THE PROCESSING OF SEEDS

Why and How Seeds Are Dried

N. ROBERT BRANDENBURG, JOSEPH W. SIMONS, AND LLOYD L. SMITH.

HIGH MOISTURE in seeds during storage is one of the chief reasons that they lose their ability to germinate.

The moisture affects the respiration rate of seeds and the micro-organisms that at moisture levels above 20 percent may produce heat rapidly enough to kill seed or start fires in a seed mass. Some seeds suffer mechanical damage in handling and processing if their moisture content is too high.

Molds tend to grow in moist lots of seed, particularly when the seed is cracked or damaged and they can enter and grow even more easily. Insect damage also is related to seed moisture, but most of the weevils and insects cannot breed properly at levels below about 8 percent and tend to die out. When fumigation is used to control insects, the danger of seed injury is increased by high levels of moisture in the seed. Finally, damp seeds tend to stick together and interfere with proper feeding and operating of processing equipment.

Seeds therefore must be dried if their processing and storage are to be satisfactory. People always have dried seeds in former times by the heat of the sun and now by artificial means, as well.

Drying basically is simply the evaporation of moisture.

Every liquid at a given temperature has a definite vapor pressure, which tends to produce vaporization. The moisture in a seed exhibits such a pressure. Water vapor in the atmosphere exerts a similar pressure.

The drying of seeds can take place only when vapor pressure of the seed moisture is greater than the resisting vapor pressure of surrounding air. The rate of drying drops as the differential of vapor pressure lessens. Drying will stop when a balance is reached between the vapor pressures. The seed moisture at this point is called the equilibrium moisture content of the seed at that atmospheric condition.

Moisture to be removed in drying is associated with the seeds in two main ways.

Surface moisture occurs on the outer surface, and the air readily absorbs it under proper conditions.

Internal moisture is distributed throughout the inner parts of the seed. Its removal involves capillary action or diffusion of the moisture to the surface, where evaporation can take place.

The many ways of drying seeds can be classed as natural or artificial.

Natural drying takes place with typical atmospheric air moving naturally around damp seed spread on trays, canvas, floors, or fields.

Artificial drying uses heated or unheated air that is forced mechanically through a drier.

All drying operations involve some movement of air through the seed. In normal vaporization, each seed tends to become surrounded by a film of saturated vapor, which obstructs heat transfer and limits the evaporation of moisture. Air movement, if nothing more than a gentle breeze, is needed to replace this wetted air continuously with drier air so the drying process can go on.

ARTIFICIAL DRYING can be done with heated or unheated air, dehumidified air, or a partial vacuum. In each, the particular air condition facilitates drying when used in conjunction with a movement of forced air through the seed. Heating air raises its saturation point and creates a thirsty medium that absorbs moisture readily.

An example: Air at 65 F., which contains 46 grains of water vapor per pound of dry air, represents a condition called 50 percent relative humidity, because the air is holding 50 percent of the maximum water vapor it can hold (92 grains) at that temperature. If the same air is raised in temperature to 95 , its moisture-holding capacity increases greatly, and the resulting relative humidity is only 19 percent. This characteristic of air is important in drying, because the moisture content of a seed varies according to the relative humidity of the air.

The use of heated air assists drying: also by heating the seed and thereby its contained moisture. The resulting rise in temperature stimulates the diffusion of internal moisture to the surface and increases vapor pressure of the liquid, thus encouraging vaporization.

Another form of artificial drying employs air that is dehumidified chemically or by refrigeration. Dehumidified, or dried, air can dry seed readily because some of the water vapor has been removed from it and its moisture-absorbing tendency thereby is increased.

Actually, since vapor pressure of the atmosphere decreases with a drop in its moisture content, the vapor pressure of liquid to be removed becomes more effective in causing evaporation., The use of dehumidified air is especially helpful when seed is to be dried to a relatively low content of moisture.

Certain chemicals that have a strong affinity for moisture are used to dry air in dehumidifying systems. Silica gel, for example, can pick up moisture from air in amounts up to 30 percent of its dry weight. When it will hold no more moisture, it can be reactivated for further use by drying at 250 -350 .

Other common drying agents are calcium chloride, activated alumina, and anhydrous calcium sulfate.

An alternate method of dehumidifying air is to employ refrigeration to drop air temperature below its dew point where moisture begins to condense. This will reduce the moisture-holding capacity of the air and remove surplus vapor, which can then be withdrawn from the system.

Vacuum drying is another way to remove moisture from seeds. It, like dehumidified air, usually is used in special situations where seeds must be dried to a very low moisture content.

Vacuum drying is somewhat like drying with dehumidified air in that the vapor pressure of the surrounding atmosphere is reduced so seed moisture can vaporize more readily. When the total pressure in a vacuum-drying chamber is lowered, the component of this pressure due to vapor is reduced proportionately. In vacuum drying, the heat for evaporation must be supplied mainly by conduction or radiation, since there is little air available for heat transfer by convection.

In drying by infrared heat, the seed, rather than the air, receives special attention. Radiant heat rays from infrared lamps will pass through air without warming it and be absorbed by the seed. The result is an increased temperature of seeds, which hastens movement of internal and surface moisture to the surrounding air. A feature of this method is its speed, especially when seeds are dried in a thin layer.

Artificial drying has advantages. It permits earlier harvest and reduces chance of loss from bad weather.

Early harvest and artificial drying usually provide larger yields of seed with fewer losses from overripening and shattering. Early harvest also tends to lower damage from insects and birds. Despite morning dews, harvesting can begin earlier in the day. Artificial drying means less worry because natural drying is not dependable every year and cuts labor costs and seed damage.

Artificial drying also has limitations in time, which reflect such considerations as mold growth in moist seed, possible drop in vigor of seeds, casehardening of seeds, and seed damage. All these conditions depend on time, temperatures, and moisture levels during the drying.

Molds are most active in atmospheres of high relative humidity and at temperatures of 80 100 . Even when suitable drying conditions reduce the moisture to about 15 percent, mold can still grow in some lots of seed. At that level also, seed respiration may be active, so that vigor and ability to germinate may decline.

A short drying period tends to combat molds and drop in vigor, but too fast drying has its dangers. During rapid drying, the seedcoats of some seeds will shrink or split and become impermeable to moisture, even though inner portions of the seed remain wet. This condition, called casehardening, can prevent complete drying, produce hard seed, and allow disease organisms to enter easily.

High temperatures in fast drying may injure seed that is moist. Free surface moisture can be removed safely at high temperatures, because rapid evaporation extracts heat from the seeds fast enough to keep the actual temperature of seeds depressed to the wet bulb temperature of the drying air. As the moisture of the seeds drops to 30 percent or lower, however, the supply of moisture for evaporation is less readily available and actual seed temperature will increase. High temperatures can injure seeds in this moisture range; for safety, temperatures of drying air should not exceed 90 110 .

So, with regard to safe drying time, it is important to find the proper balance between too rapid drying (with resulting casehardening or seed injury) and too slow drying (with deterioration of the seed).