Section 2 - Summary requirements for safe grain storage
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of cereal grains
Storage of cereal grains
by S.C. Andales
SIGNIFICANCE OF STORAGE
Storage of perishable products is a paradox. It is something that must be done and yet it is something that should be avoided as much as possible. It is a good thing to do because it will give one the assurance that something is in reserve to use or eat for tomorrow. Storage capability allows continuous supply of materials for processing or for distribution. On the other hand, it is undesirable because it means that huge capital investments in storage facilities will be required. Moreover on reception in storage, the product is subject to pest infestation and natural product deterioration.
Most agricultural commodities are produced seasonally. Their harvesting is normally done during a short period of two to three months while their consumption is constant throughout the year. For this reason, storage becomes necessary.
The primary aims of storing food commodities are normally as follows:
THE ROLE OF STORAGE IN THE RICE POST-HARVEST SYSTEM
Storage is a critical component of the rice postharvest system. In graphic form, the relationship of storage with other operations in the systems is as shown in Figure 1.
As shown in the diagram, there are two points at which the storage operation is taking place - after procurement (or drying) and just before distribution. Procurement is dictated by production in terms of timing quantity and quality. It is generally predictable and therefore the paddy storage operation can be programmed.
Local distribution is more or less fixed in terms of the constant rate of consumption. Export on the other hand is to some extent unpredictable. Delays in ship arrival and variation of quality specification by buyers demand that allowance in milled rice storage capacity has to be provided.
Between the two storage operations is the milling operation. These three operations make a problematic trio to coordinate and control. Their functional relationship is illustrated in a model shown in Figure 2.
COMPONENTS OF THE GRAIN STORAGE SYSTEM
The primary concern in storage is the safety of the product. For perishable crops, the product quality and quantity have to be maintained or deterioration has to be minimized. To be able to achieve this purpose the five aspects of storage has to be considered, attended to and understood, namely:
The interrelationship of these five components or factors may be shown graphically in Figure 3 or may well be stated as follows:
"The personnel involved in storage operation have the primary responsibility of keeping the stored product safe and secure inside the structure against storage pests and environmental factors.
FACTORS AFFECTING LOSSES IN STORAGE
A. Mechanical and Environmental
C. Biochemical Processes
C6H12O6 + 6O2--6CO2 + 6H2 + 677 Kcal.
MEASURES FOR PREVENTING OR MINIMIZING LOSSES
Losses in storage can be minimized or prevented by adopting any or combination of the following:
A. Chemical Control
B. Biological Control
C. Physical Control
D. Proper Design of Structure
TYPES OF STORAGE
Storage systems maybe classified according to storage capacity, handling method or container structural materials.
A. Farm House Storage
B. Granary (Sack or bulk handling)
C. Warehousing (Indoor)
D. Silos (Outdoor)
A. Manufacture of storage facilities and structures should be done locally by domestic technology. This would result in saving of foreign exchange and reduction in cost.
B. Government subsidy should be provided to effect the adoption of locally developed technology by farmers and processors.
Fig. 2 Model of Paddy and Rice Storage Capacity Needs with Different Potterns of Milling and Procurement
The quantity of paddy to be stored at end of harvest season is a function of the rates of harvesting, milling and sales during a 4-month harvesting season.
Sales of milled rice at 1000 metric tons per month would require 12,000 tons of paddy (in rice equivalent) to be procured by end of harvesting season.
= 0.60 for existing harvesting/milling pattern: line ac
= 0.48 for improved harvesting/milling pattern: line bed
= 0.64 for existing harvesting/improved milling pattern: line a-d
= 0.44 for improved harvesting/existing milling pattern: line b-c
Fig. 3 Components of the Grain Storage System.
Ruben E. Manalabe
The storage of foodgrains is normally done for extended periods in order to maintain a uniform supply of food for consumption, for the domestic and export market and to provide a buffer stock for contingencies such as drought floods and war. Millers, traders and other private entrepreneurs adopt it to speculate on good price.
Like any post production operation, losses in storage are considered significant. And these are attributed mainly to spillage, attacks by insects, mites, rodents, moulds and dry matter loss due to respiration. In Southeast Asia, for instance, it is reported that grain losses in storage ranges from 5% to 15% (Champ and Highly, 1981). Insect infestation accounts for the single largest component of these losses.
The storability of grains is affected by their temperature and moisture content. Either or both factors must be reduced to ensure a conducive environment for storage. This could be done through aeration. Aeration involves moving a relatively low volume of air through the grain bulk to control grain temperature. The process was found to reduce the risk of damage or spoilage of grains.
Aeration is normally done using ambient air. The cooling process is termed ad ambient air aeration or natural aeration. Otherwise, dehumidified air is used as in other countries where the ambient air may have too high a heat content or enthalpy to effect sufficient cooling.
PURPOSES AND BENEFITS OF AERATION
A. Temperature Control
There are two general objectives of aeration. These are: a) to maintain a uniform temperature in the grain bulk; and b) to keep that temperature to a low level as practical.
Like most stored products, grain is a poor conductor of heat. As such, heat does not dissipate or escape easily or quickly in some portions of the bulk. Non-uniformity of temperature keeps warm spots to remain warm and if high temperature differences exist in the bin as induced by solar radiation, air convection currents are generated causing moisture migration. In this phenomenon, the convected air picks up moisture from warmer grain and transfers this moisture to the cooler grain where condensation of moisture would likely take place. This results in grain damage which is attributed to moulding, caking, rotting, and sprouting. This is considered critical in areas where large seasonal changes in temperature exist. The respiration of insects, moulds and the grain itself also creates localized heating in the grain bulk which could also raise the temperature of the region they occupied. This center of insect activity is known as the "hot spot". The hotspot expands in size because insects migrate because of its high temperature and create identical conditions along side through further respiration. The water produced by respiration tends to rise in the warm air of the hot spot and condenses in the cold grain. Both phenomena can cause damage and loss due to mould infestation.
