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close this bookPrimary School Agriculture Volume II: Background Information (GTZ, 1985, 190 p.)
close this folderPart III: Crop storage
close this folder10. Tuber preservation
View the document10.1 Present state of tuber storage
View the document10.2 What happens to tubers in storage
View the document10.3 Preparing Tubers for Storage
View the document10.4 General storage principles
View the document10.5 Tuber stores
View the document10.6 Storage pests of tubers

10.1 Present state of tuber storage

Many farmers grow and eat more tubers than grains. This is especially true of the people in rain forest areas. The principal tuber crops are cocoyams, yams, cassava, and sweet potatoes, with some production of Irish potatoes in the higher parts of the highlands. Although all of these tubers are quite different from each other, they do have similar characteristics when they are in storage. Therefore, when this chapter speaks of tubers in general, it is speaking of the five types of tubers mentioned above.

Whereas the different kinds of tubers act almost the same in storage, storing tubers and storing grain is very different. Grain is much more durable during storage and transport. Tubers, on the other hand, bruise, cut, and spoil very easily. As experienced farmers know, it is much more difficult to store tubers successfully.

Tubers do not have as many different kinds of enemies as stored grain. However, tubers are delicate and so easily attacked that only a slight wound or cut can spread disease all through the tuber and spoil it totally. In storage the major pests seem to be:

- mould and fungus (which cause rotting and spoilage),
- insects (the weevil),
- rats.

Insects and rats do not seem to be as serious pests as they are in stored grain. Nevertheless, the problem with fungus and mould attacks are serious and widespread. There are no accurate figures for the amount of tubers lost or damaged during storage and transportation, but tuber losses are probably higher than grain storage losses. A reliable estimate of stored tuber losses would be about 15%.

The large losses of tubers after the harvest often discourages farmers from growing more. Most farmers grow enough for their families' needs, with only a small surplus, if any at all, for emergencies or for the local market. Prices of tubers rise every day because quantities of tubers are small in the markets.

With the present problems of tuber storage bothering most farmers, it would be difficult to convince farmers to increase their tuber production. They would lose too much after the harvest to make it worth their while.

Not only would better storage methods help farmers to produce more, but the quality of tubers in the market would improve. Spoilage would be controlled. Farmers could grow and sell more. The large price increases during the scarce periods would be reduced. These developments would benefit both the farmer and the consumer.

10.2 What happens to tubers in storage

It is important to understand how tubers change during storage and what actually happens to them when they spoil. If farmers can understand these changes they will also see how some of the improved methods can work to avoid the problem of spoilage.

A tuber, whether it is a yam, cocoyam, cassava, or potato, is a very starchy, fibrous root which can be peeled and the flesh inside prepared and eaten. In storage the two basic parts which concern us are the skin and the starchy interior or flesh..

- The skin is the thin outer covering which surrounds the tuberous root. In cassava and cocoyams it is a layered, corky skin, while with potatoes and yams it is not as thick. The skin is the weakest immediately at harvest time. It cuts and bruises easily. Also moisture and air can move freely in and out through the skin. The skin provides no protection against insects or rats. However, a sound, uncut skin can resist moulds and fungus better.

- The fleshy interior makes up over 90% of the tuber. This is the stored food of the tuber. It is a root or stem of the tuber plant. This is the part which is eaten. Each tuber species and variety has its own distinctive colour, taste, and texture. However, in general the flesh is starchy and moist. Just under the skin the flesh is sometimes tougher. But it cannot protect itself against cuts, insects, or fungus and mould.

Tubers are living things. While in storage they must be kept alive. If a tuber ceases to live it will begin to spoil immediately. What does it mean when we say, "The tuber is alive"?

First of all the tuber is breathing: Just like other living things, it takes or breathes in oxygen from the air and uses it inside its body, in this case the fleshy interior. To complete the breathing process the tuber must give off or breath out the air that is used. This used air is called carbon dioxide.

Secondly, to stay alive the tuber must ´'eat'' or nourish itself. When it breathes in air it also consumes a tiny part of its stored food in the fleshy part. When it consumes or eats part of this food it gives off heat and moisture as wastes.

The entire process of breathing in air and consuming its stored food and then giving off used air carbon dioxide - and heat and moisture is called respiration.

All living tubers, even in storage, respirate. For safe storage the tuber should respirate slowly. Therefore the good tuber store will have the ability to make the tuber "breathe" at a very slow and steady rate. Now we will look at the conditions which control this important rate of respiration.


A living tuber respirates

Moist, warm places make tubers respirate faster. A very cool, dry storage place keeps the tuber living and respirating, but at a very slow rate. If the tuber is cool and dry it is taking in a small amount of oxygen from the air and is using or eating only a very small amount of its stored food. Therefore, it is giving off or breathing out only a very tiny amount of heat, moisture and carbon dioxide, the used air. This is a good storage situation.

