In this article we will discuss about: 1. Introduction to Grain Storage Problems 2. Analysis of Airtight versus Ventilated Storage 3. Analysis of Bag Storage versus Bulk Storage 4. Analysis of Underground Storage versus Above-Ground Storage 5. Storage 6. Recommendations 7. Chemical Control Operations.
Contents:
- Introduction to Grain Storage Problems
- Analysis of Airtight versus Ventilated Storage
- Analysis of Bag Storage versus Bulk Storage
- Analysis of Underground Storage versus Above-Ground Storage
- Storage for Different Periods
- Chemical Control Operations for Grain Storage
- Recommendations for Grain Storage
1. Introduction to Grain Storage Problems:
Stored grains are usually damage. Mainly from four sources, viz., rats, insects, mites and microorganisms. The problems of rats, being basically different from those of the other three, these three can be talked together first.
The safety of stored grains from damage caused by insect’s mites and micro-organisms largely depends on the proper management of three factors, viz.:
(1) The moisture content of the grain,
(2) The availability of oxygen, and
(3) The development of “temperature gradient” within the stored grain.
For its rapid development, each of the different insect and mite species requires particular humidity which in stored grains depends on the moisture-content of the grain a certain percentage of oxygen in the air and a certain range of temperature. The requirements of the micro-organisms are also somewhat similar although some of them are capable of sustaining them self even in the absence of oxygen.
Hence, for ensuring the safety of the grain from insects, mites and micro-organisms, all these three factors have to be properly manipulate, firstly, through the design and construction of the storage structures and storage godowns and, secondly, through storage practices. If, however, these ecological safety measures are not practicable under a particular set of conditions, then recourse to the chemical or physical control of infestation becomes necessary.
(1) Moisture-Content:
The moisture requirement of some storage pests are given in Table 5.1. This table will show that if arrangements can be made to maintain the moisture-content of the stored grain below about 8 percent, then most of the insect infestation can be avoided except that of the Khapra beetle which can be avoided by the proper manipulation of the oxygen content of the air (videinfia). The moisture-content has also to be kept low for ensuring freedom from micro-organisms and the quality and viability of most of the grains. The question is how best manage the moisture-content of the grain.
The initial moisture content before storage can be brought down by sun-drying or by means of different types of grain-dries. But, if this dried grain is stored under conditions wherein it keeps coming into direct contact with atmosphere moisture, it will soon adjust its moisture in accordance with atmospheric humidity.
It will also absorb the moisture if the grain is in direct or indirect contact with a moist surface. A third source for the increase of moisture is the metabolic water produced within the grain through insect and / or the grain respiration but this source remains insignificant if the above two sources are taken care of.
Also at times there is increase of moisture in different portions of the stored grain due to the development of a kind of temperature-gradient within the grain, the moisture evaporating from the warmer portions and condensing in the cooler spots of the grain stock(vide infra). All these points have to be kept in view in designing storage structures and storage godowns and also in finalizing the schedules of storage practices.
(2) Oxygen Availability:
All animals and plants require oxygen for their respiration. When a grain like wheat is stored in an airtight storage structure, the oxygen- content of the air in this enclosed microcosm goes on getting reduced as it is consumed in respiration both by grain and by the insects, if any, infesting the grain. Weight for weigh, the oxygen consumption by insects is 130,00 times the consumption in grain respiration.
Hence, the reduction in oxygen-content is much quicker if the grain is infested by insects. And, as each insect species requires a particular minimum concentration of oxygen, it is killed once the oxygen-content goes below that minimum.
The minimum concentration of oxygen required by the different stages of two species of storage pests has been determined at the IARI by keeping these stages confined along with their food in airtight containers and analysing the oxygen content at the time when all the individuals are dead. The data thus collected are given in Table 5.2., it will be seen from this table, that different stages of even the same species can stand different amounts of reduction in the oxygen content.
