Principles of Crop Storage and Methods of Storage of Crops

December 15, 2022November 16, 2023 Agric4Profits

High temperature and high moisture are the most significant factors affecting grain quality in crop storage. Each can cause rapid decline in germination, malting quality, baking quality, colour, oil composition, and many other quality characteristics.

When dried to moisture contents below the safe moisture level, cereals and pulses can be stored for periods of a year or more under a wide range of temperatures, provided that during storage the moisture level does not rise, and precautions against insects are taken.

Crops producing root or stem tubers, have special requirements with respect to storage, because of the high moisture content of the tubers (60 – 80% when fresh).

Two major factors, that is the moisture content of the product when it comes from the field and the relative humidity of the outside air during the storage period, determine the choice for the best storage.

Methods of Crop Storage

Insects and moulds impair the quality of grain directly by their feeding and development, and indirectly through generation of heat and moisture. High temperatures and moistures favour development of insects and moulds.

Development of insects is limited by temperatures below 15°C, and by moistures below 9% in cereal grains. Development of moulds is limited by high temperature.

High moisture and high temperature are the most significant factors affecting grain quality in storage. Each can cause rapid decline in germination, malting quality, baking quality, colour, oil composition, and many other quality characteristics.

Spraying with insecticides or fumigating minimises insect problems but leaves chemical residues in grain, which break down with time. Presence of residues, and their concentration, affects acceptability of the grain to markets.

Some markets prefer grain without residues. Grain buyers will not knowingly accept grain treated at rates higher than those specified on the label, or within the specified withholding period.

1. Cereals and Pulses

Conditions for Storage

Table: Safe moisture content for any particular grain may vary slightly depending on the variety (valid for temperatures up to valid for temperatures up to about 27 °C.

Higher Temperature

	Product	Safe moisture contents%
Cereals	Maize threshed yellow	13
	Maize threshed white	13.5
	Maize flour	11.5
	Paddy rice	14.0
	Husked brown rice	12.0
	Sorghum	13.5
	Millet	15.0
	Wheat	13.5
	Wheat flour	12.0
Pulses	White and kidney bean broad	15
	Cowpea	15
	Lentil,pea	14

The safe moisture content for any particular grain may vary slightly depending on the variety. The mentioned safe moisture contents are valid for temperatures up to about 27 °C. Higher temperatures require lower moisture content maxima.

Methods of Crop Storage

Principles of Crop Storage and Methods of Storage of Crops

The following two factors determine the choice for the best storage method:

The moisture content of the product when it comes from the field

The relative humidity of the outside air during the storage period

1. Temporary Storage Methods

Such methods are quite often associated with the drying of the crop, and are primarily intended to serve this purpose. They assume the function of storage only if the grain is kept in place beyond the drying period.

2. Aerial Storage

Maize cobs, sorghum or millet panicles are sometimes tied in bundles, which are then suspended from tree branches, posts, or tight lines, on or inside the house.

This precarious method of storage is not suitable for very small or very large quantities and does not provide protection against the weather (if outside), insects, rodents, or thieves.

3. Storage on the Ground, or on Drying Floors

This method can only be provisional since the grain is exposed to all pests, including domestic animals, and the weather.

Usually it is resorted to only if the producer is compelled to attend to some other task, or lacks means for transporting the grain to the homestead.

4. Open Timber Platforms

A platform consists essentially of a number of relatively straight poles laid horizontally on a series of upright posts. If the platform is constructed inside a building, it may be raised just 35-40 cm above ground level to facilitate cleaning and inspection.

Platforms in the open may be raised at least 1 m above ground level. They are usually rectangular in shape, but circular or polygonal platforms are common in some countries.

Grain is stored on platforms in heaps, in woven baskets or in bags. In humid countries fires may be lit under elevated platforms, to dry the produce and deter insects or other pests.

Instead of being horizontal and flat, the platform may be conical in shape, the point at the bottom.

Up to 3 m in diameter, such platforms facilitate drying because of their funnel shape: at the top they consist of a frame of horizontal poles which is square, circular or polygonal in shape, against which the timbers which form the cone rest; these timbers meet at the bottom on a wide central supporting post.