If the tuber is packed in a warm, wet place it begins to wake up and breathe in more oxygen and burn up more of its food. This makes the tuber give off more carbon dioxide, heat, and moisture, which makes the tuber even wetter and warmer. This will make the tuber respirate even faster again. Soon, this process of faster and faster respiration can cause the tuber to spoil.

To better understand why fast respiration is not good for tuber storage one should look at what really causes the tuber to spoil.

Tubers spoil because they are attacked by moulds and fungus which feed off the tuber's flesh and eventually consume or contaminate the tuber until it spoils.

Moulds and fungi are tiny plants which grow and multiply very rapidly on all kinds of objects: wood, grain, plants, clothes, and stored tubers. Since moulds are a special type of fungus which is still very similar to a fungus we will call all of this type of pest a fungus. Fungi are plants which cannot make their own food. They must grow on an object which can supply them with their food. Fungi are everywhere. They grow and multiply best under warm, wet conditions. Fungi do not spread in cool and dry places. The tropical climate is perfect for the growth of fungi. It is warm and wet many months during the year, especially in the rain forest zones.

Fungi are spread by the wind. An adult fungus plant will release thousands of tiny seeds, called spores, into the wind. These seeds or spores are so tiny that the wind carries them everywhere. Only a few spores will find warm, wet places. They will grow and multiply very rapidly. This is when fungus becomes dangerous.

When fungi begin to grow on a tuber, for instance, they respirate as well. This means they breathe in air and they consume part of the tuber's stored food. Then the fungi give off heat, moisture, and carbon dioxide. Many fungus plants are poisonous. When they begin to grow on an object they can contaminate it with their poison. If humans or animals eat the contaminated thing they can get sick.

Tubers are easily attacked by fungi. The protective skin is very thin and weak. It cannot resist fungi. If the tuber is cut or bruised the tuber's wet flesh is exposed to the air. The fungi spores (seeds) will grow very rapidly in the moist flesh of the tuber. The fungi will respirate quickly, giving off heat and moisture as it grows. This makes the tuber respirate even faster, making it warmer. This allows the fungi. to multiply even faster. Soon they have spread all over and even in side the tuber. The tuber gets discoloured, black streaks and stains running all through it as the fungus begins to cause the tuber to rot or spoil. Very soon the fungi can even spread to another tuber in the store. The whole process can be repeated until many tubers are suffering from rotting and spoilage caused by the mould and fungus at" tacks.

As you can see, tubers are very delicate and once they are wounded either by insects or rats or man, they can be attacked and easily spoiled by fungi. This must be avoided at all times if safe storage is to take place. Tubers must be handled carefully from the moment of harvest to the time of consumption if spoilage is to be avoided.

10.3 Preparing Tubers for Storage

a) Harmful Harvest Techniques

As we have shown, tubers are very delicate food stuffs. The most vulnerable stage of the tuber's life is just when it has been harvested. At this time the skin, especially of the sweet potato and yam, is the weakest. The following practices at harvest time are very harmful to tubers which are to be stored. If the harvesters knew that what they were doing was harmful to the tuber they might change their method of harvesting.

- Careless removal from the ground: When the tuber is moved from the ground at harvest it is often cut with the cutlass or other digging tool. Even stones can cut or bruise the skin when the tuber is being removed. When the crown of the yam head is removed with the sett stem piece make certain that the cut is clean and not too deep.

- Packing the tubers in the field: Once the tuber is lifted or dug from the ground many harvesters carelessly throw or drop the fresh root into the carrying baskets. Even a short drop of 30 cm can bruise a potato enough to make it spoil after two weeks of storage. Potatoes, cassava, and yams are especially sensitive to this type of rough handling.

- Exposure to the sun and rain: Tuber harvesting often takes all day long. As they dig the tuber roots many farmers leave the fresh tubers lying in the sun. This sunning is bad for freshly dug tubers, especially Irish potatoes, sweet potatoes, and cassava. The white flesh of the Irish potato turns green just under the skin. This makes the potato bitter. Exposure to the direct sunlight makes the tubers lose too much water too fast. This is bad.

In the same manner, rain can quickly damage freshly dug tubers, especially yams and cocoyams. Fungus attacks can begin immediately after rainfall has wetted the tubers. The wet skins can be penetrated very easily and they take a long time to dry out.

Farmers should avoid doing these careless practices which harm tubers before storage even begins.

b) Some Harvesting Hints which Aid Storage

Be careful not to cut, bruise, or wound the tubers when they are dug up. When white yams are harvested wood ashes (ashes from fire) should be rubbed onto the cut which was made on the tuber's crown when the tuber was removed from the yam plant.

Cocoyams and cassava should be cut away from the stem so that a part of the woody stem is attached to the root. This way the tender flesh is not exposed to the air.

