The storage environment can be influenced in two main ways:
● The building (or structure) in which the seed is stored.
● Its environment (i.e. temperature and relative humidity).
It is important that the seed is prepared for storage in a purpose-built structure as quickly as possible after harvesting and a system organized, so that the seed remains in the storage environment for as long as is practical prior to distribu- tion for sowing.
Construction of seed stores
Seed stores should be designed to maximize security, minimize fire risk, exclude birds and rodents, and keep the entry of insects and microorganisms to a minimum.
The ideal building should have a raised and smooth-finished reinforced con- crete ground floor with a rodent-proof lip. Entry can be by removable ramp and the raised floor designed to match up with the loading level or height of delivery vehicles. The roof should be pitched and overhung to offer the best possible runoff of storm water and provide shade and extra protection for the ventilator open- ings. There should also be a pitched canopy roof over the entrance chamber.
The extent to which rodent proofing, measures to deal with excessive storm water (including surface runoff water from higher levels) and extremely high temperatures are included as design features will depend on the location, topography and prevailing conditions throughout the year.
A double entrance door system incorporating an entrance lobby, or ante- chamber, should be included in the design, and ideally there should be no other openings or windows except those connected to environmental control systems.
The walls and ceiling should be constructed of smooth-finished stone, mor- tar and concrete which are lined with a moisture barrier of tar, aluminium foil or polythene. Wood should either not be used or kept to a minimum, as in time it can be attacked by rodents and other pests and present maintenance prob- lems. The final finish of the interior walls should be of a material which will protect the insulation from damage made by trolley, pallets or other handling, transport or storage devices. Any ventilation openings which are incorporated must be efficiently screened to exclude insects. The overall interior finish must be smooth with electrical conduit channels and other cracks smoothly sealed.
External finishes, especially the roof, should aim to minimize absorption of solar heat and exclude water. Particular attention to these specifications and details are necessary in parts of the world subject to high temperatures and rainfall. Seed stores should be provided with an adequate lighting system but with a low heat emission. There should be a regular cleaning and maintenance programme to ensure that all the design features of the structure are upheld during the life of the store.
Additional features of conditioned seed stores
The design and materials used in the seed store’s construction should mini- mize the absorption of solar radiation and act as an effective vapour barrier.
In some parts of the world, these are the only features necessary to ensure a satisfactory storage life of seed. However, in many other areas further control of the storage environment must be achieved by air conditioning. This becomes a major requirement where the climate is such that the design of the seed store is insufficient to ensure satisfactory potential life for the seed.
The actual design and structure of conditioned stores follow the specifica- tions outlined above. But where conditioning is required, it is extremely impor- tant to ensure that the best use is made of insulating materials in order to achieve the maximum possible efficiency from temperature controlling equip- ment and value from operating costs. This factor is continuing to increase in importance with the progressive rises in energy costs and the need for conser- vation of fossil fuels and protection of the environment.
Vapour proofing of the structure is also important and is achieved by build- ing a continuous polythene film sealed with bitumen or other suitable sealant into all floor, wall and ceiling areas. This vapour barrier forms a seal completely enveloping the store and is installed on the ‘wet’ or exterior side of the heat insulation barrier. All doors must also be fitted with gaskets and there should be an air lock at the entrance.
An adequate power supply for operating the apparatus which is to control the store’s environment is necessary. Control appliances should be positioned so that the heat they emit is exhausted to the store’s exterior. An important
consideration is positioning of air exhausts; hot air should be emitted from the store just below the roof line, while moist air should be expelled from near ground level if conditioned separately.
Temperature control
Storage temperature can be reduced by ventilation and refrigeration in addition to insulation and structural features. Complete temperature control is expen- sive in any part of the world and is unlikely to be used in commerce for seeds, regardless of their value. Total temperature control by refrigeration is, however, used in the long-term storage of germplasm collections and breeders’ material and was outlined above, under ‘Storage of germplasm’. Work by Ellis et al.
