Grain Storage
Grain Storage
Grain storage on a subsistence farm is primarily based on minimizing grain loss. In modern agricultural practices there are methods of managing less than 1% grain loss, but small subsistence farms can see 20% - 100% of grain loss. This causes starvation and an unstable food supply. Grain loss can be caused by mould growth, bugs, birds, or any other contamination. One method of preventing loss is hermetic grain storage. Hermetic grain storage strives to eliminate all exchange of gases within the storage system. This mitigates bacterial activity and prevents rodents and bugs from being able to breathe inside the storage systems. The introduction of hermetic grain storage to subsistence farms can create a more stable food supply in the area and reduce the risk of starvation..
Requirements for safe storage Crops left standing un-harvested start to show diminishing quantitative and qualitative returns through shatter losses and attacks by insects, mould, birds and rodents. It is therefore important to complete harvesting as soon as possible. In addition, it is necessary to remove dust and contaminants, which can include insects, and vegetable material, such as bits of straw and chaff and weed seeds. These will fill up pore spaces
Within the crop, inhibiting air movement and adding to any possible spoilage problems. The crop must therefore be clean. One of the most critical physiological factors in successful grain storage is the moisture content of the crop. High moisture content leads to storage problems because it encourages fungal and insect problems, respiration and germination. However, moisture content in the growing crop is naturally high and only starts to decrease as the crop reaches maturity and the grains are drying. In their natural state, the seeds would have a period of dormancy and then germinate either when re-wetted by rain or as a result of naturally adequate moisture content. Another major factor influencing spoilage is temperature. Grains are biologically active and respire during storage. One of the products of respiration is heat, and reducing the temperature of the crop can help to diminish the rate of respiration, thereby lengthening the storage life by lessening the possibility of germination. Another major temperature effect is on the activity of insect and fungal problems. With lower temperatures, the metabolic rate of insects and fungi decreases and consequently so does the activity causing spoilage. A damp or warm spot in grain will increase the rate of respiration. In addition to heat, another product of respiration is moisture. The heat and moisture from such a ‘hot spot’ can spread by convection, encouraging moulds and bacteria, which in turn respire and give off more heat and moisture. It therefore becomes a self-generating process. Insect activity also increases with a rise in temperature. These spoilage mechanisms can also affect the viability of grain required for seed or malting, where the inability to germinate would render it unmarketable. Figure 16.1 shows how the relationship between moisture content and temperature affects the storability of crops. It can be seen that the moisture content of grain must be reduced at higher temperatures
Parameters
The major objectives of crop conditioning and storage have been discussed before. To be able to achieve these objectives, the store must satisfy the following parameters as far as possible:(a) the grain must be kept dry;
(b) The grain should be kept at a uniform temperature;
(c) The grain should be protected from insect attack;
(d) Rodents and birds should be excluded.
It is evident from previous sections that, in many cases, facilities for drying and storage are found in one and the same structure. Combining these functions is economical and allows further conditioning at later stages if required. For example, if a hot spot develops in a storage bin, it can easily be ventilated again. It may also be possible to provide some low-volume ventilation in an otherwise pure storage system. However, there are situations where storage is considered quite separately from drying, ranging from the storage of naturally dried crops to the storage of grain from a continuous-flow or batch dryer.
The size and type of a storage facility is likely to be dictated by: • total volume of crop/produce to be stored; • the storage requirements for the crop/produce to be stored; • the unit cost of various types of storage; • the form in which the crop/produce is stored, i.e. cob maize versus shelled maize, or bagged wheat versus bulk wheat.
The volume of the store required can be estimated from the expected yield and the land area. A comparison between different forms of storage is normally made by calculating costs per tonne of capacity. The form of storage depends not only on how the crop is harvested, the volume and the way it is delivered to the market, but also on the overall cost. Where drying is a problem, bag storage has the advantage of allowing a higher moisture content than bulk storage. For maize, the requirement for safe storage is a maximum of 15 percent and 12 percent moisture content respectively. In general terms, the respective advantages and disadvantages of bag and bulk storage are:
Bags Bulk
1. Flexibility of storage Inflexible storage
2. Partly mechenizable mechenizable
3. Slow handling Rapid handling
4. Considerable spillage little spillage
5. Low capital costs High capital costs
6. High operating costs low operating costs
7. Easy inspection Inspection more difficult
Solid - wall bins and silos for bulk storage
Solid-wall bins may be anything from a small plastered basket to large steel or concrete silos holding several thousand tons. The traditional bins used by African farmers are small with a capacity of 2–3 tonnes, including gourds, clay pots, mud-plastered baskets raised off the ground and mud-walled silos (‘rumbus’). Many of these solid-wall bins or silos have limitations, particularly in terms of durability and protection against rodents, insects and moisture from ambient air. Solid-wall bins or silos should be used only in areas where the produce can be dried sufficiently prior to storage. Several attempts have been made to improve on traditional stores to make them more suitable for long-term storage.
Improved traditional
Bins many traditional stores perform excellently in their appropriate climatic conditions and others can be made to do so with minor changes. Efforts should be made to prevent cracking of the surface of the walls and to seal the entrance to the bin. This can be done, for instance,
by adding lime or cement to the mud (i.e. a stabilized soil technique) or by incorporating an airtight lining (e.g. plastic) in the wall.