All cuts should be rubbed with wood ashes as soon as possible. This makes it difficult for fungus to attack. Wood ashes are very dry. Fungus cannot grow well on wood ashes.

Line the carrying basket with grass and leaves. This makes the basket soft. Place each tuber in the basket gently. Do not throw or drop the tubers inside the basket. If there is time cover each layer of the tubers with grass so that the next layer of tubers does not touch the bottom layer.

Harvest on dry days. Do not let the rain fall on the fresh tubers. Also do not let the sun shine directly on the harvested roots. All full baskets and piles of tubers should be shaded from the sun. The sun and rain can harm the freshly harvested crop.

Clean all dirt and debris from the tubers. Make certain that the tuber is well cleaned. Insects, fungi, and other diseases can hide in the dirt and be carried to the storage area if the dirt is not removed.

Get the tubers to the storage area as fast as possible. Leaving the tubers outside where you cannot control the weather or the fungus is dangerous.

The hints given above can do all to make certain that the tuber is correct for storage. Most losses of stored tubers occured because the tuber was not properly handled or prepared for storage. It is wise to be very careful when handling tubers.

c) Curing the Tubers

Some tubers can store better if they are cured before they are stored. This is true of the Irish potato and the sweet potato. Sometimes yams (especially the white yam) can benefit from three days of curing. Curing is a process of resting under certain conditions which will make the tuber's skin get a bit thicker and tougher. This makes the tuber store better.

Potatoes should be spread out in a shaded, cool dry place where air breezes can pass. They should not lie on the earth or touch each other. After one week or so the potatoes can be put in the storage place.

Yams should be treated like this for about two days, followed by one day in the sun for four hours, turning the yams occasionally, to complete the curing process. Then place them in storage.

d) Selection of Tubers for Storage

This is one of the most important elements in tuber storage. It is very true that one bad cocoyam spoils all the achu. Diseases can rapidly spread from one tuber to another inside the tuber store.
Therefore, all tubers which are diseased, infested by insects, or have a portion of their root spoiled should not be chosen for storage. Also, all tubers which are cut, bruised, or wounded should be separated from those tubers which will be stored. The unsatisfactory tubers should be used as soon as possible. It is this selection process which can save you many lost tubers.

e) Transportation

A few words about transporting tubers could be useful at this time. Follow the same rules about tuber handling at harvest time when you transport the tubers.

- Always handle them carefully. Small bumps or drops can bruise or cut the tuber. This will open it up for attack by fungus. Do not throw or drop tubers while loading or unloading them.

- Avoid sunlight and rain. The tubers should be well covered so that sunlight and rain cannot strike them.

- If you are carrying a very large load of tubers in a lorry or a pick-up it is a good idea to make a soft bed of grass and sticks on the floor of the lorry so that air can pass under the tubers and bumps on the road do not damage them. Also, do not pack the tubers more than two or three feet (60 or 90 cm) high without putting a layer of grass and leaves in between so as to provide a cushion for the tubers.

Following these basic principles will make transporting tubers a safer operation.

10.4 General storage principles

In general, all tubers store best under similar conditions. However, there are a few differences which should be noted. Therefore tuber storage principles will be talked about under three groups. The first group will be sweet and Irish potatoes, the second group will be yams and cocoyams, and finally, cassava storage will be discussed.

a) Sweet Potatoes and Irish Potatoes

Once the curing and selection process is finished the. potatoes should be kept under the following conditions:

- dark,
- dry,
- cool,
- ventilated.

Sweet potatoes do not store as well as Irish potatoes. Many people leave sweet potatoes in the ground until they wish to eat them. This does not work very well during the rainy season. The Sweet Potato Weevil is a serious pest in some areas which makes it difficult to follow the practice of leaving the potatoes in the ground until they are needed.

In the store the potatoes should not be piled too high on top of each other. One meter is the limit. Potatoes can be stored in cold storage or refrigerators.

The store should be dark, dry, and cool because it is hard for fungus to live in these conditions. Also, the tuber respirates in dry, cool, dark places. The store should be ventilated so that when the tuber gives off heat and moisture during respiration the air can pass and carry it away. The potatoes should not lie directly on the earth floor - water can come and spoil them.

b) Yams and Cocoyams

The main principles to maintain in yam storage are the same as for potatoes. The store should be:

- dark,
- dry,
- cool,
- ventilated.

Such a store keeps the yams and cocoyams respirating slowly and it makes it difficult for fungus and disease to enter. The ventilation makes it possible for the air to keep moving away any heat which is given off during respiration.

Whereas potatoes can be laid on top of each other up to one meter, yams do not store well when they are touching each other. Yams should not touch or lie on each other in storage. They do not dry evenly and air does not pass as well when the yams are piled like this. Yams and cocoyams should not lie directly on dirt floors.

