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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

10.2 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.