Safe storage of grains
STORAGE OF GRAINS
(Dr. Girish Chandra)
The importance of grain storage as part of the marketing, distribution and food security system is well recognized. In 1978, following the resolution of the UN General Assembly which called for the reduction of post-harvest losses, FAO launched the Special Action Programme for Prevention of Food Losses (PFL). Since then more than 250 projects have been implemented worldwide under this programme. During recent years, as a result of privatization and liberalization of trade, the organization and management of grain storage has changed in many developing countries.
GRAIN STORAGE PRINCIPLES
High temperature and high moisture affect grain quality in storage and cause rapid decline in germination, colour, oil composition, and many other characteristics. Insects and fungus impair the quality of grains directly by their feeding and development, and indirectly by generating heat and moisture. High temperature and moisture favour development of insects and moulds. Insects cannot develop at temperatures below 10°C and moisture below 9% in cereals.
If the moisture is very high the grains should be dried before long term storage. Hot-air drying is necessary to maintain the quality of high moisture grain. However, holding grain at too high a temperature for too long in the dryer will reduce grain quality.
Stored grains should be inspected frequently. By operating the aeration system and smelling the air coming through the grain, storage problems can be detected. Insect activity can often be detected by looking for powdery material under the grain.
In India, about 70% of farm produce is stored by farmers for their own consumption. Farmers store grains in bulk using different types of storage structures made from locally available material. The major construction materials for storage structures in rural areas are mud, bamboo, stones and plant materials. They are neither rodent-proof nor safe from fungal and insect attack. Some of the major considerations for constructing a storage structure to minimize losses are:
- The structure should be elevated and away from moist places in the house.
- As far as possible, the structure should be airtight after loading and unloading.
- Rodent-proof material should be used for construction of rural storages.
- The area surrounding the structure should be clean to minimise insect breeding.
- The structure should be plastered with an impervious clay layer to prevent attack by termites and other insects.
Buying agencies often collect the bagged grain from the farm or market and the producer has to store the sacks of grain for some time before they are sold. The bagged grain must be kept off the ground to prevent seepage of water or attack by termites. If there is risk of rain during the temporary storage period the bags should be covered with waterproof sheeting. Alternatively, the sacks of grain should be stacked waterproof sheet away from walls in a rodent proof room. Plastic sheet lined jute bags have added advantage of being both insect and moisture proof and are now preferred.
In humid countries where grain cannot be dried adequately prior to storage and needs to be kept well ventilated during the storage period, traditional granaries are usually constructed entirely out of locally available plant materials: timber, reed, bamboo, etc. elevated at least one metre above ground level.
Peri for grain storage in Himachal Pradesh
Food grains like maize, wheat and paddy are stored in special structures made of bamboo called Peri or Peru. Prior to use, these structures are plastered on the inner side with a mixture of cow dung and clay. These containers are placed on the ground floor and grain is loaded into them from a hole made on the roof of the first floor. To take out grains, as per need, a special opening is provided near the bottom.
The use of bamboo containers allows the free exchange of gases inside the grain and keeping containers on the ground floor ensures cool temperature for storage. Loading from top and unloading from bottom offers easy material handling. Keeping storage structures away from main living room protects grain from fire etc.
Coal Tar Drum Bin
This bin was developed at the Central Institute of Agricultural Engineering (CIAE) and compares very well with other metal structures. Basically it is a used or empty bitumen drum, which is heated on fire to remove excess tar. A layer of tar remains inside, and serves as an insulator as well as a protective coating for the galvanised iron sheet. At CIAE the local artisans have been trained to fabricate these bins to suit village requirements. Inaccessible to rodents, efficient against insects, sealed against entry of water, these drums make excellent grain containers.
Chittore Stone Bin
As described earlier, locally available materials should, wherever possible, be used in the construction of grain bins. In Rajasthan, stone slabs are naturally available in abundance. At the College of Agricultural Engineering, Udaipur, a stone bin called the Chittore bin has been developed by using 40 mm stone slabs. It is a rectangular bin of 250 kg capacity and is constructed by farmers themselves using mud as a cementing material.
Polyethylene-lined bamboo bin
Conventionally, the bamboo bin fitted with a lid and with a plastering of mud inside and outside is a very common storage structure used by farmers in India. They are not impervious to attack by insects and rodent pests. The modification of these bins by lining them with polyethylene has been found to be very effective. One such bin, developed at the College of Agricultural Engineering, Akola, can store about 500 kg of grain. The bin is mounted on a metal tripod with rat barriers. It ensures air tightness due to the constant pressure of grain.