(1996) has indicated that storage at −20 °C rather than 20 °C is beneficial to seed survival, and that hermetic storage at 20 °C of seeds first dried at 20 °C to moisture contents in equilibrium with about 10% RH provides greater longevity than 5.5–6.8% moisture content in the five species studied (carrot, groundnut, lettuce, oilseed rape and onion).
Temperature control during the storage of ‘short-term’ and ‘carry over’
seed stocks is usually achieved by ventilation in conjunction with refrigeration, the degree of which will depend on the outside temperature.
Ventilation
The ventilation of seed stores should be considered in conjunction with the rela- tive humidity of the ambient air, because it would be more harmful to the seed to lower its storage temperature, if the result is to increase the seed’s moisture content. Conversely, ventilation can be used to lower the storage temperature and the seed’s moisture content when the relative humidity of the outside air is relatively low.
Storage engineers can design systems controlled either manually or auto- matically which operate ventilating fans according to the temperature and rela- tive humidity of the outside air. Ventilation also enables a gentle air flow to be passed through bulk lots of stored seed as and when required, thus ensuring that hot spots do not develop which would endanger the stored material.
Another form of localized heating sometimes coupled with moisture migra- tion can occur as a direct result of convection currents. These are most likely to occur in unattended stores with relatively poor insulation and are caused by drier warm air moving from a warm spot to a cooler one within the store. On cool- ing, moisture is condensed which is subsequently absorbed by the drier seeds.
Refrigeration
The use of refrigeration in controlling seed store temperatures is generally confined to long-term storage of high-value material, for example, germplasm collections and breeders’ stocks (Astey, 2007; Cromarty et al., 1982). How- ever, refrigeration is also useful in the tropics for valuable categories of other seed stocks.
Extra care and attention must be given to thermal insulation and structure of the store when refrigeration is to be included in the control systems.
There are four sources of heat within a seed store which the refrigeration has to cope with; these are derived from:
● Leakage from outside (despite insulation).
● Field heat of seed and associated materials.
● Respiration of seed in the store.
● Incidental heat derived from lighting, other equipment, workers and exter- nal heat which enters when doors are opened.
The cooling coils of a refrigeration unit should be situated within the storage area but the compressor must be sited so that its heat is given off to the exterior of the storage room. The relative humidity of the air is reduced during the refrigeration process. The moisture condenses on the cooling coils which have to be defrosted at intervals. Although this reduction in relative humidity is an advantage, in practice, the store’s relative humidity is inversely proportional to its temperature and at temperatures below about 13.0 °C, the relative humidity is too high for open storage.
Dehumidification
An alternative system to refrigeration of seed stores is the use of a suitable chemical desiccant in a dehumidifier.
There are two types of chemical dehumidifiers generally used in seed stores:
the bed and the revolving drum. In each system, the apparatus will add to the interior heat load if not carefully sited and it is therefore important that dehumidi- fiers be placed in the structure so that their heat is given off to the outside of the store. Silica gel, which can absorb up to 40% of its own dry weight in water, is usually used for seed store systems. In the bed system, the silica gel is dried up to about 175 °C to drive off all the absorbed moisture. After cooling, air from the storage area is blown through the dried silica gel bed. When the silica gel is again in moisture equilibrium with the air, it is reheated to dry it before further reuse.
Some bed systems use two beds per unit, in which case one bed is dehumidifying the store’s atmosphere while the other is being dried to reactivate it. The opera- tion of alternating beds is normally controlled by a time clock.
The revolving drum system has a desiccant bed which is divided between two air streams. Different sections are dried or used for absorption as the bed rotates. The revolving drum systems are capable of removing more moisture from a given air flow than the bed systems.
The choice of system will depend very much on local conditions and stor- age requirements, so in all cases qualified and experienced environmental con- trol engineers should be consulted.