It is obvious that further multiplication of a particular species will stop as soon as the oxygen-content goes below- the level required by the most susceptible stage and not the one required by the most resistant stage. Thus, for example, the Khapra eggs lose their viability at 16.8 per cent oxygen-content, while full-grown larvae can stand up to about 1 percent. Naturally this pest cannot multiply when the oxygen content goes below 16.8 per cent.
The insects generally die much before the oxygen gets completely exhausted and before the question of real anaerobic respiration by the grain of the micro-organisms can be expected to arise although some micro-aerophilic bacteria grow in reduced oxygen tension. It is also clear from this date that complete airtightness is not needed.
It will be enough if the storage structure is sufficiently airtight so as to bring down the oxygen-content of the enclosed air to the desired levels, as shown in Table 5.2 Further, it will be seen that the Khapra pest, which can thrive even at much lower moisture content, is very susceptible to the reduction in oxygen-content.
Hence, if the grain is dried up to about 8 per cent moisture content and then kept in a reasonably airtight enclosure, most of the species will not multiply due to the storage of moisture and those that will multiply for some time will get annihilated due to the shortage of oxygen.
The time taken for the reduction of oxygen to the lethal level will depend on the extent of infestation, the volume of empty space in the storage structure etc. For example, if the empty space is more, the amount of available oxygen will also be more and more time will be needed to reduce the oxygen content to the lethal level. This explains why some times insects can remain alive in practically empty structures for quite long periods even if they are airtight.
(3) Temperature Complications:
Temperature is the most important factor controlling the development and multiplication of insects in general. Besides this simple and universal temperature effect, there are a number of harmful side complications arising out improper or ineffective manipulation of the temperature factor.
These are mainly due to:
(a) Heat spot developing within the stored grain, and
(b) Development of temperature gradient generally from the middle towards the periphery.
The development of the heat spot is due to the excessive heat produced either by insect multiplication or by microbial infection in some localized pocket. Insect respiration produces both water and heat and due to comparative non-conductivity of the grain, heat dissipation does not take place effectively.
The result is that both temperature and moisture increase, leading to greater multiplication, first of insects and later of micro-organisms. Sometimes these heat spots can develop even without insect infestation in case the moisture-content of the grain happens to be high enough for microbial activity.
Also sometimes this chain activity goes on to such an extent that the heat developed in pockets of infestation kills the insects there but this self-sterilization of the grain is not of much help because by the time this stage is reached, the grain becomes very much damaged and spoiled. Lumps of grain are formed which give off offensive smell. The process is commonly known as ‘caking’.
Hence all efforts have to be directed towards the prevention of such heat spots and to cool them down in case some heating has started in some pockets. This can be done by various means, such as forced aeration turning the grain, control of infestation by fumigation, etc. The development of the temperature gradient is due to unequal heating in the different portions of the stored grain.
Generally, it is from the middle where the temperature rises due to the respiratory activities of grain, insect infestation microbial infection, etc., to the periphery where the temperature may be low particularly if the wall of the storage structure is made of some good conductor (say metal) which dissipates that heat outside.
Also if a metallic storage structure like a big silo happens to be under direct sun, the surface facing from south-east to south-west may get comparatively more heated and a temperature -gradient may develop from south to north inside the grain. The main harm done by these temperature-gradients is in what’s sometimes known as sweating of the grain.
The moisture evaporating from the grain in the warmer region migrates to the cooler region and condenses there. This lead to the increase in the moisture-content in the cooler region sometimes even to the extent of the grain getting wet; the dam grain then provides better environment for the multiplication of insect infestation and microbial and microbial infection, for increased grain respiration and even for caking of the grain.
Thus, it will be seen from the above, that the grain suffers both from a poor conductivity of its own and from the good conductivity of its container. It is, therefore, advisable to make the wall of the storage structure from a material with poor conductivity and also on the other hand to make suitable arrangement to overcome the harm due to the poor conductivity of the grain itself.