In most cases cocoyams do not store as well as yams. However, this varies as to the locality and the species of cocoyams and yam.

Yams are also left in the ground in many areas and harvested upon need. However, the success of this method depends on the local problems with pests in the field: monkeys, insects, rats, and termites. This practice apes not succeed during the rainy season once the tubers are mature.
Usually yams which take the longest time to mature and are harvested late have the strongest flesh and seem to store better. The white yam and the yellow yam store better than the water yam and the Chinese yam.

c) Cassava

Cassava is very difficult to store. Cassava tubers generally cannot last more than a few days once they are harvested. It should be left in the ground and harvested as it is needed. The best way to preserve cassava is in the form of gari.

10.5 Tuber stores

There are so many different types of tuber stores which are used locally throughout Africa that to talk about them all would be impossible. Here are three methods which are fairly common. Most other stores use the same principles.

a) The Yam Bam

This store is a small simple building with a thatched roof: zinc roofs are sometimes too hot. The walls can be of any construction: wood plank, bamboo, etc. Mud blocks or mud packed walls are preferred because they keep the inside of the barn much cooler. The floor should be raised off the ground about 30 cm. This allows air to pass all through the barn. The floor can be made of bamboo or wood.

There should be small spaces in the walls or under the eaves to let the air pass freely through the barn. Tubers are generally spread evenly on the floor. Make certain that the barn is filled carefully so that the tubers are not damaged.

An improved method using the same store is to build bamboo racks or shelves along the walls of the store. The tubers are then placed on the racks. The racks or shelves should be constructed at 60 cm (2 feet) intervals along the side of each wall. This way each wall would have about four racks attached to it, all spaced 60 cm from each other.

Yams, cocoyams, and potatoes would store well in this manner. The tubers would not be piled up on top of each other. They would not be touching each other. Disease could not spread. The room is cool and dry and the sun cannot enter. Furthermore, air can pass all around the tubers to keep them cool. The tubers can be easily inspected for insect or disease problems as they are not piled on top of each other. The barn can be locked at night to discourage thieves.

b) Clamps or Tuber Pits

This method is used mostly for potatoes. It does not seem to be as successful with yams.

A shallow hole is dug in a shady, cool place. The hole is then lined with sand. Grass, leaves, and sticks are then packed over the sand. The tubers are then carefully placed in the hole and covered with ashes or sand. Grass and banana leaves are then placed over the filled pit. A small sun/rain shelter of bamboo and thatch is built over the clamp. One must make certain that the drainage is good so that water does not fill the pit and spoil the tubers.

This method keeps the tubers cool, dry, and dark. However, there is no ventilation. If heating takes place the heat cannot escape. Instead it will build up inside the pit and could cause spoilage. It is also difficult to inspect the tubers for storage problems. Termites could bother the pit if they are a problem in your area. The wood ashes sometimes discourage them.

c) Box or Basket Storage

Tubers, especially the potato, can be gently packed into baskets or boxes and then stored in a cool, dry place in the house. Sometimes it is good to pack the tubers with wood shavings, sand, or wood ashes. This not only cushions the tubers, but it also stops the spread of fungus diseases.

Make certain that the box has a few holes in it so that air can circulate. A basket should have a loose weave. The major difficulty is that the tubers cannot be easily inspected. The area is small, not too many tubers can fit into a box or basket.

10.6 Storage pests of tubers

We have already discussed the rotting and spoilage problems caused by fungi and moulds. Now the insect and animal pests will be discussed.

The major insect pest of the potato tuber is the Sweet Potato Weevil. It looks like the Rice Weevil shown on p. 170. The weevil often begins to attack the tuber in the ground before it is harvested. Then, when the tuber is carried to the store, the larvae, eggs, and adults are all carried to the store. Once in the store, they move into other tubers and spoil them. This weevil makes tiny, hard-to-see holes in the skin of the potato. However, fungus and other diseases can enter easily through this holes.

The best control method is to rotate the crop every two years so that the eggs which are laid in the soil will not find food when they hatch into larvae. They will die and soon the field will be free of the pest. Harvesting earlier than normal is helpful. The weevil likes mature, ripe sweet potatoes. Leaving the potatoes in the ground can be dangerous - the weevils can fly to fields where they know the potatoes are growing.

Insecticide - Malathion or Actellic - can be used if it is necessary. Check up in section 7.2, for information about chemical control of the Sweet Potato Weevil. The same rate of application should be followed with sweet potatoes as with grains.

Yams sometimes have a problem with termites in the field. Harvesting earlier is helpful. In storage yams sometimes are bothered by a weevil entering the tuber through the cut crown (where the sett was taken from). Rub wood ash on the cut before you store it. This helps to keep the cut free of fungi and insects.
If rats and other animals bother the tubers, trapping or harvesting early is the only control method.