The major improvements are:
• The floor is raised above the ground to avoid moisture.
• Supporting legs of hardwood are made 90 cm long and equipped with baffles to protect against rats.
• Instead of mud, the walls may be plastered with cement or mud mixed with cement/lime.
• Inlets and outlets should be made with airtight and lockable covers.
• Thatched roof to protect the bin from rain and strong sun.
Underground pits
In a few countries (e.g. India, parts of Africa and Latin America), it is claimed that underground pits are able to store grain without damage for many years. The pits keep grain cool, and some of them are relatively airtight. However, the grain on top and around the sides often becomes mouldy. There are several types of pit, most of them flask shaped and covered with sticks, cow dung and mud, or a large stone embedded in soft mud. The area should be free from termites and relatively dry. Improvements to the pit may include:
• Better lining of straw and mat;
• Plastic sheets and concrete or Ferro cement;
• Use of plastic bags in the pit;
Brick-walled silo
Brick-walled silos or bins are suitable for small- and medium-size stores. The need for reinforcement makes them uneconomical when the height exceeds 7–8 metres. The wall may be made of bricks or blocks of mud, stabilized soil, burnt clay, stones or cement. To withstand the pressure from the grain, the wall will need reinforcement commensurate with the size and strength of the building materials. Reinforcement can be reduced, and even omitted, by building thick, heavy walls (gravity walls). Figure 16.21 shows a silo with gravity walls where the bricks are Mud or soil, Cow dung, Sticks, Straw lining placed radially. While no reinforcement is needed for this size, more building material is required. Walls made of bricks, mud or cement will absorb moisture from the ambient air. In areas with high relative humidity it is therefore necessary to protect the grain by adding a moisture barrier to the silo walls. It will help considerably to bag-wash or plaster the walls on the outside with a mortar of cement–lime–sand (1:1:5) for burnt bricks or cement, and cement–sand–mud (1:2:6) for mud walls. The walls can then be painted with plastic paint or coal tar if better protection is needed.
An alternative to plastering and painting the silo is to incorporate a lining of plastic sheeting in the middle of the wall, floor and roof to make the container airtight. The Pusa bin is such a structure and has been developed by the Agricultural Research Institute in New Delhi. Originally the bin was rectangular with walls of two layers of brick; the floor and the roof are made of two layers of mud. The system can be used for silos of any shape and, if properly constructed, will give good protection against air and moisture.
Reinforced concrete silos Concrete can take very little tension and needs to be reinforced when used for silos. Small silos suitable for an individual farm may be reinforced with chicken-wire. The Ferro cement store or ‘ferrumbu’ is a typical example.
One or two layers of 12 mm chicken wire are tied to vertical sticks or rods placed in a circle. The chicken wire is then plastered from both inside and outside. The verticals are removed after the outside is finished. Taller silos of 3–4 metres or more may be framed by welded mesh wire and with 12 mm chicken wire tied to the outside. With bags or plastic tied to the outside, it is possible to plaster the silo from the inside first and then from the outside a few days later after removing the bags. These techniques make it possible to construct walls with a thickness of 3–6 cm. Larger concrete silos are built using a sliding mould, which is moved upwards continuously or step by step. Reinforcement and concrete are supplied from the top. Concrete silos can be made airtight if openings are properly sealed
Storage warehouses
A warehouse is built for the storage and physical protection of goods or bagged grain. It may also include materials and equipment required for the packaging and handling of bagged grain, and chemicals to control storage pests. Factors such as topography, soil characteristics, accessibility, orientation and proximity to human dwellings should be considered when locating the warehouse.
When determining the dimensions of the warehouse, the following information is important:
• The specific volume of the principal product to be stored (m3/tonne)
• The maximum tonnage of the product to be stored
• The maximum stack height desired
• The extent to which separation of lots is desired
.Steel bins
Steel bins range from thoroughly cleaned steel oil drums to commercial stores with a capacity of several thousand tonnes. In most cases, steel silos are more expensive than concrete silos but have the advantage of being easy to erect and, for the smaller sizes, possible to move. The welded steel silo is normally airtight if the openings are properly sealed, but even a silo assembled of corrugated iron sheets can be made airtight if all joints are sealed with rubber gaskets or bitumen
Bag storage
The most commonly used method of grain storage in many countries is bag storage in any of a variety of buildings, e.g. stone, local brick, corrugated iron and mud and wattle, with or without plastered walls, and with an earth, stone or cement floor and a corrugated iron or thatched roof. As mentioned before, the form in which the produce should be stored will depend on the quantity, harvest method, handling method, moisture content and costs. The advantages of bag storage were listed earlier. The disadvantage is that jute bags give no protection against insects, which means that an insecticide has to be used. In some countries with a dry climate, it is common practice to stack the bags on plinths and cover them with a tarpaulin for temporary storage. Examples are the hard stands used in Zambia and the groundnut pyramids used near Kano in Northern Nigeria. However, if the grain is to be kept for some time, it is recommended to store the bags in a building. A simple store makes use of the ventilated maize crib that is used for drying, the only difference being that the walls should be covered for protection against rain (see Figure 16.24). If the bags are stored in a multipurpose farm shed, or even in the farmer’s dwelling, they should be kept out of reach of rats and mice. A raised, freestanding platform equipped with rat guards will serve this purpose.
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