Keeping in view the few basic principles initiate above, it should become fairly easy to come o rational decisions regarding various disputed points about storage structure, storage godown and storage practices. The main points for which it is necessary to have quite clear-cut ideas are being discussed below.
2. Analysis of
Airtight versus Ventilated Storage:
It has been demonstrated again and again that insects do not multiply long under airtight storage. However, as grain is also a living material it cannot be theoretically kept indefinitely under completely airtight conditions because ultimately the oxygen is bound to get considerably exhausted and thereafter the grain will also die and deteriorate.
Under practical conditions, however, it has been observed that insects die long before oxygen gets completely exhausted and thereafter the grain which consumes very little oxygen can remain healthy for quite long-periods, provided its moisture content is not high, i.e., above 10 percent or so under Indian conditions.
Also in the case of airtight storage, there is no fear of increase in moisture-content from out-side source. The one disadvantage of this type of storage is that it is comparatively difficult to maintain the airtight conditions ventilated storage on the other hand provides a very good environment both for insects and the grain. But on the whole, insects multiply quickly and damage the grain very much unless special disinfestation operations are resorted to.
Despite this distinct disadvantage, some specialists on pest control have expressed opinion in favour of ventilated storage. Obviously when the storage conditions are such that the airtightness is not enough to keep the pest infestation down, then it is certainly better that the storage to be kept thoroughly ventilated. Under such circumstance, the advantage of full ventilation is that the storage conditions remain more hygienic for the workers and the chances of the development of heat spots remain somewhat reduce.
However, on the whole the above analysis of the pros and cons of ventilated and airtight storage leads to the conclusion that whenever possible and practicable airtight storage should be preferred to ventilated storage and all planned efforts should be made in that direction.
3. Analysis of
Bag Storage versus Bulk Storage:
Bag storage is largely in vogue, particularly in the Government and trade godowns, mainly because of the ease in handling and transport; otherwise there are quite strong scientific considerations in favour of bulk storage which is in vogue both at the level of cultivators who construct various kinds of storage structures like bukhari, Kothari, morai etc., and also at the level of very advanced storage organizations making big silos and underground pits.
The points in favour of bag storage are:
(a) That each bag constitutes a definite unit which as such can be brought in, sent out, purchased or sold,
(b) That under present Indian conditions it is easier to load and unload grain packed in bags,
(c) That bags can be neatly arranged in stocks of different sizes, and
(d) That if any individual bag is noticed to be infested it can be easily segregated and treated, (ex) that the surface of the bag being practically exposed to atmosphere the problem of sweating is less etc.
On the other hand, the main scientific considerations in favour of bulk storage are:
(a) That the peripheral exposed surface area per unit weight of grain is much less in the case of bulk storage and consequently the damage from external sources is reduced, and
(b) That because of the slow movement of air in the inter granular space the conditions in the deeper layers of the grain stored in large bulk are nearer to those of airtight storage and that results in the consequent reduction in infestation which tends to remain confined to the periphery. These two main considerations in favour of bulk storage are on the whole much weightier than several of the smaller considerations in favour of bag storage, particularly if adequate precautions have been taken against sweating of grain in bulk storage.
4. Analysis of
Underground Storage versus Above-Ground Storage:
There is no intrinsic scientific difference between underground and above-ground storage provided the basic requirements of safe storage enunciated earlier have been provided for. However, there are certain practical advantages and disadvantages of these two storage practices as they are generally in vogue in the country at present.
These practical considerations in favour of underground storage are:
(a) That the grain in underground storage is more free from the seasonal vicissitudes of temperature and humidity provided of course adequate precautions have been taken against seepage etc., in the underground pits particularly in regions where the water table is likely to rise; this is very desirable if the grain has been brought to the proper condition of moisture and temperature before storage,
(b) That the underground storage pits are nearer to airtight storage with its consequent advantages,
(c) That in underground storage the grain is generally stored in bulk which has its own advantages, and
(d) That underground storage is safer from various external sources of damage including theft etc.
