Principles of grain drying
All cereals are hygroscopic living matter; their moisture content will change in relation to changes in relative humidity and the prevailing temperature. If the surrounding air is saturated (100% relative humidity), it can hold no more water and therefore cannot take up any water from the grain. But, if the relative humidity of the air is only 75%, for example, it will absorb water from the grain until the grain moisture content is about 14%. Then, if the relative humidity of the air would increase to 90%, the grain will absorb moisture from the air until its moisture content reaches 13%. The drying of grains depends on this natural balance between relative humidity of the air and moisture content of the grain. The grain moisture content will adjust itself to balance a given relative humidity of the air.
The drying process is basically a transfer of heat by converting the water in the grain to vapor and transferring it to the atmosphere. Heat is transferred to the kernel for the evaporation process by convection, radiation, or conduction. Convection drying requires the heating of the air to lower its relative humidity sufficiently to absorb moisture from the grain (De Datta 1981, De Padua 1970).
In drying grain with forced air, a vapor pressure difference between the moisture in the air flowing around the grain and the moisture in the kernels of the grain itself is necessary for drying to occur. When the vapor pressure of water in the grain is higher than that in the air, moisture will move from the grain and cause a reduction in grain moisture content. The rate at which grain may be dried is dependent upon the difference in vapor pressure between water in the grain and that in the surrounding space. To obtain fast drying, a large amount of vapor pressure difference is required. This can be accomplished with heated air since the vapor pressure of the moisture of the grain is increased while the vapor pressure of the drying air remains constant (Casas 1980).
The basic concept in drying is the removal of moisture from the grain until the moisture content of the product is in equilibrium with the surrounding air, usually 12–14% moisture. Moisture content is the amount of water present in the grain expressed as a percentage of the weight. At equilibrium state, the product does not gain or lose moisture. However, when the surrounding air is either high or low in moisture, the product will either gain or lose moisture to the surrounding air as it is no longer in an equilibrium state (De Padua 1970).
Simple Methods of Drying
Freshly harvested rice spoils rapidly because of high tempera-ture, high moisture content, high rate of respiration, high micro-bial action, and presence of fo-reign materials. Cleaning rice to remove foreign materials (straw, chaff, weed seeds, stones, etc.) will speed up drying and lessen the danger from spoilage. Rice, to be stored safely for an indefinite period of time, should be dried within 6–12 hours after harvest down to 12 or a maximum of 14% moisture content.
Sun drying
This is a method of drying grain using a flat outdoor flooring (usually a cement floor) where the grains are spread in an even layer and are directly exposed to the heat of the sun. The grain, when heated up, will release its excess moisture, which could be carried away by the moving wind. In localities where labor is cheap, the rainy season is short, and the amount of grain to be dried is not much, this method is the most economical. One must exercise caution when sun drying for seed purposes because of the possible overheating of the grains, which are directly in contact with the cement floor. The cement floor receiving direct radiation from the sun could build up a temperature so high (above 100 oF) that it may destroy the seed germplasm. This method of drying takes 10–14 days, depending on the initial moisture content and the weather. Farmers will either dry the harvested grain in bundles, especially if labor is lacking, or they would thresh immediately and then dry.
Flat bed drying
The other method of drying is by mechanical means. Here, the air is heated, then passed through the harvested grains for a certain period of time, allowing the grain moisture to reach equilibrium with the air. During this process, mois-
ture from the grain evaporates through its surface. This means that the outer surface is the first to dry. Considerable time must be provided to allow the mois-ture from the center of the grain to diffuse evenly toward the grain surface. Some dryers have tempering bins to cool off the grains and thereby achieve moisture equilibrium, instead of subjecting the grains to continuous drying. If the drying temperature is above 140º F and there is no tempering, surface evaporation of the grain will exceed internal water diffusion; the outside surface thus dries out excessively and this will result in the cracking of the unmilled grain. There are other designs of mechanical dryers that could accommodate bigger volumes of grain. Examples are the continuous flow dryers, which are practically automated in operation. It makes use of alternate drying and tempering (cooling) process. The major components of these types of dryers include burners, blowers, holding bins, conveyors, drives, and parts for safety control.
