Storage and Processing

Contents: [ Storage Capacity | Preparing the Storage | Storage | Problem Potatoes | Frozen Processing |

Related WWW Sites ]

Storage Capacity, Temperature Conversions

The following are useful figures to bear in mind if your are building, operating or renting new storage space. Pile dimensions are expressed in feet.

• 1 cwt of potatoes occupies 2.3 cu.ft.
• 1 cu. ft. of storage holds 0.42 cwt of potatoes
• Total cwt of bulk stored potatoes = (pile length in feet x pile width x pile height) / 2.3
• °C = (°F - 32) / 1.8
• °F = (°C x 1.8) + 32

See also OnlineConversion.COM and Weights and Measures for additional values and conversions.

Preparing the Storage

The storage season should begin well before harvest. That is, all components of the storage should be examined, cleaned and repaired as needed and operated so as to establish a satisfactory storage environment before the first potato is introduced. Important steps in preparing the storage include these:

• Thoroughly clean the storage and machinery of all potato debris and excess dirt using a pressure washer and steam as needed.
• Disinfect equipment and storage structure with recommended materials (see below).
• Perform routine maintenance and repairs on ducts and structure as needed.
• Clean dust and dirt and sprout inhibitors from fan blades.
• Check dampers and louvers for free movement and functioning of limit switches.
• Be certain all motors are lubricated and working and that belts are in good condition.
• Check all thermostats, humidistats and controls.
• Wet the storage floor to help maintain high humidity if needed.
• Operate the entire system so as to humidify and precool the storage to 55-60F a few days before potatoes are introduced.

Materials shown in the following table are commonly recommended for disinfecting potato handling equipment and storage structures for both seed and commercial operations. Be certain to follow current label directions. In general, disinfect by wetting the surface with the product for 10 to 45 min and then thoroughly remove all residues with water.

Characteristics of Some Common Disinfectants

Comments:

Quarternary Ammonium Compounds -- Diluted solutions are relatively safe but concentrated form is poisonous. Slightly corrosive, use stainless.

Hypochlorites, 5.25% bleach -- Quick acting, inexpensive; caustic to skin and clothing. Use at 1:50 when mixing with water only. For maximum effectiveness, use 1 part 5.25% bleach: 200 parts water: 0.6 parts white vinegar. Very corrosive.

Iodine Compounds -- Not for internal use. Becomes ineffective as yellow-brown color is lost. Tamed iodophor compounds work best.

Phenolic Compounds -- Provide residual action. These compounds show "phenol" in the list of of ingredients.

Formaldehyde -- Use may be canceled. Produces irritating, choking fumes. Not generally recommended.

Copper Sulfate -- Not widely used; mostly for soaking crates and bags.

Storage

The purpose of potato storage is to maintain tuber quality and provide a uniform flow of tubers to fresh market and processing plants throughout fall, winter and spring. Good storage should prevent excessive dehydration, decay and sprouting. It should also prevent high sugar concentrations which result in dark colored fried products. A potato storage should have adequate insulation, outside waterproofing, inside vapor proofing, ventilation, air distribution, adequate humidification, and properly designed controls for precisely maintaining the storage atmosphere.

Temperature, humidity, and air movement are the most important environmental factors affecting storability. Temperature requirements are determined by the intended use of the potatoes. Tubers should always be kept in the dark since very small amounts of light will gradually cause greening. Lights should not be used more than absolutely necessary. Surface greening is due to chlorophyll formation and is harmless. However, its presence in potatoes is undesirable because of marketing restrictions and the fact that at times an alkaloid called solanine increases with the chlorophyll. Solanine and other glycoalkaloids cause potatoes to have a bitter, undesirable flavor. Greening develops slowly in the light at 40 F or below but develops rapidly at 68 F.

Potatoes are usually held in bulk piles 8 to 20 feet deep. Some are stored in pallet boxes for short periods. Pressure bruise and internal black spot are substantially lower with pallet storage but decay is often increased because of poor air circulation within boxes. Long-term pallet storage is not recommended. Because of the large number of cultivars grown, only general storage recommendations can be given here. Growing and harvesting conditions influence the behavior of potatoes in storage (See "Storing Problem Potatoes," below).

Early harvested potatoes are usually stored only briefly if at all. Such tubers are quite perishable and damage easily because of immature skins. Early potatoes free from serious bruising and decay can sometimes be held 4 to 5 months at 40 F for table use if they are cured 4 or 5 days at 55 to 65F to heal wounds before storage. However, early crops should usually be sold immediately because of poor storability and typically high early season prices.

Part of Oregon's early crop potatoes are chipped directly form the field. Holding these potatoes in cold storage even at moderate temperatures of 50 to 55 F for only a few days can cause excessive reducing sugar and undesirable dark chips.

Most late potatoes are stored. Storage temperature depends on crop use (see text below). A relative humidity of 90 to 95% is typically recommended regardless of temperature regimes. Most of the crop is held in non- refrigerated, common, air ventilated farm storages. In general, comments offered below relate specifically to common storage.