The considerations in favour of above-ground storage on the other hand are:
(a) That the above-ground storage can be maintained in more hygienic conditions
(b) That the above-ground storage is more convenient for inspection and for operations like turning the grain, and
(c) That the danger of the grain heating up due to internal sources is somewhat less.
There has not been much controversy about other aspects like dunnage etc. It should, however, be kept in mind that there is no point in making temporary dunnage in a permanent storage godown, as is generally found to be the case. It is better to provide permanent dunnage so as to avoid annual recurring expenditure.
5. Analysis of Storage for Different Periods:
The period for which the storage is to be arranged is of vital importance in deciding about various storage practices.
On the basis of this criterion, storage can be classified into the following categories:
(a) Transit Storage:
In this category comes shortest-term storage in which case the grain is practically on the move or where some kind of rotation in practised so that the old stock moves out as the fresh stock comes in. The examples are many of the Government godowns, godowns at the seaports, godowns of retailers and also of some wholesalers. In such cases, when the grain is handled at very short intervals, bag storage has to be preferred for the ease of handling and transportation; this has to be preferred till such time as modern means of handling and distribution are provided for in this country.
(b) Short-Term Storage:
Sometimes the storage is practiced by cultivators who generally like to store their seed grain from harvest to sowing and food grain from harvest to harvest, if not for longer periods. The storage in the Warehousing Corporation may also be included in this category. These people generally and rightly use various types of storage structures like bukhari, Kothari, morai etc.
Which are all examples of non-airtight bulk storage? The necessity of bringing about the desired improvements in these structures has been realized for sometimes past and the Indian Standards Institution has formulated standards for these different types. The Indian Agricultural Research Institute has, on the other hand, devised what is called Pusa Bin for such storage and the combines in itself the various ideal storage requirements mentioned.
(c) Long-Term Storage:
Sometimes the storage is made for such long periods as required by:
(i) Large-scale trade stockiest, and
(ii) Government agencies desiring to deep bufler stocks or to maintain food banks.
It is for such long- term storage that we must make very carefully thought-out planning and recommendations.
6. Chemical Control Operations for Grain Storage:
If the recommendations given above for safe storage are properly followed and provided for, there should be no necessity to use insecticides in grain storage.
If, however, it has to be done, the following points may be kept in view:
(a) Mixing of Insecticides:
It has been shown beyond doubt that persistent insecticides mixed with grains can keep the grain safe from insect infestation for varying and quite long periods. Although this provides quite a potent method for storing grain meant solely for seed purposes, a recommendation to this effect cannot be free from serious hazards particularly because the grain though originally meant solely for seed can always find its way to the food market.
Also if the general public becomes well conversant with this method, it will become quite difficult to check the unscrupulous people from applying the same method to food grain. Hence, it is advisable to desist from this otherwise tempting recommendation.
(b) Impregnation of Bags with Insecticides:
This recommendation is also quite tempting and has been made by a number of workers but there is a subtle limitation due to which this method generally does not work. The storage pests, particularly the beetles, take just a few minutes to enter the weave of the bag unless it is very close and tight and during this short period, they generally do not pick up the lethal dose. Due to this limitation, this method cannot be much depended upon.
(c) Dusting of the Bags:
This is likely to be more effective than impregnation because in the case the dust can stick to the insect body and some of it may be carried along with the insect. Of course, this may also lead to greater contamination than impregnation. The safest dust to be used for this purpose is the one based pyrethrum as an active ingredient fortified by a suitable synergist. However, this insecticide is not of persistent type and breaks down within a very short time.
(d) Fumigation and Fumigants:
This is the best and most dependable technique for disinfesting grain. One has, however, to be careful about the choice of the proper fumigant. In this connection it may be mentioned that a fumigant like carbon tetrachloride is comparatively less toxic to insects but its good point is that it is less absorbed by the grain.
