Drying is a complex operation involving heat and mass transfer along with physical and chemical changes. Physical changes include shrinkage, puffing, crystallization. Chemical and biochemical reaction includes changes in color, texture, odor or properties of solid product. Storage of high moisture grain in long term causes insect infestation, mold growth, development of off odor, lowering nutritive value and germination of grain. This causes quality deterioration of grain and in long term, make unfit for human consumption. Therefore, drying is the most crucial part for grain preservation. Not only to increase shelf life of grain and maintain quality, drying of grain facilitate handling and processing.
For drying of cereal grain, heat may be supplied by using direct dryers (convection), indirect dryers (conduction) or radiation (grain is heated up by electromagnetic radiation). Most of the industrial driers are direct dryers that uses convective heat transfer. In such dryers, hot air or direct combustion gas are the drying medium. However, the principle objective of all types of dryer is removal of water. Following are the commonly used drying systems for grain drying.
1. Radiation drying:
Electromagnetic radiation with wavelength ranging from 0.25 μm and 0.2 m can be used for drying different materials. The sources of radiation are solar, infrared, dielectric and microwave.
a. Sun drying: Sun drying is inexpensive and most of the crops are still sun dried throughout the world. Sun drying is an ancient method of drying that is very simple, requires no fuel and preserves viability and baking quality of dried grains. However, it cannot be denied that it is highly labor intensive, weather dependent and has high chances of contamination. This drying can be achieved either as drying of standing crops, drying of grains in stalk or drying of threshed grains.
In some practices, grains are allowed to dry on the standing crops and are harvested and threshed thereafter. This is a slow drying process and takes 2 – 3 weeks after maturation of grain. This cannot be the better option for grain dying because of low quality and low output.
When crops are harvested at their maturity, stalk of grains is left in the field until the grains in it are dried to desired level. Doing so, grain losses due to shattering, birds and rodents are unavoidable. It requires 7 – 20 days for drying depending on weather condition.
After grain are harvested and threshed, grains are spread in thin layer (3 -5 cm thick) either on the clean field, concrete floor or straw mats and sun basked. Regular spreading of the grain is required to achieve uniform drying. Generally, it takes continuous 2 – 3 days of sun basking to bring down moisture content from 20 – 22 % to 13 –14 %.
b. Solar drying: The heat transfer unit (absorber) in solar dryer absorbs the solar spectrum of radiation and converts radiation into thermal energy. In the direct mode of drying, solar energy is received directly by small batch of materials placed inside the dryer. Moderate air flow rate is maintained by natural convection where air is heated at 50 – 60°. In indirect mode of drying, materials inside the dryer do not get heat direct solar heat. First of all, drying air is preheated by sucking the air which then is passed to bed of material (grain) inside bin or storage house.
c. Electromagnetic radiation: This includes dielectric or microwave drying and infrared drying. Radio frequency and microwave system are the example of dielectric drying. The main limitation of this system is the economy of the whole process. The advantage of this process is volumetric dissipation of energy throughout the product and uniform drying. Unlike other drying process, using hot air, drying by dielectric and microwave system is quite fast due to rapid generation of vapor within the material.
Certain frequency range called Industrial, Scientific and medical (ISM) bands are agreed internationally for use in heating. The basic principle behind dielectric drying is absorption of energy by water present in the material (grain). The absorbed energy causes water in grain to evaporate thereby reducing the moisture content of the grain.
2. Conduction drying:
In this technique, heated material surface is bought in contact with high moistened grain. This can be further classified as below.
a. Direct contact with heated surface: Here, the material to be dried is placed over heated plate (stationary or vibratory) that supplies heat for vaporization of moisture from grain. Such type of dryer have thermal efficiency as compared to convective dryer. Uniform drying can be achieved by using these dryers and are mainly used to handle high moisture grain kernels.
b. Direct contact with heated particles: This technique uses heated sand for drying paddy (20 – 29% moisture). Here, paddy is mixed with heated sand (120 150°C) in the ratio of 1:2 for about 5 minutes . After heating, sand and grain is discharged and is sieved to separate grain and sand.
3. Convection drying:
This is the most popular grain drying technique. Here, heat transfer is achieved by direct contact of moistened grain with stream of hot air. The vaporized moisture is carried away along with air. This is further classified as below.
a. Natural air drying: Generally, natural drying of grain takes place in original place (in situ) where it is contained in bins. The bins are equipped with fan or blower, perforated base and exhaust vents for moist air to escape. The hot air can be blown upward or downward for moisture removal. The rate of drying depends on weather conditions and air flow rate depends on initial moisture content of grain. For initial moisture content of 20 %, minimum air flow rate for wheat, oat, shelled corn, sorghum is 2.4, 1.6, 2.4, and 2.4 m3/m3 min-1 respectively .
b. Drying with supplemental heat: This involves drying of grain in storage bin by constant use of heated air. Generally air is heated 4 – 12°C above ambient temperature. Such drying method is preferred in regions where weather remains moist most of the times. Moreover, it is simple to use and economical. However, the grain may have chance of over drying and possess risk of fire hazard to some extent. This dryer have slow rate of drying.
c. Heated air drying: In this method, air is heated considerably and this method is usually practiced when grain need to be marketed immediately or contain very high moisture such as in parboiled paddy. This drying method have faster rate of drying, high drying capacity, can also be used for long term storage of grain and is suitable for bulk drying. However, using this method, here is still risk of fire hazard if not handled properly, need skilled manpower and is high fuel consumption. Heated air drying can be achieved in following ways
Thin layer grain drying: Baffle type and Louisiana State University (LSU) dryers are example of thin layer drying system. A number of inclined sheet of metal plate baffles are used in path of grain movement in downward direction wile hot air flows in crosswise manner .
Deep bed grain drying: It is practiced for drying of grains in farm. In this technique, not all grains are exposed to same drying condition. Natural air or heated air is blown from perforated base of the dryer. Above drying zone, the moistened grain that are at its initial moisture level are in equilibrium and exhaust air leaving the bin. Whereas in drying zone, exchange of moisture takes place due to evaporation. Due to effect of evaporation, the escaped air is cooled which is also manifested by temperature drop of supplied heated air. The depth of drying zone depends on air velocity height of bed.
Fluid bed drying: Dry air (fluid) with high velocity is passed through the perforated bottom of dryer, particles start to move and suspend in the air (fluid). The process by which particles (grain) are fully suspended in fluid and behaves as dense fluid is called fluidization and the bed (of grain) is called fluidized bed. Food particles of size ranging from 20 μm – 10 mm are suitable for fluidized bed drying. Cereal grains can be easily fluidized over wide range of moisture contents.
Spouted bed drying: This is a particular design of fluidized bed dryer which can handle coarse solids (above 5 mm) that are too large to be fluidized. Here, high air velocity is allowed to enter through nozzle in a conical base of dryer. As the name says, zone of fast moving particle at the center creates spout and the grains moves downward, surrounding the central spout at slower rate. Thus, downward moving grain comes in contact with hot air that is forced towards upward direction (counter current direction) and drying takes place.
 Iengar, N. G. C., Bhasker, R., & Dharmarajan, P. (1971). Studies on sand parboiling and drying of paddy. Journal of Agricultural Engineering.
 Mujumdar, A. S., & Beke, J. (2003). Grain drying: basic principles. Handbook of Postharvest Technology: Cereals, Fruits, Vegetables, Tea, and Spices, 119–139.
 Sumner, P. E., & Williams, E. J. (2009). Grain and soybean drying on Georgia farms.