Shell-Cooled Potato Storage

by ALFRED D. EDGAR RESEARCH into problems of storing potatoes used to have the rather simple aims of keeping potatoes above freezing and below sprouting temperatures so that they would meet local requirements for seed and eating. Now the marketing of potatoes and, therefore, the objectives of research in their storage have become more complicated.

A larger proportion of the seed potatoes for central and southern areas is grown in the North, and northern-grown seed potatoes may have to be stored under one of three sets of conditions : At a relatively cold temperature for 7 or 8 months, when intended for seed to be used locally; at moderate temperatures for 6 or 7 months, for planting in the Central States; or at a relatively high temperature, to get them through the dormant period quickly, for early planting in the South.

The storage of table-stock potatoes also has become more complex. Potatoes for early consumption can be kept in warm-temperature storage with minimum air circulation, and it is now known that a higher vitamin content is retained by this treatment. Low temperatures, however, are required for long-period storage. It is obvious, therefore, that the temperatures at which potatoes are stored will depend largely on the specific use to which they are to be put and the time at which they are to be marketed. Potatoes for dehydration and those used in the manufacture of potato chips require special storage conditions.

There is a growing tendency also for areas of different climates and different harvesting dates to grow and store potatoes for overlapping markets. For example, potatoes harvested early in the North may be kept in low-temperature storage for planting in the South, while those harvested farther South later must be stored at a higher temperature to get them through the dormant period in time for southern planting.

For many years, potato storage construction, particularly for the humid part of the country with average temperatures of 25 to 30 F. during January and February, featured bins having double slatted floors and bin partitions, so the air could circulate through the mass of potatoes. Such through-the-bin circulation, however, reduces the vitamin content and increases the shrinkage of stored potatoes.

Shell cooling of potato bins to reduce shrinkage and preserve better table quality has proved effective in the potato storage investigations of the Department in cooperation with several experiment stations in the late-crop potato States. The shell-cooling process consists of circulating air under and around the. bins with tight walls and floors, rather than through the mass of potatoes. It is most satisfactory in large storages where power-operated blowers and thermostatic controls are used.

In the fall, outside air is drawn into the building and circulated to remove field heat from the potatoes. A differential thermostat can be set to start the blower whenever the outside temperature is lower than that of the potatoes. As the temperature drops, a second thermostat protects the potatoes from freezing by breaking the circuit. During the winter further cooling of the potatoes is not necessary, but air circulation is still required to maintain uniform temperatures and reduce condensation of moisture. Blower dampers are therefore arranged to recirculate air within the storage house around, under, and over the bins. In smaller houses that have a capacity of fewer than 10,000 bushels, gravity air circulation is usually satisfactory.

Potatoes kept in storages built and operated according to recommendations of agricultural engineering research show shrinkage losses from 1 percent to 10 percent less than other types of houses. At least 10 million bushels annually of late-crop potatoes are now stored in buildings of improved design, and the annual saving in shrinkage alone approximates $120,000. Heavy losses from freezing and decay are prevented, and the potatoes for table use have higher nutritive value.

Additional savings are effected by shell circulation because the storage houses last longer, being less subject to damage by moisture. The new designs in these houses eliminate the condensation of moisture on the structural parts without allowing the potatoes to dry out.

Another problem that has come up in connection with the storage of potatoes in the past few years is the need for centralized washing and grading in storages. Twenty years ago most of the crop was sold ungraded and none of it was washed. When grading first became common, it was done by portable graders, which were moved right into the bins. Until recently, therefore, the principal handling problem was to design storages so that potatoes could be put into the bins with the least injury. Now, however, many storages use stationary washing and grading equipment and large stationary brushers. Some means must be worked out for moving potatoes from bins to graders, and various methods are being compared. Forty-bushel portable bins can be handled with industrial lift-trucks; potatoes in sacks or small boxes can be stacked on pallets and moved with industrial or hand-operated lift-trucks; or conveyors designed to avoid bruising the tubers can be used both from truck to storage and from storage to graders.