Curing and Wound Healing

Immediately after harvest, healthy potatoes should typically be cured by holding at 50 to 60 F and high relative humidity with good air movement for 10 to 14 days to permit suberization and wound periderm formation (healing of cuts and bruises). Although wound periderm formation is most rapid at about 70 F, lower temperatures are recommended to reduce decay. Curing reduces subsequent weight loss and decay by preventing the entry of Fusarium, soft rot and other decay organisms. The relative humidity should be about 95%.

Final Holding Conditions

After curing, potatoes for seed or table stock are held at 38 to 40 F. Respiration is minimal at 37 F and actually increases as the temperature either falls or rises. Storage at 38 to 40F is considered optimum for many cultivars for maximum storage life because sprout growth is absent or negligible, decay and shrinkage are low, and other losses are usually minimized. At temperatures below 38 F, potatoes are chilled and tend to become too sweet for most uses. Storage at 32 to 34F for 20 weeks or longer causes some cultivars to show mahogany browning, a symptom of chilling injury. Potatoes are easily damaged by freezing; therefore, low temperature storage permits little leeway if air circulation is inadequate to maintain uniform temperatures throughout the storage. Tubers can freeze at temperatures as high as 30 F. Most potatoes will remain dormant during 5 to 8 months of storage at 39 F. If longer storage is desired, as it often is with seed stock, potatoes should be stored at 36 to 38 F. A range of 40 to 50 is suggested for potatoes to be marketed by December, depending on intended usage.

Table stock: Storage at about 40 to 42 F with 95% relative humidity is recommended for most table stock potatoes. Under these conditions Russet Burbank tubers treated with a sprout inhibitor will remain sprout free and of high quality for 10 to 12 months. Most other varieties tend to have a shorter dormancy and store less well than Russet Burbank.

Potatoes that are particularly susceptible to fresh bruises or internal black spot are injured less if warmed to about 50 F before handling. However, the rise in temperature may be detrimental to other potatoes in the storage.

Frozen processing and chipping: The optimum temperature range for storing most cultivars of potatoes to be processed into chips or french fries is 45 to 55 F. Relative humidity should be 95% to minimize weight loss. Russet Burbank potatoes for frozen french fry processing are held at 45 F. Potatoes stored at much lower temperatures (i.e., 40 F or below) are seldom suitable for processing without first being reconditioned at higher temperature to reduce the quantity of reducing sugars. Reconditioning is accomplished by holding potatoes at about 65 to 70 F until trial cooking tests show that they have recovered sufficiently for use. The length of the conditioning period depends on the cultivar and on the amount of reducing sugars that have accumulated; usually it is 1 to 4 weeks. Some lots and cultivars of potatoes will not recondition satisfactorily.

Compared to chipping potatoes, slightly higher reducing sugar levels are acceptable in french fry stocks. Potatoes for frozen processing can usually be stored at temperatures of 42 to 45 F and fried without reconditioning or at 39 to 40F with reconditioning. Some varieties can be fried directly out of 39 to 43 storage. Desirable storage temperature depends not only on the variety but also on the particular method used by the processor.

Chipping potatoes are usually stored at temperatures approaching 50 F or even higher, depending on the variety, due to the need for very low reducing sugar levels. Because of these high temperatures and the fact that most chippers ae round whites with relatively thin skins, chipping potatoes are more prone to shriveling, sprouting and decay than most russets, and Russet Burbank in particular.

Processing from cold storage would minimize shrinkage and decay, reduce the need for sprout inhibitors and otherwise improve the quality of stored potatoes. Because of these cold-storage advantages, breeders and geneticists are developing so-called "cold chippers" and russet varieties which will produce acceptable french fries from 40F and even colder.

Dehydrating stock: Relatively little information is available on storage temperature requirements for potatoes grown for dehydration. For flake production, potatoes are usually held at about 39 to 43F, treated with a sprout inhibitor and then reconditioned for processing; alternatively, stocks are held at 50F continuously. These practices lower reducing sugar levels. High sugars adversely affect the shelf life of flakes.

Seed potatoes: Seed potatoes are usually held at 36 to 38F and high humidity. Seed stocks are often warmed to 50F for several days just before shipping or during transit to stimulate healing after cutting and rapid sprouting after planting.

Sprout Control

Potatoes usually do not sprout until 2 to 3 months after harvest even at 50 to 59 F. However, after 2 to 3 months of storage, sprouting can be expected in potatoes stored as cold as 39 F and much more so at 50F. Although limited sprouting does not seriously damage potatoes for food purposes, badly sprouted tubers shrivel and are difficult to market.

Humidification

High relative humidities (90 to 95%) are required in all potato storages unless poor crop condition (see "Storing Problem Potatoes" below) or storage design (inadequate insulation and excessive condensation, for example) determine otherwise. Humidifiers are typically used to add water to the air. A humidifier is normally placed in the plenum chamber immediately down wind from the circulation fan. In this location, all of the air, whether for recirculation or ventilation or both, will pass over the humidifier. A humidity of 95% reduces weight loss and shrinkage markedly over humidities of 80 to 85% and generally results in better quality regardless of the end use.