Platforms with roofs (but no walls), of whatever shape or form, may be regarded as transitional types between temporary and long-term stores.

In southern Benin, Togo and Ghana, for example, maize cobs in their sheaths are laid in layers on circular platforms with their tips pointing inwards.

The platforms are usually between 2 and 3 m in diameter, but some may be more than 6 m wide, with a maximum height of 2.5 m at the centre and 1.5 m at the periphery.

5. Long-term Grain Storage Methods

Storage baskets (cribs) made exclusively of plant materials;

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 (cribs) are usually constructed entirely out of locally available plant materials: timber, reeds, bamboo, etc.

Under prevailing climatic conditions most plant material not fairly quickly, and most cribs have to be replaced every two or three years – although bamboo structures may last up to 15 years, with careful maintenance.

Basically similar to the outdoor type of platform described above, in all its variations, the traditional crib differs in always having a roof and wall(s).

It may even be elevated at least one metre above ground level, with a fire maintained underneath to assist drying of the contents and, allegedly, to reduce insect infestation. However, such cribs (especially the larger ones) are more commonly raised only 40 to 50 cm above ground level.

Access to the interior of a crib is gained usually over the wall. This may involve raising the roof, but some cribs have a gap between the top of the wall and the roof to facilitate entry. Relatively few cribs have sealable gaps in the wall or floor for the removal of grain.

6. Calabashes, Gourds, Earthenware Pots

These small capacity containers are most commonly used for storing seed and pulse grains, such as cowpeas. Having a small opening, they can be made hermetic, by sealing the walls inside and out with liquid clay and closing the mouth with stiff clay, cow dung, or a wooden (cork?) bung reinforced with cloth.If the grain is dry (less than 12% moisture content) there there is usually no problem with this kind of storage.

7. Jars

These are large clay receptacles whose shape and capacity vary from place to place. The upper part is narrow and is closed with a flat stone or a clay lid: which is sealed in position with clay or other suitable material.

Generally kept in dwellings, they serve equally for storing seeds and legumes. So that they may remain in good serviceable condition, they should not be exposed to the sun and should not be either porous or cracked.

8. Solid Wall Bins

Such grain stores are usually associated with dry climatic conditions, under which it is possible to reduce the moisture content of the harvested grain to a satisfactory level simply by sun-drying it. Solid wall bins are therefore traditional in the Sahel region of Africa, and in southern African countries bordering on the Kalahari Desert.

The base of a solid wall bin may be made of timber (an increasingly scarce resource), earth or stone. Earth is not recommended because it permits termites and rodents to enter. The better base is made of stone.

Mud or clay silos are usually round or cylindrical in shape, depending on the materials used Rectangular-shaped bins of this type are less common, because the uneven pressure of the grain inside causes cracking – especially at the corners.

Clay, which is the basic material, varies in composition from one place to another. That most commonly used for such construction work is obtained from termitaries, because the termites add a secretion which gives it better plasticity.

To give it added strength, certain straw materials such as rice straw may be mixed with it. The diversity of materials used explains why the capacities of such silos can vary from 150 kg to 10 tonnes.

In West Africa, when only clay is used, the walls are 15 to 20 cm thick. The shape is then more or less cylindrical and the construction is similar to the walls of a house.

However, when the clay is strengthened as described above, the bin is usually rounder in shape and resembles a jar; with walls only 2.5 to 5 cm thick, but very strong, so that it is possible to climb on top to enter the silo for regular withdrawal of grain.

The interior is often compartmented by vertical internal walls, joining at the centre on a central column which serves to support the foot when one enters the silo.

The walls are rendered as smooth as possible, inside and out in such a way as not to offer refuge for insects and their larvae; fissures are sealed with liquid clay before each loading.

Similarly, the angles formed by the internal partition walls and external wall are rounded for the same reasons.

In southern Africa, where the bins are commonly rectangular in plan, internal compartments are usually covered with mud-plastered timber ceilings and are accessed via sealable ‘windows’.

These face a short corridor leading to the exit, which may be fitted with a standard lockable door.