Underground Storage in Khattis
Practiced in northern India this method of storage is used in dry regions where the water table does not endanger the contents. They are long term storage pits of varying capacity from a few hundred kg to 200 tonnes. The entrance to the pit may be closed either by heaping earth or sand on a wooden board cover, or by a stone sealed with mud.
The advantages of khattis are:
- Few problems with rodents and insects.
- Low cost of construction.
- Ambient temperatures are relatively low and constant.
- Being indoors and invisible it is relatively safe from thieves.
- No need for continuous inspection.
The disadvantages are:
- Construction and digging are laborious process.
- Storage conditions adversely affect the germination capacity of grains.
- The grain can acquire a fermented smell after long term storage due to humidity.
- Removal of grain is laborious and can be dangerous because of the accumulation of carbon dioxide in the pit.
- Inspection of the grain is difficult.
- Risk of penetration by water, insects and rodents is high and the grain at the top and in contact with the walls often becomes mouldy.
Developed by the Indian Agricultural Research Institute (IARI), New Delhi, these silos are made of earth or bricks. They are rectangular in shape and have a capacity of 1 to 3 tonnes. A typical Pusa bin has a foundation of bricks, compacted earth or stabilised earth. A polyethylene sheet is laid on this, followed by a concrete slab floor 10 cm thick. An internal wall of the desired height (usually 1.5 to 2 metres) is constructed of bricks or compacted earth, with a sheet of polyethylene wrapped around it. This sheet is heat-sealed to the basal sheet, and the external wall is then erected. The concrete slab roof is supported by a wooden frame and is constructed of two layers separated by a polyethylene sheet. During its construction, a man-hole measuring 60 x 60 cm is built into one corner.
The bin is constructed on a hard surface to prevent rodent attack. If the surface is not hard, a platform of burnt bricks is built. The black polythene sheet is spread over the platform, extending 60 mm from all four sides. Another platform of unburnt bricks is constructed over the polythene layer. The inside wall is built to the required height, depending on the capacity of the structure. The surface of the wall is plastered with mud.
A wooden frame with an additional pole at a distance of 450 mm from the end of the structure is prepared and is placed at the top of the inner wall to support the roof. An additional pole is placed 250 mm from the outer side of the structure to provide further support. A small hole, 90 mm in diameter, is cut near the bottom in the middle of the front wall for delivery of the grain. A mud slab, 50 mm thick, is placed over the raised inner wall to serve as roof, leaving a manhole of dimensions 500 x 500 mm at one comer. The structure is then plastered with mud on top and on all four sides, and is left to dry. At this stage, a pouch made out of galvanised iron or plastic pipe 90 mm in diameter with a cap, is fitted into the delivery hole.
The outer wall of the structure is erected using burnt bricks up to 450 mm and unburnt bricks for the rest of the portion. Alternatively, a band of metal is provided at 450 mm to make it rodent proof. The whole structure is again plastered with mud on top as well as on all four sides and allowed to dry before use.
After filling the structure with grains, the diagonal cut on the film covering the manhole is sealed with adhesive tape and the manhole is plugged with mud. For efficient performance, the Pusa bin is used only after it is completely dried and filled completely so that minimum free space is left.
The rural godowns are primarily meant for providing warehousing facilities to the farmers. The godowns are of 100 to 1000 tonne capacity. They are owned by FCI, central and state warehousing corporations, market committees or cooperatives that look after the storage facilities for agricultural produce. The Government of India (GOI) appointed an expert committee (1979-1980), which concluded that there was a need for storage facilities for 2 million tonnes of food in rural areas. The govt. of India gives 50% subsidy for the construction of rural godowns. Therefore, godowns are being constructed on a large scale in Indian villages.
A silo is a structure for storing bulk grain or fermented feed known as silage. Silos are more commonly used for bulk storage of grain, coal, cement, carbon black, wood chips, food products and sawdust. Three types of silos are in widespread use today: Tower silos, Bunker silos and Bag silos.
Storage silos are cylindrical structures, typically 10 to 90 ft in diameter and 30 to 275 ft in height and made of concrete or metal. Silos store grain, cement and woodchips that are typically unloaded with air slides. Tower silos containing silage are usually unloaded from the top of the pile, originally by hand using a pitchfork. An advantage of tower silos is that the silage tends to pack well due to its own weight, except in the top few feet.