Another fumigant like ethylene di-bromide is comparatively highly toxic to the insects, but its bad point is that it is highly absorbed by the grain. Other fumigants like ethylene dichloride occupy an intermediate position in both respects. Hence great care has to be taken in the choice of a fumigant. An ethylene dichloride-carbon tetrachloride mixture in the 3:1 ration has been much in vogue because of its comparative safety to operators.
Methyl bromide is a very good general-purpose fumigant but it has to be handled with utmost care and with special fumigation appliances. During recent years, phostoxin has become quite popular, but due to the extremely hazardous nature of phosphine gas, generated by the tablets, its use by untrained individuals has not been recommended.
(e) Disinfestation of Godowns:
Fumigation is the best technique for disinfesting godowns, provided the godowns are structurally fit for this operation. Otherwise a good spray of the walls with a suitable persistent insecticide can be resorted. The safest spray is the one based on pyrethrums fortified by a suitable synergist. At present Malathion, which is also a relatively safe insecticide, is use widely for this purpose.
7. Recommendations for Grain Storage:
1. Grain Dryers:
As stated already, the moisture-content of the grain is the most vital factor to be taken care of in order to ensure safe storage of the grain. In dry months and in dry areas, the grain can be dried directly in the sun but the same is not possible in wet areas and in wet months even in dry areas. Hence, it is highly advisable that a suitable grain dryer is provided for each storage godown without exception. This should be considered as an item of highest priority for any storage organization.
2. Storage Structures and Godowns:
These have to be somewhat different for short-term, long-term and transit storages.
(a) Short-Term Rural Storage:
For this purpose and ideal storage has been devised Pusa Bin. The rationale of the structural design of this bin is as follows- Earthen structures of various shapes and size provide the easiest and most economic methods for the storage of grains under rural conditions in many parts of our country. However, in this method of storage, one often finds the grains infested with insects.
In some localities rats also pose a serious problem in this kind of storage, since they easily cut through the mud walls and do considerable damage to the grains stored inside the mud structure. In some wet regions of the country, grains are also found to be affected by the high humidity conditions prevailing outside and leading to a process which ultimately results in the caking of the grains.
As a result of all these troubles, the loss of food grains in storage is often quite considerable will) a view to reducing these losses, a thin sheet of polythene film (0.17 to 0.18 mm thick) is embedded in the mud wall of an ordinary earthen structure. The idea behind the sandwich in of the polythene film within the body of the wall is to combine the mechanical strength of the mud wall with the just effective imperviousness of polythene films to vapors and gases.
The polythene film being fairly impervious to water vapors does not allow the atmospheric moisture to enter the grains. Also this film is impervious enough to oxygen with the result that the oxygen tension within the structure is reduced to such an extent that insect multiplication becomes impossible.
On the other hand, the earthen layers both inside and outside the polythene film keep the film safe from mechanical injuries due to abrasion and handling stress and strain. When a polythene film is used as an inner or outer lining instead of being embedded within the wall, it gets damaged quickly and becomes ineffective. Also earthen layers provide very goods walls for the storage structures because they have poor thermal conductivity and reduce the dangers of sweating at the periphery.
Thus it will be seen that the Pusa Bin combines in itself all three major requirements of safe storage, viz.:
(i) It is moisture- proof,
(ii) It is just sufficiently airtight, and
(iii) Its walls have poor thermal conductivity.
The earthen layers can be replaced by any other material of poor thermal conductivity; 4 of course, wood has the danger of being attacked by termites.