The roof is usually made of thatched grass, with a generous overhang to protect the mud wall(s) from erosion. Where a side door or a detachable ‘cap’ is not provided, the roof has to be lifted for access to the bin. Such silos can serve for 30 or even 50 years.

9. Underground Storage

Practiced in India, Turkey, Sahelian countries and southern Africa, this method of storage is used in dry regions where the water table does not endanger the contents. Conceived for long term storage, pits vary in capacity (100 kg to 200 t).

Their traditional form varies from region to region and they are usually cylindrical, spherical or amphoric in shape, but other types are also known. The entrance to the pit may be closed either by heaping earth or sand onto a timber cover, or by a stone sealed with mud

The advantages of this method of storage are:

Few problems with rodents and insects;

Low cost of construction compared to that of above-ground storage of similar capacity;

Ambient temperatures are relatively low and constant;

Hardly visible, and therefore relatively safe from thieves;

No need for continuous inspection.

The disadvantages are:

Construction and digging are laborious;

Storage conditions adversely affect viability; the stored grains can only be used for consumption;

The grain can acquire a fermented smell after long storage;

Removal of the grain is laborious and can be dangerous because of the accumulation of carbon dioxide in the pit, if it is not completely full;

Inspection of the grain is difficult;

Risks of penetration by water are not small, and the grain at the top and in contact with the walls is often moldy, even if the rest of the stock is healthy.

1. Cassava Conditions for Storage

Root crops, crops producing root or stem tubers, have special requirements with respect to storage, because of the high moisture content of the tubers (60 – 80% when fresh).

On the one hand desiccation should be avoided; on the other hand one has to guard against too much humidity around the tubers, which may cause rotting.

Living tubers continue to breathe fairly intensively, and this increases at higher temperatures. When high tuber temperatures are combined with airtight storage, lack of oxygen occurs, which results for example in potatoes with black hearts.

As the temperature of the product is higher better ventilation is necessary. During storage chemical changes take place in the tubers which may influence the firmness and the taste.

Tubers have a dormancy of a certain period. After this dormant period they start to sprout. This period varies with the crop and the variety and the temperature of storage of the tubers.

Yams can be stored for about 4 months at a normal temperature without sprouting but potatoes already start sprouting a little after 5 weeks at 15 °C. Also the tubers are often attacked by rodents during storage.

Without cooling the keeping quality of tubers decreases in the following order: Yam, cocoyam, sweet potato, cassava. Also for each root crop, varieties are known that keep well and others that do not.

Fresh cassava roots suffer very heavy losses when stored for more than a few days. These losses are not caused by the insect pests but by microbial infection and physiological factors. Post-harvest storage of cassava has been a major problem for production, marketing, utilisation and industrialization.

However, it is a general belief that fresh cassava roots cannot be stored for more than a few days and so farmers prefer to keep the cassava roots in the ground until they are required. When the cassava roots are allowed to remain in the soil as a means of preserving them, the following problems arise

The economic value of the land on which it is cultivated is drastically reduced for as long as the matured cassava roots continue to remain in the soil.

Internal discoloration takes place rendering the cassava and cassava products, such as gari unacceptable for human consumption and for livestock feed.

Microbial rotting occurs.

Rodent and Nematode damage occurs

Storage Practices

Traditionally, the problem of storage has usually been overcome by leaving the roots in the ground until needed and once harvested, it is immediately processed into a dry form which gives it a longer storage life

The roots can be left in the ground for several months after reaching maturity but a disadvantage of this system is that large areas of land are occupied by a crop which is already mature and is thus unavailable for further use.

Also the roots become more fibrous and woody and their starch content and palatability declines and in addition, susceptibility to pathogenic losses increases

One means of storing fresh cassava roots which has been used since ancient times by the Amerindians of Amazonia is to bury the harvested roots in pits or trenches, a technique probably derived from the common practice of leaving the cassava un harvested.

For a short period cassava roots can be kept fresh by being heaped and watered daily and a coating of paste made from earth or mud to preserve the roots for four to six days.

2. Yam

Curing

Yam can be cured at temperature of 29 – 32 °C and relative humidity of 90 – 95% for 4 days.

