Temperature Control in Seed Storage
As mentioned earlier, temperature is one of the most important environmental factors, which influence seed viability and vigour during storage, the lower the temperature, the longer the seeds maintain germination capacity, thus temperature control is an important consideration in building a seed storage.
Temperature control may be achieved in one of the following ways:
These methods are not mutually exclusive, and normally used to supplement each other.
Ventilation could be used to reduce seed temperature and seed moisture content, if used judiciously. In addition, it also helps prevent hot spots from developing, the formation of convection air currents; and maintenance of uniform seed moisture content and temperature.
Time of Ventilation:
Whenever the outside temperature of air and relative humidity are low enough to benefit the seeds, either by reducing seed temperature or seed moisture content, the ventilating fans ( exhaust fans) can be turned on.
First determine the temperature and relative humidity of air inside and outside the storage. The temperature is measured with thermometers and the relative humidity with a psychrometer. After determining these values it could readily be determined from table when it is safe to ventilate a storage in order to cool it without increasing seed moisture.
Precautions to be Taken Ventilation:
1. The moisture content of the seed should not be allowed to increase to a value in equilibrium with air relative humidity above 65 percent and
2. The seed temperature should not increase above 33 0 C for more than few hours.
Insulation of seed storage is done to reduce the flow of heat from the warmer exterior , through the walls , roof, floor of the storage to the cooler air and seeds in storage. Heat flow depends upon:
1. Temperature difference between the two places in the material. Heat flow is twice as fast with a 20 0 temperature differences as with a 10 0 Difference.
2. Distance the heat must flow. Heat flows twice as fast through one inch of insulation as through two inches of the same material.
It would appear that air is the best insulator. However, it has serious defect in that if the air space is more than a fraction of an inch, a convection current occurs moving the heat from one surface to another. Therefore, the air , to be of these. Moreover, they absorb moisture readily and may mold and rot. Neither are they fire-proof, nor do they have any mechanical strength. Thus, although they are cheap, they have some severe restrictions. Foam polystyrene has the advantages of being light in weight. Because the air bubbles are enclosed , no convection currents are possible. It has considerable resistance to moisture penetration. It is easy to install, has mechanical strength and dimensional stability. It can become water-logged, losing its insulation value if the storage develops leaks. Certain fumigants may affect it. It does deteriorate slowly with age.
Glass wool is rat – proof , but insects can harbour in it. It is odourless. It has a low water absorptivity. It is fire-proof. It is unaffected by fumigants. However, it allows a free flow of moisture , so an additional moisture barrier is imperative. People can be allergic to it. Convection currents are possible particularly in the lighter grades. It has less dimensional stability and mechanical strength than rigid insulation material, but is more easily worked around the curves and corners.
The basic objective of refrigeration is to keep the storage temperature below the usual ambient temperatures. An alternative to refrigeration is storing the seeds dry, either by using dehumidification or by drying and storing in sealed containers. Refrigeration often becomes necessary for carry-over seeds, special kind of seeds, foundation seed and nucleus seed/ breeder seed.
The amount of refrigeration needed depends upon the removal of heat generated from various sources. There are four important sources of heat:
1. Heat leakage :
This is the flow of heat through the floor, walls, and roofs into the storage when the outside temperature is higher than the temperature in the storage. This is directly proportional to the temperature difference and the distance heat must travel.
Insulation as discussed above reduces heat leakage but does not eliminate it. The better and thicker the insulation, the less refrigeration is required because leakage of heat is reduced.
2. Field Heat:
Field heat refers to the heat of seeds, packaging material, pallets, bales of bag and anything else brought into and put in the storage in excess of the heat at the storage temperature. Refrigeration capacity must be sufficient to remove field heat in a reasonably short time.
3. Heat of Respiration:
Heat of respiration in seed storage is not a serious heat input because dry seeds have an extremely low respiration rate. Below 11 % seed moisture, seed respiration is so low as to be almost immeasurable. Respiration , however ,is higher temperatures and decreases with decreasing temperatures in the seeds of the same moisture content. At higher seed moisture contents, the respiration jumps astronomically.
4. Incidental Heat:
The incidental heat includes heat from electric lights, and external heat that enters a storage when the door is opened. Care should be taken not to leave the door open.
The amount of refrigeration needed is usually arrived at by calculating heat packaging, field of respiration and then arbitrarily adding ten percent more as incidental heat.
Refrigeration seed storage should be done by a competent company. However, seed technogists have to specify the temperatures and relative humidity requirements of the seeds. Their guidance on specific technical requirements is also necessary.
Although theoretically it is possible to obtain lower relative humidity by refrigeration, the practical minimums depending on temperature are as follows:
Minimum Relative Humidity
90 0F ( 32 0C)
81 0F ( 27 0 C)
74 0 F (23 0 C)
70 0 F ( 21 0 C)
67 0 F ( 19 0C)
62 0 F ( 16 0 C)
57 0 F (14 0 C)
Below about 15 0 C , the relative humidity possible with refrigeration alone is too high for proper seed storage. Hence refrigeration alone is not considered sufficient for storage of seed.