This silo contains 27 variations of stone, sand and gravel, usually with concrete walls, that are filled and packed with tractors and loaders. The filled trench is covered with a plastic tarp to make it airtight. These silos are usually unloaded with a tractor and loader. They are inexpensive and especially well suited to very large operations.
Bag silos are heavyweight plastic bags, usually around 8 to 12 ft in diameter, and of variable length as required for the amount of material to be stored. They are packed using a machine made for the purpose, and sealed on both ends. They are unloaded using a tractor and loader or skid-steer loader. The bag is discarded in sections as it is torn off. They can be used as a temporary measure when growth or harvest conditions require more space, though some farms use them round the year.
Modern steel silos
Both galvanized and glass coated steel are used for silage and grain storage. Their adaptation is primarily one of installing aeration equipment, modifying unloading if necessary, and making provision for aeration air discharge in the top of normally sealed units. Older style steel silos, especially those that show severe corrosion in the lower sections and those that have not been used for a number of years, should be viewed with extreme caution as safe grain or silage storages. Corrosion on a very thin metal wall can markedly reduce the metal area to sustain the storage load.
Warehouses are intended for the storage and physical protection of grain, mainly bagged grain. It may also include materials and equipment required for the packaging and handling of bagged grain, and pest control.
Standard Warehouse Design
All warehouses consist of a floor, walls, a roof, and one or more entrances and may include ventilators, windows, artificial lighting, etc. The warehouse should also be easy to clean and maintain and it should provide good working conditions.
The floor must be able to bear the weight of the grain which will be stacked upon it, and it must also be impermeable to ground water. For these reasons the floor should consist of a slab of reinforced concrete laid upon well compacted hard core, with a moisture barrier sandwiched between the two. This moisture barrier should consist of a layer of bitumen or asphalt, bitumen felt, or a polyethylene film. The floor level must be sufficiently above ground level to ensure that water will not enter the warehouse, even after the heaviest rainfall.
Most modern warehouses are constructed with a framework, usually of reinforced concrete. The supporting pillars are linked together by lower tie-bars, which are themselves secured to the floor slab, and by upper tie-bars, which hold the frame firmly together. The walls of the warehouse are built between the supporting pillars.
The walls may be made of breezeblocks, or stabilised earth bricks 15 to 20 cm thick, and should be rendered smooth on both sides. They should be painted white, on the inside to facilitate the detection of insect pests, and on the outside to help keep the warehouse as cool as possible.
Roof frames made of wood or bamboo is only suitable for warehouses not more than 4 or 5 metres wide. The wood used must be well dried and treated with a preservative. Roof cladding may be of galvanised steel or aluminium sheeting, or asbestos cement; the latter being more fragile but having better insulating properties. The roof should overhang the gables by 1.0 metres to ensure that rainwater does not drip on the walls. The overhang also helps to keep walls cool and protects ventilation openings from rain.
Ventilation openings are necessary for reducing the temperature and moisture in the warehouse. They should be fitted on the outside with anti-bird grills and on the inside with 1 mm mesh screens to deter most insects.
The number of doors will vary according to the size of the warehouse. If possible there should be at least two doors. Double sliding doors are recommended made of steel, or at least reinforced along their lower edges with metal plate as protection against rodents, they should be sufficiently large and closely fitting. It is recommended that the doors be protected from rain by an extension of the roof or a separate cover.
Grain elevators are buildings or complexes of buildings for storage and shipment of grain. They were invented in 1842 in Buffalo, New York by Joseph Dart, who first developed a steam-powered mechanism called a marine leg for scooping grain out of the hulls of ships directly into storage silos. Older grain elevators and bins often were constructed of framed or cribbed wood and were prone to fire. Grain elevator bins, tanks and silos are now usually constructed of steel or reinforced concrete. Bucket elevators are used to lift grain to a distributor or consignor where it flows by gravity through spouts or conveyors and into one of a number of bins, silos or tanks in a facility. When desired, the elevator's silos, bins and tanks are then emptied by gravity flow, sweep augers and conveyors.
An interesting problem the old elevators had was that of silo explosions. Fine powder from the millions of grains passing through the facility would accumulate and mix with the oxygen in air. A spark could spread from one floating grain to the other creating a chain reaction that would destroy the entire structure. To prevent this, elevators have very rigorous rules against smoking or any other open flame. Many elevators also have various devices installed to maximize ventilation.