Satisfactory healing only occurs around deep wounds such as knife cuts. Bruised tubers (with superficial wounds) do not respond to curing. Such tubers can only be preserved by cutting off the bruised parts before curing.

Storage

The production of yam, unlike that of cassava, is very seasonal and therefore the tubers have to be stored for several months. The termination of dormancy (when sprouting starts) is the main constraint to long term storage. Removal of the shoots extends the storage life.

Storage conditions for cured tubers: approximately 16 °C and 70%relative humidity. Above 16

°C the tubers can be stored for 3 to 4months. Uncured tubers should be stored at a relatively low humidity Below 12 °C chilling injury occurs.

There are three common methods of storing yam tubers;

Barns
On platforms
Underground

The barn is the commonest form of yam storage in West Africa. It is erected in an open place and consists essentially of a series of vertically-oriented poles to which the yams are tied with rope.

In platform storage, the yams are laid horizontally on an elevated platform.

Underground storage the tubers are placed together in a large ditch and then covered with soil or dry vegetation

Yam Tubers Storage

It is essential that the tubers should be well aerated and well shaded

Tubers should be inspected frequently so that rotten ones can be removed

The sprout should be removed from those that begin to sprout

Causes of Loss during Yam Tuber Storage

Rotting
Tuber respiration
Tuber sprouting
Moisture loss from the tubers
Cold storage for yam tubers
Reduces storage losses due to rotting, sprouting and respiration Limitations
High cost of refrigeration
Yams stored at temperatures below 10^OC tend to become brown and unsuitable for human consumption

3. Sweet potatoes

Curing

Tubers are cured under temperature of approximately 30 °C, relative humidity, 85 %– 90% for 5-7days.The tubers can be left in the field in small heaps that are covered at night with straw or jute sacks if the temperature drops below 25 °C.

Curing after harvesting but before storage: the tubers are subjected to curing, a process which promotes the healing of wounds inflicted during harvesting.

Curing is best done by subjecting the freshly harvested tubers to a temperature of 27- 30^oC and relative humidity of 85-90% for 4–7 days in a well- ventilated ware house.

Curing can be accomplish by simply leaving the tubers exposed to ambient conditions for a few days before they are packed for storage.

After curing the best condition for the storage of sweet potatoes tubers is temperature of 13-^1o and relative humidity of 85-90%. The attainment of such condition requires refrigeration which may be uneconomical or unavailable.

Most farmers in Africa simply store their sweet potatoes under ambient conditions in baskets, in underground pits or in platforms.

Storage

The sweet potato has low storage potential in the tropics. Optimum storage conditions after curing: 13 – 16 °C and high relative humidity (85 % – 90%). Higher temperatures promote sprouting and increase respiration, leading to heat production and dry matter loss. Ventilation during storage is of great importance.

The tuber is highly susceptible to physical damage and subsequent deterioration. Red varieties seem to be stored better than white varieties.

Storage losses in fresh tubers can be lessened by processing: peeled tubers are sliced and sun dried to produce chips which can be stored intact or ground into flour.

Storage Methods include;

Clamp Storage

Pit storage whereby, the pits are lined with straw or bamboo and covered with a tight fitting (wooden) cover and a roof for protection from the rain. Only possible in areas of good drainage.

Hut Storage

The cured tubers can be stored by wrapping them in newspaper or packing them in dry sawdust. (The sawdust must be dry in order to minimize regrowth and rotting.) They can also be stored in crates lined with plastic. Holes in the plastic allow for ventilation.

During the first week curing is allowed to proceed at ambient temperature (18^oC – 31 °C). After curing, excess moisture must be removed to prevent sprouting.

4. Irish Potato

Storage Condition

Optimum storage of Irish potato is at temperature below 10 °C. Potato is a sensitive crop with respect to oxygen need, damaging and infection by fungi, etc. Potatoes should not be exposed to the sun for too long (maximum 1 hour).

They should be stored in the dark in a dry and well ventilated place. Under certain tropical conditions potatoes can be left in the ground for a period beyond the time at which it would normally have been harvested under temperate conditions Methods of storage

Storage in clamps or (partially) underground pits. Because the rate of respiration is still very high in the beginning the final layer of sand is sometimes applied to the straw after one week.