Maintaining a low temperature in the grain bulk can deter the development and growth of fungi and could also inhibit insect infestation. Generally, at low temperature, the rate of multiplication of insects is very low. For instance, the saw-toothed grain beetle will not breed if the temperature is below 18°C. The grain weevil, on the other hand, can breed at temperatures as low as 13°C. Other studies have shown that cooling the grain to 63° F (17°C) or below prevents granivorous insects from completing their life cycle quickly enough to cause significant build up and damage to grain (Burges and Burrel 1964). Furthermore problems on insects are limited to temperatures in the approximate range of 15-40°C. In tropical climates, however, storage temperatures are normally in the range of 2030°C. Hence, cooling or heating the commodity to temperatures near or beyond the limiting temperatures gives a measure of control over insect development.
Low temperature in the grain bulk can significantly reduce the amount of pesticide required to give longterm protection from insects. Also, the rate of decay of pesticides is independent of the initial condition of grain. This means that the final concentration of pesticides on aerated grain after a given storage period is about the same, whether the grain be initially warm and wet or cool and dry. (Thorpe 1985).
Microbial growth is enhanced with increase in temperature. Reports have shown that an increase in growth of about 2.5-4.5 fold can be expected with 10°C increase in temperature if the temperature does not exceed the optimum for fungi. The optimum range for a variety of storage fungi is about 23 to 40°C. Other storage microorganisms have high optima and if the grain moisture is high, they can grow at temperatures of 65 to 75°C. Aeration with sufficiently low ambient temperatures could pervert this cyclic effect.
B. Other uses of Aeration
COMPONENTS OF AN AERATION SYSTEM
The components of aeration system basically consist of the following:
If recirculatory fumigation is to be carried out, a return duct is also included. Except for this return duct, the above basic components are found for both the bulk and bagged grain aeration systems. Fig. 3 and Fig. 4 show the typical bulk storage and bagged grain storage system, respectively, with the principal components of an aeration system.
AERATION PRINCIPLE AND SYSTEMS DESIGN
The temperature of the grain stored in bulk is reduced by allowing cool air to flow through the warm grain mass. The grain is cooled by displacing the warm air and by contact cooling. Ideally, the moisture content does not significantly change as the relative humidity of the aeration air is in equilibrium with the moisture content of the grain. When the cooling air has a low relative humidity, however, drying may take place. If this condition persists, then the grain may be cooled to below the incoming air temperature due to the evaporation from the surface of the grains. Conversely, the delivery of air above the equilibrium relative humidity of the grain at a given moisture content will result in rewetting the grain. Furthermore, climate and moisture content determine the requirement for aeration (Teter, 1981).
There are three principal considerations in the design of aeration systems. These are 1) airflow rate, 2) fan selection and 3) air distribution. Automatic controls which are now widely used mayle part of the system.
Airflow rate - This is the volume of air desired to maintain uniform conditions in the stored bulk and to remove the generated heat and water. The recommended rate depends on the purpose of aeration, the type of grain being aerated, the size and type of storage structure, and climatic conditions. In the United States, an airflow rate of 2 liters per second per ton is widely used for shelled corn and soybean stored in farm bins and in flat types of storage. For wheat and other smaller seed grain, one (1) liter per second per ton is more common. A range of 1 to 0.5 liters per second per ton is widely used especially when cost limitations for fan power is considered.
Fan selection - The selection of fan is normally based on the airflow rate used for a particular grain, the kind of grain handled and the grain depth. These factors determines the resistance of grain to airflow and the static pressures against which the fan must deliver the required airflow.
Two types of fan are used for aeration. These are the centrifugal and the axial flow fan. generally, the axial flow fan will deliver more air than centrifugal fans at a static pressures up to about 4 inches of water (1,000 Pa). For higher static pressures, the centrifugal fans are recommended.
Air distribution - This includes the ductings, false floors, etc. which are used to move the air to the desired points. The proper sizing of the ducts, the sizing and spacing of the openings in the ducts to let the air move between the duct and aerated grain, the layout of the duct system are important to maintain the entering (or exiting) air at an acceptable velocity and provide uniform airflow through the grain. Teter (1981) has outlined a detailed procedure in the design of aeration in bulk storage. Part of this procedure is shown in Annex A.
AERATION SYSTEM OPERATION
In controlling temperature, continuous ventillation is necessary during the early stages unless air temperatures are excessive. But for economic considerations, fan operation is selected only at a particular time of the day when the ambient relative humidity is lower or equal to the equilibrium moisture content of the grain. The use of humidity controllers which is interlocked with the fan controls provide convenience in the operation of the fan.
Generally, the grain temperature is reduced to below 15°C as soon as possible. Once the desired temperature is reached, only minimal ventillation is required. In this aspect, temperature sensors are likewise convenient to monitor and check the temperature in the bulk.
1) FOSTER, G.H. and TUITE, J. F. Aeration and Stored Grain Management. Storage of Cereal Grains and Their Products. pp. 117 -142.
2) TETER, N. C., 1981. Grain Storage. southeast Asia Cooperative Post-Harvest Research and Development Programme. SEARCA, College Laguna, Philippines.
3) Preserving Grain Quality by Aeration and In-Store Drying. ACIAR Proceeding No. 15 of an International Seminar held in Kuala Lumpur, Malaysia. 9-11 October 1985.
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