THEAUTHOR D. Edgar, an agricultural engineer in the Bureau of Plant Industry, Soils, and Agricultural Engineering and a graduate of Kansas State College, started specializing in potato storage research in his present position in 1931, when he was put in charge of the potato storage research project that the Department has maintained continuously in various States, so that findings in one area can be tried in another, and new developments in the later locations taken to the earlier locations for refinement. He was one of eight Department specialists sent by Congress to Alaska in 1946 to determine agricultural problems upon which research is most needed.

Pests in Stored Products

by R. T. COTTON UNTIL RECENTLY the only really effective insecticides available for controlling insect pests of grains and food products stored in bulk or packages were those that could be used in the vapor stage as fumigants. Contact sprays did little good. Insects harbored in the woodwork of farm and elevator grain bins, warehouses, storerooms, flour mills, ships, and railway cars are not readily controlled by fumigation, and constitute a source of infestation whose importance to our food industries can scarcely be overestimated. Newly harvested grain placed in wooden bins, boxcars, warehouses, and such quickly becomes infested with insects that emerge from the cracks in walls and floors.

Now the so-called residual spray has assumed an important role in the control of these pests. The residual spray, when applied to the surface of walls, floors, partitions, et cetera, leaves a protective film of a toxic chemical that causes the death of insects that touch it.

Several chemicals with residual properties have been developed, many of them highly effective against the insect pests of stored grain and milled cereals. The best known is DDT, which can be had in many different forms and concentrations, and can be used in either a water or an oil-base spray. Two analogs of DDT, which possess quite similar characteristics, are di ( paramethoxyphenyl) trichloroethane and dichlorodiphenyl-dichloroethane. Other residual chemicals are a chlorinated campene with the empirical formula GoHioCls, and a chlorinated hydrocarbon, with the empirical formula C,oH,,Cls.

A 5-percent concentration of any one of the first four compounds or a 2-percent concentration of the last in a deodorized kerosene are highly effective against even the most resistant type of stored-product insect. They should be applied at the rate of not more than 1 gallon per 1,000 square feet of surface area. These residual poisons are relatively slow in action. The addition of a "knock-down" agent, such as pyrethrum or one of the thiocyanates, improves the efficiency of their performance.

Because all the residual insect toxicants are poisonous to warm-blooded animals, they must not be used to spray grain or milled cereal products intended for human food or animal feed. Precautions must also be taken to avoid spraying food containers and to prevent foodstuffs from coming into direct contact with sprayed surfaces.

DDT may also be useful for incorporation into wrappers for preventing the entry of insects into bagged and packaged foodstuffs. The adaptation of DDT to this use is still in the experimental stage, but it shows promise of being invaluable for the purpose of protecting many types of susceptible foodstuffs from invasion by insects after manufacture and packaging.

As a dust, DDT has been found highly efficient in protecting bagged and packaged seed from insect attack and provides one of the cheapest and best methods of long-time protection known. Effective when mixed with seed at the rate of only 15 parts per million, it is best used in combination with a carrier dust. The carrier, by increasing the volume, insures a better distribution over the seed. Dust composed of 3 percent of DDT in magnesium oxide or pyrophyllite has been found satisfactory, although any commercially available dust containing 3 percent DDT can be used.

DDT dust of this concentration should be applied at the rate of a half ounce to a bushel of seed. Seed-treating machines can be used efficiently to coat the seeds with a uniform layer of the protective dust. The treatment does not affect germination in any way. DDT is a chemically active dust that kills insects by contact, so it is effective regardless of the moisture content of the seed.

Certain chemically inactive dusts have also been found useful for protecting seed. Their effectiveness is thought to be due to their abrasive action that breaks the waterproof, fatty covering of insects, so that the insects die as a result of the evaporation of excessive amounts of body moisture. Because of their mode of action, the effectiveness of inert dusts decreases as the moisture content of the seed rises above 12 percent.