Hut Storage

The stores are ventilated during the night when the temperature is lower. The stores may be built partially underground with air ducts under the tubers to utilize cool night air.

Storage in Bulk or Preferably in Small Boxes

Oil-containing Products

Oil-containing products art consumed directly (groundnuts, soybeans, sesame, coconut) or for the extraction of oil. Often the remaining material (the “press cake”) can be used as cattle feed. The value of the oil is determined for a large part by the free fatty acid content.

This influences the smell and taste of the oil in a negative way. This is especially important if the oil is used locally. The process that liberates these free fatty acids (lipolysis) goes faster at a higher temperature and humidity.

The enzymes that play a role in this process are already present in these products naturally, but are produced in greater quantities when infection by insects and fungi and mechanical injury occurs.

Apart from influencing taste and appearance of the dried products as well as the oil to be pressed from it, fungi can also form toxic substances like aflatoxin in groundnuts. Fungal growth on oil-containing products takes place at moisture content above 7 – 8%.

Table: Safe moisture content of oil-containing products.

Products	Safe moisture content
Groundnuts shelled	7%
Copra	7%
Palm kernels	5%
Cotton seed	10%

Drying

Groundnuts, soybeans and sesame are first dried in the field with the leaves and stems still attached. Afterwards they are threshed or picked by hand. A moisture content of 15% appears be the most suitable for picking by hand and threshing with flails or simple stripping machines.

Threshing with hands and the use of simple stripping machines however, give a high percentage of broken groundnuts, which increase the chance of infection by fungi and insects. When threshing mechanically, it is possible to thresh at a higher moisture content, which results in less damage.

The picked or threshed products can be dried further on mats or canvas. If these products are stored in jute sacks, some additional drying will take place, provided the sacks are piled loosely. In the rainy season the additional drying should be done artificially. Storage of too moist products increases the possibility of internal heating.

Palm kernels are very difficult to store without loss of quality and have to be dried very well. For prolonged storage of coconuts the coconut meat should be dried from the original moisture content of about 50% to about 6%.

The nuts are cut in half and are dried in the sun or artificially or by a combination of both. For sun-drying 60 – 80 hours of sunshine are required; if it takes longer than 10 days the coconut will spoil. They are covered at night against dew formation and during rain, or the drying racks can be stored under a roof.

The meat is removed from the shell after about 2 days and 3 – 5 days are then necessary to complete the drying. Artificial drying can be done with hot air. Temperatures above 77 °C should not be used, except in the initial stage; later, the temperature is reduced to 65 °C or less. Smoke affects the quality. Before drying artificially it is advisable to dry the halved nuts in the sun for 1 or 2 days.

Storage Methods

Earthen pots and gourds, baskets, jute bags and silos are suitable for storage of oil-containing products. When using the airtight methods, it is important to have a very well dried product at the beginning of the storage period. In the humid tropics methods that allow ventilation have to be used where airtight storage is not possible.

Groundnuts have to be stored in the shell as much as possible, as this gives protection against insects and fungi. After shelling, quality and viability deteriorate rapidly. If jute sacks are use, the coarsely woven sacks prove to be better from the point of view of ventilation, but they increase the possibility of insect attack. Storage in big baskets is satisfactory.

In summary, high temperature and high moisture are the most significant factors affecting grain quality in storage. Each can cause rapid decline in germination, malting quality, baking quality, colour, oil composition, and many other quality characteristics.

Development of insects is limited by temperatures below 15°C, and by moistures below 9% in cereal grains. Development of moulds is limited by high temperature.

Root crops, crops producing root or stem tubers, have special requirements with respect to storage, because of the high moisture content of the tubers (60 – 80% when fresh).

On the one hand desiccation should be avoided; on the other hand one has to guard against too much humidity around the tubers, which may cause rotting. Living tubers continue to breathe fairly intensively, and this increases at higher temperatures.

Groundnuts, soybeans and sesame are first dried in the field with the foliage still attached. Afterwards they are threshed or picked by hand. A moisture content of 15% seems the most suitable for picking by hand and threshing with flails or simple stripping machines.

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Principles of Crop Storage and Methods of Storage of Crops