Authors: *Kana Ram Kumawat, Ravi Kumawat and Madhu Choudhary
Department of Plant Breeding and Genetics, S.K.N. Agriculture University
Jobner-303329, Jaipur, Rajasthan, India
The process of elimination of moisture from the seed is called drying. Seed drying should reduce the seed moisture content to safe moisture limits to maintain its viability and vigour during storage, which may otherwise deteriorate quickly owing to mould growth, heating and enhanced microbial activity. Seed drying also permits early harvesting, long term storage of seeds, more efficient use of land and manpower, the use of plant stalks as green fodder and production of high quality seed. Depending upon the climate and method of harvesting adopted the threshed seed may or may not be dry enough for safe storage. Under less favourable conditions, threshed seed needs further drying.
Principle of seed drying:
The migration of moisture to the surface is slower than the evaporation and a moisture gradient is developed in the kernel. Elimination of moisture from the seed depends upon the relative humidity and temperature of the environment surrounding the seed. When RH of the atmosphere is less than the seed, moisture is eliminated from the seed. While drying, care should be taken to minimize / prevent oxidation and decomposition and volatilization. In this process there will be loss of dry weight of seed, which is widened when the processes take place at high temperature. Hence, greater the seed moisture content lesser should be the drying temperature and vice versa. The rate of drying depends mainly on-
- Initial seed moisture content
- Size of the bin and capacity
- Depth of spread of seed
- The rate of air blow
- Atmosphere air temperature and relative humidity
- Static pressure
- Drying temperature The moisture from the seed is eliminated two stages viz., (i) surface moisture of the seed that initially removed by the drying air and (ii) the removal of the moisture in the surface cause an imbalance in the moisture potential in the surface of the seed and the inner portion of the seed which leads to the migration of moisture from the inner organ to the surface.
METHODS OF DRYING
- Traditional sun drying / physical drying / natural drying
- Mechanical / artificial drying
This is the common conventional method in which drying of the harvested crop is carried out in the field or threshing floor by the radiant energy of the sun. This does not involve any expenditure. To achieve uniform drying, the seed should be spread in thin layer. High moisture content seed with a moisture content of more than 17% should be dried first under shade to reduce the moisture content less than 17% and then dried under heavy sun i.e. noon drying. Sun dried seeds should not be allowed to remain open in the floor during night, since seed will absorb moisture from air. Two to four days are needed to reduce the moisture content to 10-12%. Direct sunlight also can adversely affect seed germinability owing to high temperature and ultraviolet radiation, especially if the moisture content of the seed is high.
Advantages of traditional sun drying:
- This method is easy and cheap
- This does not require any expenditure or fuel.
Mechanical drying is mostly done using forced air drying. In forced air drying, natural air or air supplemented with heat is blown through a layer of seed until drying is completed. Generally ordinary seed godowns are provided with two types of ventilators for free movement of air circulation. In modern godowns, provisions are to be made for forcible circulation of air with the help of an electronic blower. The outside air which is comparatively dry is circulated in the godown and thereby the seed get dried up in this process. This is possible only in dry months.
Two types of driers i.e., batch and continuous flow driers may be used for forces air drying. In batch drier, relatively dry air is blown through a layer of seed until the seed is dried completely, after which it is removed and replaced by another batch of seed. The method is simple and well suited to small quantities of seed, allows easy cleaning and is recommended for farm drying. The air blown through a batch must not be too hot, because the seed at the bottom may be overheated by direct exposure to the entering hot air. In Continuous flow drier, the seed moves horizontally or vertically through a stream of hot air and then into a cooling chamber. These driers are however difficult to clean when there is a change of cultivar. These driers can use air temperature higher than those of batch dries, because the seed is heated for a much shorter time.
Advantages of mechanical drying:
- Quick method, timely and uniform drying is possible
- Makes early harvest possible
- It reduces the chances of losses due to over ripening and shattering of seed
- Losses due to rodents and birds are prevented.
- Less damage during processing operation.
About Author / Additional Info:
Post-Graduated in subject of Plant Breeding and Genetics from SKNAU, Jobner