Humidities above 90% relative can be hazardous in poorly designed/operated storages due to potential condensation on storage structures, particularly on the roof, and free water on tubers which can lead to increased decay. Moisture condensation on tubers and on walls and ceilings should be avoided at all costs. Such condensation can usually be controlled by adequate ventilation and airflow unless the storage is extremely short of insulation. In years when late blight, leak (Pythium spp.), or field frost is severe (see "Problem Potatoes" below), it is usually desirable to maintain a low humidity to keep the decaying tissue relatively dry and reduce disease spread to other potatoes.

Ventilation and Recirculation

Ventilation or air circulation in potato storages is needed to control temperature and humidity, remove carbon dioxide and provide adequate oxygen for respiring tubers. Oregon recommendations are to provide 90 to 95% R.H. air at 15 to 20 cubic feet/minute/ton of potatoes. In Idaho, excellent results have been obtained with Russet Burbank potatoes by ventilating with air at 95% relative humidity at a rate of 10 cubic feet/minute/ton of potatoes on an intermittent basis. In Maine, an airflow of 10 to 12 cubic feet/minute/ton is recommended. In the U.S. Midwest, because popular cultivars are usually thin-skinned round whites susceptible to decay and shriveling, storages normally are designed for an airflow of 20 to 22 cubic feet/minute/ton for seed and table stock and 24 to 30 cubic feet/minute/ton for chippers. Rapid air circulation may result in lowering the relative humidity of the air immediately surrounding the potatoes and may promote drying and weight loss. These effects may be desirable if there are disease problems but are undesirable with sound potatoes because of increased shrinkage. Since conditions vary so greatly among producing areas, ask your Cooperative Extension Service or other experts for guidance on storage ventilation and recirculation.

Storing Problem Potatoes

Blighted, frozen, wet or otherwise compromised potatoes call for extraordinary storage measures. Common storage rules must sometimes be bypassed in order to save the crop. The following suggestions may help you make the best of a bad storage situation.

• Market straight from the field if feasible; if not, sell at the earliest reasonable opportunity.
• Avoid problems by harvesting dry, sound potatoes with a pulp temperature between 45 and about 65F.
• For frozen or blighted potatoes, if possible wait a few days for symptoms to fully develop before harvest.
• Leave rotten, frozen potatoes and debris in the field insofar as possible. This may call for additional people on the harvester.
• Sort into storage. Provide sufficient light, people and time to do the job properly.
• Have your storage ready (and at the proper temperature) with the air delivery and control systems in good order. Be certain you provide adequate flow rates in all areas of the storage. Add portable systems to otherwise airless storages. Good air movement is absolutely essential for storing problem potatoes.
• Omit the traditional curing period. Since problem potatoes are usually wet and infected with decay organisms, your goal is to cool and dry the crop as quickly as possible.
• Quickly cool the pile to the final storage temperature (about 38F for seed, 42F for tablestock, 45 for french fry processing and 50F for potato chips). It may be necessary to cool and hold processing potatoes well below the 45 (frozen processing) and 50F (chips) levels typically recommended.
• Do not humidify. The potatoes are probably much too wet already.
• Run fans continuously until the crop is dry and decay is under control. Bear in mind that running fans does not necessarily call for ventilation. Recirculate air through the potatoes at all times during the problem period, even when you are not pulling in outside air.
• Keep the pile as shallow as possible to promote air movement and easy removal of problem hotspots. Rotting potatoes and dirt sometimes form barriers to air movement.
• Monitor the storage daily. Thermometers suspended at various depths in the pile provide a good indication of the average temperature. Infrared "guns" are helpful in locating hot spots before they begin to sink and spread.
• Do not expose cold potatoes to warm outside air. A layer of free water will condense on the tubers. Try to use air no warmer than about 5 degrees F above the desired tuber temperature. Water on the tubers tends to suffocate the tubers while at the same time favoring soft rot bacteria.
• Do not expose tubers to air at or below freezing.
• When unloading the storage, do not wash dry seed potatoes unless they are covered with dried slime. Washing will probably help wet, slimy seed. If you do wash seed, use sprays as opposed to a dip tank. Try to use multiple nozzles so that all surfaces are washed clean. Misting washed potatoes with a 10% sodium hypochlorite solution is recommended (check the labeling for your area). Add one gallon of bleach to 9 gallons of water and mix well before applying in a well ventilated area. Problem seed which requires washing should be planted as soon as possible provided soil conditions are suitable.
• Ask for help when you need it. Extension workers can provide answers to most of your questions. Private consultants and technical reps are also good information sources.

Related WWW Sites

• Related Storage Sites
• Processor Sites
• Sprout Control
• Production Management
• Potatoes, General