New Ideas on Problems of Storage

Madeline G. Lambou, Marjorie Z. Condon, Aaron M. Altschul.

The durability of seeds in storage is the key to their usefulness to man and animal. Many seeds have potential value for food, feed, and industrial products, but in any climate only those that can be stored without much deterioration have economic value. In temperate climates, cereal grains need only to remain in the field for a short time after harvesting to dry. In more humid areas, they have to be dried and cooled artificially to prevent deterioration during storage.

The seed-crushing industries in the South have long operated under conditions that imposed losses in the quality and quantity of their products in spite of all they might do to control them. In 1943, the industries asked the Department of Agriculture for help in solving their problems. Aaron M. Altschul and Melvin L. Karon proposed a new approach, that of chemically treating the seed to minimize deterioration during storage. The industries supplied seeds and mill facilities for the research and they set to work on the problem.

The suggestion was based on the fact that a seed is a living entity capable of the many types of biological activity that are characteristic of all living things. The chemical reactions involved in this activity are controlled by enzymes, which in a dry, dormant seed are relatively inactive. They are segregated in the cells in such a manner as to allow no intimate contact with other chemical compounds which they affect. When water is added, the constituents of the individual cells become mobile, contact occurs between the enzymes and the compounds with which they react, and the rates of biological activity increase. Under ideal conditions, the diverse activities are coordinated to produce germination in the seed. But if sufficient moisture for germination is not present, or if the enzymes have lost their activity, or the compounds with which they would normally react have been destroyed or rendered inactive in some manner, degradative processes will set in and result in the production of heat and destruction of the constituents of the seed.

In cottonseed, deteriorative processes cause formation of free fatty acids, from the fat stored in the seed, and darkening of the seed and oil. Deterioration in rice shows up in the form of discoloration of the kernels, in- creased breakage of the kernels during milling, an increase in free fatty acids in the oil in the pericarp and aleurone layers (which constitute the bran removed during milling) , reduction in vitamin content, and development of off-odors and off-tastes in the white milled rice.

Comparatively little is known about the enzyme systems involved in the metabolism of oilseeds and consequently about the type of chemicals that might be expected to inhibit them. It is possible for chemicals differing greatly in molecular structure and properties to inhibit deterioration, because each reacts at a different point in the complex chain of reactions that undoubtedly are involved in spoilage of seeds. Therefore, the problem of selecting protective inhibitors is enormous and cannot be solved by resorting to the use of only the chemicals that have heretofore been reported to be biologically active. A wide variety of chemicals must be tested individually until the inhibitor activity of any given chemical can be correlated with its molecular structure.

THE TIME required to test the activity of chemicals under field conditions and the cost of materials would be prohibitive. The investigators needed a rapid method whereby deterioration of seed could be produced in the laboratory in a few days. Such a method, developed by Madeline G. Lambou, makes it possible to make preliminary tests of the inhibitory activity of chemicals in 6 days, using only a few ounces of seed.

The method comprises conditioning the seeds to a high moisture content, storing the seeds under conditions that will promote rapid heating and deterioration, and comparing stored untreated controls with the treated samples, as to the effect on the temperature of the seed and the formation of free fatty acids in the oils during storage.

The process is carried out by using a simple type of calorimeter in the form of a Miter vacuum bottle. The moistened seeds are placed in several bottles, and air is drawn through them at a constant rate, during which time the change in temperature is automatically recorded. After storage for 6 days, the samples are removed and analyzed for their content of free fatty acids. Flaxseed was found to be satisfactory for the rapid testing of a large number of chemicals. Therefore most of the preliminary testing was made with flaxseed. The chemicals that possessed inhibitory activity with it were tried on other seeds.

The first chemical found to be effective in the laboratory was ethylene chlorohydrin. When ethylene chlorohydrin in a concentration of 0.38 percent, based on dry weight of the seed, was applied to high-moisture flaxseed, no heating or formation of free fatty acids occurred during a 6-day period.

Other chemicals have since been found to compare favorably with ethylene chlorohydrin in ability to inhibit heating and the formation of free fatty acids.