Results of comparative tests have shown that freshly-harvested and dried wheat could be kept safe for a long time in improved type of storage structure. It has also been shown that none of the four principal storage pests, viz. Sitophilus oryzae, Phizopertha Dominica, Trogoderma granarium and Tribolium castaneum, could breed well in this improved type of storage structure (even when they were introduced), provided the initial moisture-content of wheat was at a level of 10 percent or below. The viability of the wheat seeds is also not affected when stored in this type of structure for more than three years. There is no obvious factor which is likely to limit the size of the storage structure.
However, the details of the constructional procedure and the cost involved in making a structure of about 2,000 kgs, are given below:
The structure is constructed over a pucca brick floor. It a pucca brick floor is not available, it should be made at a suitable site. Next, a mud platform of roughly 1.7 cm × 1.2 cm × 7 cm (thick) sizes is made and a polythene film of 1.8 m × 1.4 size is placed over it. A 7 cm thick layer of mud is then applied over the film covering the same size as the platform under the polythene film.
The inner wall (111 cm thick) of the structure covering the four sides is then constructed. The inner layer of the mud roof of the structure is then made by using 5 cm-thick mud slabs prepared earlier and placing them over a wooden frame. An area of 50 cm x 50 cm is, however, kept open in one corner to serve as a manhole.
The entire structure is then covered with the polythene film carefully, the polythene cover having been prepared earlier by the method of heat-sealing; thereafter, the free edges of the polythene sheet near the base are also similarly sealed. At this stage, the outlet is fixed by making a hole in the inner mud layer as well as the polythene film.
Finally, the outer layers of the walls of 11 cm thickness are created all-round the structure covering the polythene film. A 5 cm thick mud plaster is also put over the polythene film at the top, leaving out the manhole. The portion of the polythene film covering the manhole is then cut diagonally for making the necessary passage.
When the structure is filled with grain, the manhole is finally sealed with a square piece of polythene film. Normally, grains are to be taken out as and when necessary through an outlet fixed near the base of the structure except that at the end of the storage season, the manhole at the top can also be used for this purpose.
For making the structure rat-proof, the outer wall of the structure may be constructed using pucca (burnt) bricks upto a height of 45 cm. Alternately, a tin-band (from old kerosene tins) may be provided round the structure upto the same height. By this way the structure is protected from damage by rats.
(b) Long-Term Storage:
Airtight bulk storage is best suited if the stock has to be maintained really on a long-term basis. It is generally in the form of modern silos above ground or as airtight moisture-proof underground pits. Of course, there are often storage arrangements which are far from satisfactory. Large-sized structures constructed on the basis of the Pusa Bin will also serve well for long-term storage.
Of course, these structures have to be located under fairly rainproof conditions. Where fresh arrangements for long-term storage are under consideration, the best course will be to provide both for a good storage godown and for a good storage structure in this godown. The godowns should have adequate provision for making the whole godowns sufficiently airtight for fumigation with poisonous gases and also for proper aeration after fumigation.
This can be easily managed by having ventilators fitted with both proper exhaust fans and also a suitable arrangement for closing the ventilator airtight. The doors should also be suitable for this purpose. Then inside the godown there should also be permanent structure for airtight bulk storage. The structure based on the principle of the Pusa Bin will be best suited for such godowns.
If these permanent provisions are made from the very beginning, the cost of storage is bound to be quite cheap in the long run. If both the godown and the storage structures are properly made, there may be no necessity for taking any chemical control measures. If, however, some infestation somehow creeps in, on can disinfest either the individual storage structure or the whole godown as such.
(c) Transit Storage:
In the past, transit storage has been maintained in a variety of sheds available on emergency basis instead of proper storage godowns. In these sheds, generally bagged storage has been practised, the bags being arranged in stacks.
In these godowns, there should be permanent cubicles in which the bagged grain can be suitably stacked. The cubicles should be such as can be made airtight both for storage and for fumigation, if need be. This will mean a proper combination of airtight storage and bag storage. This combination will provide both ideal storage conditions and all handling facilities of bag storage specially needed for transit storage. The structure based on the principle of the Pusa Bin will do very well for this purpose also.