processing and fresh market through the winter and spring months. Many of the older storages were of the dugout type, partially below ground without any ventilation or humidity and temperature control. Many modern potato storages are entirely above ground, of concrete or corrugated metal exterior, insulated and equipped with a system of temperature, humidity, and atmospheric control. Sizes range from 500 ton to over 20,000 ton capacity. In some production areas, the trend is away from small storages located on individual farms. With the expansion of the processing industry, particularly in the northwestern U.S., many of the larger storages are clustered near processing plants where better control of storage conditions can be maintained and season long accessibility to the stored crop is assured. With the increased consumption of processed potatoes and a resultant decrease in fresh market sales, storage conditions have become more critical. Mechanization in handling, both placement into and removal of potatoes from storage, has necessitated larger structures without obstructions such as upright supports within the storage area.
The design of storage structures varies considerably, primarily in methods of air distribution. Large air plenums are located on either or both sides, or in the middle with ducts running across or lengthwise, depending on shape and size of the storage. A common type of duct is galvanized corrugated pipe located above ground, although air ducts can be located below ground. Dirt floors are the most popular although many storages have concrete floors with wood-covered flumes in the center. The wooden covers can be removed as potatoes are flumed out. Moisture proof barriers with insulation are required on the walls and ceilings, the thickness of the insulation varying with storage location. In most of the northern production areas above the 43rd parallel and in areas of high elevation, a minimum of 2 inches (5 cm) of urethane or its insulating equivalent on walls and 3 1/2 inches (9 cm) or its equivalent on the ceilings is required. Because high humidity is required for potatoes in storage, condensation of water on the ceiling sometimes occurs. Proper design, adequate insulation, and air flow directed across the ceiling keep this problem to a minimum. Also the placement of heat cables along the eaves on the inside of the storage can reduce, if not eliminate, these problems. Requirements for storage in a specific geographic area should be obtained from the agricultural resource people in that area.
Physiological and Biochemical Changes in Storage
The objective of the storage environment is to keep the deterioration of the external and internal quality of potato tubers to a minimum. The quality of potatoes coming out of storage will not be any better than the quality of the potatoes placed into storage. Storage ability of potatoes is influenced considerably by production and harvesting practices. The maintenance of potato quality in storage is enhanced by care in growing, harvesting, and placement of tubers into storage.
The storage period can be divided into three periods. The first is the curing period or the period of suberization and maturation. The second, more lengthy phase, is the holding period, and the third is the warming period before removal. During the curing period, bruises incurred during the harvest operation are suberized to prevent entry of rot organisms and immature tubers are allowed to mature (set and thicken the periderm). Temperatures of 50° to 60°F (10-15.6°C) are used, depending upon eventual use of the tubers, variety, relative maturity. and conditions of growth. Curing temperatures of approximately 50°F (10°C) are used when most of the potatoes are processed into frozen French fries, made into dehydrated products. or sold on the fresh market. Where potatoes are processed into chips, higher curing and holding temperatures may be used because of differences in varieties and a lower maximum allowance of reducing sugars. Length of the curing period is generally one to two weeks. Relative humidity above 95 percent is a necessity for the suberization process and to keep weight loss to a minimum.
Proper curing of tubers is necessary to ensure storage without loss of quality. In relatively warm fall areas a considerable amount of field heat as well as heat of respiration must be dissipated, requiring forced ventilation with cool night air. Removing heat is necessary to keep rot to a minimum and to keep tubers from physiologically aging. which eventually results in greater development of reducing sugar and premature sprouting. If tubers are relatively immature, they should be kept at curing temperatures for longer periods to ensure maturation of the skin for minimum weight loss.
Tubers are kept at holding temperatures of between 40 and 50°F (4.4-10°C) depending on ultimate use. Chip potatoes are generally stored at a minimum of 50°F (10°C) while those used for making French fries are kept at 45°F (7.3°C) and fresh market and seed potatoes are maintained near 40°F (4.4°C). The accumulation of reducing sugars and deterioration of texture preclude storage of processing potatoes at low temperatures. Constant storage temperatures are more desirable than fluctuating temperatures. High relative humidity is desirable to keep weight loss to a minimum and to prevent the occurrence of pressure bruises.
When potatoes have been kept at low holding temperatures, they should be warmed to approximately 50°F (10°C), before removal from storage to reduce bruising. When excessive reducing sugars have accumulated in storage, the amount of sugar can be reduced by holding the tubers at higher temperatures for 3 weeks or longer.
Most varieties of potatoes are in the resting stage for two to three months after harvest. To prevent sprout development in potatoes stored longer than this, chemical sprout inhibitors are used. The two most common inhibitors are maleic hydrazide (MH) and chlorpropham (CIPC). MH is applied to the green foliage in the field two to three weeks after full bloom stage. The inhibitor is translocated to the tubers and will keep potatoes sproutfree through the storage season. CIPC is applied through the ventilation system in an aerosol form after the period of suberization and maturation, usually in December.
Ventilation and Humidity Control
The primary purpose of ventilating potato storages is to remove field heat and heat of respiration. Forced movement of air through potatoes also maintains uniform temperature and humidity throughout the pile. A good ventilation system can also be used to control progress of wet rots by drying out areas of the pile which look wet as well as drying tubers which are harvested in a wet condition. Excessive accumulation of carbon dioxide especially during the high respiration period early in the storage period can be prevented by adequate ventilation.
Air distribution through potato piles is attained by various methods according to areas of production. Air ducts are spaced from eight to ten feet apart beneath the pile. These ducts are made of various materials. Corrugated steel or aluminum pipes of different sizes according to size of storage with outlet holes spaced from 8 to 12 inches (20-31 cm) apart are used in some areas. In other areas, wooden A-frame slotted air ducts are used. The air is distributed to lateral ducts by a main plenum connected to the fan house. Variations of this basic distribution system exist in the various producing areas.
The capacity of ventilation systems varies depending on the availability of low night temperatures for cooling. The variation in capacity is from 0.5 cfm/cwt to 2.4 cfm/cwt. Higher air capacities are sometimes used to control soft rot and for drying tubers which were harvested wet. After the initial cooling period. high air velocities are not generally required.
In arid climates the addition of moisture to air used for cooling is a necessity. particularly during the initial cooling period. Evaporation of moisture in the air has the additional benefit of increasing cooling capacity. Many storages have small centrifugal type humidifiers which distribute a fine mist into the air stream. Generally this type of humidifier is inadequate when large amounts of cool air are brought in during the cooling period. Large air washers are becoming more popular and provide more adequate humidification. Refrigerated water chiller units can give additional cooling capacity to air washers if necessary. Refrigerated storages are used to keep potatoes for prolonged periods into the early summer months.