The chemicals are: Propylene chlorohydrin; ethylene bromohydrin; allyl alcohol; B-chlorallyl alcohol ; diethyl oxalate; diethyl malonate; glycol diacetate; propylene glycol dipropionate; propylene glycol diacetate; vinyl propionate; tributyl borate; ethyl chloroacetate; ethyl chloropropionate; methyl chloroacetate; ethyl isovalerate; triethyl phosphite; diethyl phosphite; propionic acid; B-chloropropionic acid; acetic acid; butyric acid; valeric acid; crotonic acid; sodium cyanide; 1,3-dichloropropene-2; 1,3-dichlorobutene-2; 1,3-dichloropropene1; phenol: salicylaldehyde; p-tertiaryamyl phenol (Pentaphen) ; sodium pentachlorophenate (Santobrite) ; 2- chloro-4-phenyl phenol (Dowicide 4) ; chloro-2-phenyl phenol (Dowicide 30) ; o-vanillin; 1,3-dimethyl-2, 4-bischloromethyl. benzene; 1,3-dimethyl4,6-bischloromethyl benzene; benzotrichloride; Chloramine T; Dichloramine B; sulfanilamide; p-toluene sulfonamide; Hyamine 1622; Hyamine 10X; 2-aminothiazole; chloroacetamide; acrolein; and methyl vinyl ketone.

Some trade names appear in the list; it is to be understood that it is not the policy of the Department of Agriculture to recommend the products of one company over those of any others engaged in the production of the same or similar products.

Even though a chemical might be effective in a 6-day test, its use in industry would depend on its behavior during a storage period lasting several weeks or months. Therefore, while a test method made possible the selection of the most promising chemicals, it had to be supplemented by laboratory tests of longer duration to determine how long the chemicals would retain their effectiveness in preventing heating and deterioration.

These tests were conducted in the same manner as the original ones, except that they were not concluded until the temperature of the treated seeds began to rise above room temperature. For example, with flaxseed of 22 percent moisture content, it was shown that vinyl propionate, when used in a concentration approximately equivalent to that of ethylene chlorohydrin, prevented heating of the seed for 21 days. The untreated control lot of the same seed heated to 110 F. in 4 days. Grain sorghum, treated with ethylene chlorohydrin, did not heat for 10 days. With ethylene bromohydrin, no heating occurred for 14 days.

The most interesting observation was on the effect of mixing two or more chemicals, each of which had been found to possess inhibitory activity. When two "active" chemicals were mixed, the resultant inhibition was greater than that obtained by treatment with either chemical alone, even when each was used in a higher concentration. For example, heating of flaxseed was inhibited for 62 days by a mixture of propylene glycol dipropionate and 1,3-dimethyl-4,6-bischloromethyl benzene. If twice the concentration of one of them ( propylene glycol dipropionate) was used alone, inhibition was maintained for 18 days, while double the concentration of the other one (1,3-dimethyl-4,6-bischloromethyl benzene) was effective for only 15 days. Treatment with the mixture of the two chemicals had an analogous effect on the rate of formation of free fatty acids.

THE OBJECT OF STORAGE is to provide conditions that keep to a minimum the activity within the seed. Obviously, that is best accomplished by drying of the seed in the field before harvesting. If that cannot be done, the seed may be dried in the field immediately after harvesting, as, for example, in shocks of grain or stacks of peanut vines on poles. Both effect drying at atmospheric temperature by free circulation of air of low relative humidity through the seed.

The mechanization of harvesting changed the requirements for handling and storing seeds. Harvesting grains with combines has resulted in a substantial increase in the average moisture content of the seed leaving the fields, and has increased the need for artificial drying before storage. For example, widespread application of mechanical methods for harvesting peanuts will depend on the development of practical methods for drying green or uncured seed.

If the climatic conditions or the method of harvest preclude field drying, the seeds may be artificially dried by the circulation of warm air. Such an operation, if carried out carefully, prevents deterioration during storage. If, however, the moisture is removed too rapidly or the seeds are heated to too high a temperature, irreparable damage will be done. In rice, for example, drying at too high a temperature increases the proportion of broken kernels in milling.

The ideal artificial drying procedure is one that approximates most closely the action of the sun and air in the field. It has never been completely attained because it has never been possible to stop the damage that takes place in seeds during such drying. An example of this difficulty was given in an experiment on storing cottonseed. Twenty tons of cottonseed of 17.6 percent moisture content was stored in a bin, and air at atmospheric temperature was drawn through it continuously for 20 days. In that time, the moisture content of the seed was reduced to 11.5 percent. Nevertheless, the content of free fatty acids of the oil present in the seed increased from an original value of 1.2 to 3.5 percent after 35 days of storage and to 6.8 percent after 71 days of storage. The rate of drying was too slow to prevent damage to the seed during the time that the moisture content was being reduced to one considered safe for storage. Even after the moisture content was reduced, damage continued to occur because seed already injured is more susceptible to additional damage.