Roots and tubers are storage organs for plants and are also important to man as food.
They rank next to the cereal grains as the major source of carbohydrate in Nigeria and provide a significant part of the total food supply to the people of the tropics.
They also contribute to the economies of many developing countries in Africa, Asia and Latin America as food staples. Some species of yam and taro have potential in traditional medicine as precursors of certain pharmaceutical steroids and alkaloids.
The major tuber and root crops grown in the tropics are cassava, yam and cocoyam. Irish potatoes, sweet potatoes and carrots are also grown in the tropics but are primarily temperate crops. These crops are highly perishable and huge losses can occur after harvest due to their high water content.
COMPOSITION AND NUTRITIONAL VALUE OF ROOTS AND TUBERS:
Nutritionally, roots and tubers contribute about 20-40% of the total calories and about 7.1% protein to the diets of the people of sub-saharan Africa.
In Nigeria, they are main sources of calories accounting for over 50% of the caloric intake of the people. Yam, a popular staple food in Nigeria, has less than 6% protein while cassava, the major source of energy is a poor source of protein (less than 3%). Cocoyam is fair in protein (7.9%) and calcium. Sweet potatoes are poor in protein but fair in their supply of the B-vitamins and ascorbic acid. The yellow varieties are rich in carotene. Irish potato is an excellent source of ascorbic acid and fair in protein and the B-vitamins.
Cassava, particularly the bitter cultivars are associated with toxic cyanide in bound and free forms. Yam and cocoyam in their fresh states also contain substances which cause irritation to the skin and throat.
Cooking alone or in combination with other processes such as roasting, fermentation or soaking in water are able to render toxic substances associated with roots and tubers harmless.
UTILIZATION OF ROOT AND TUBER CROPS
1. YAMS:
Yams can be prepared as food in many ways. They may be boiled in pieces of convenient sizes or sometimes boiled whole if the tubers are small.
Boiled yam may be eaten with meat, fish, stew, vegetable soup or palm oil.
Yam tubers may be rosted in an oven or over a slow fire or baked in the ashes of fire. Small slices or cubes of the raw tuber are also fried in hot oil in a manner similar to French fries. Softer and more palatable products are however, obtained if the slices are pre-boiled and strained off from the water used for boiling before being fried.
Although, considerable sizes and quantities of yam tubers are used as already expressed, the most important culinary products made from yam are: Yam chips, yam flakes and yam flour.
YAM CHIPS:
Yam tubers are cleaned, peeled and cut laterally or lengthwise into small pieces.
The slices are then soaked in water for 2-4 hours. Parboiled (60-72°c; 12-18 minutes). Left to steep in the hot water until cool.
Sun or air dried to 10-14% moisture content. The yam chips is ready.
YAM FLAKES:
To produce yam flakes, yam tubers are cleaned, peeled, washed, cut into thin slices and cooked under pressure for about 30 minutes.
They are then mashed and the product drum dried to thin flakes which are then packaged in polythene bags.
Yam flakes offer a high potential for processing and storing yams.
The preparation time from flakes to a white doughy mass is much shorter than preparing pounded yam from fresh yam tubers.
Unlike pounded yam, yam flakes requires no pounding; all that is needed is stirring and kneading in hot boiling water.
YAM FLOUR:
Yam flour is made by:
- Cleaning
- Peeling
- Cubing/slicing
- Blanching and sulphiting
- Steaming for 30 minutes
- Drying
- Mechanically grinding to form the yam flour.
- Packaging in polythene bags for use.
Yam flour for fufu is considered slightly interior to yam flakes for "poundo yam" because of its light brown dough compared to that from yam flakes which is white.
Yam flour can be stored for a considerable period of time and can be reconstituted with hot water followed by stirring to make fufu.
2. COCOYAM:
The cocoyams- Colocasia(taro) and Xanthosoma(tania) are the most important genera of the family, Aracea used for food in West Africa.
Alocasia, Amorphophallus and Caryotosperma are three other genera common in the tropical areas of the Pacific.
Cocoyam may be pounded either pure or mixed with yam and cassava and eaten with vegetable soup.
They can also be boiled or roasted and eating with palm oil, stew, meat or fish.
Two main products, cocoyam chips (achicha) and the cocoyam flour are produced from cocoyam(taro).
PRODUCTION OF COCOYAM CHIPS:
The raw corms are cleaned, washed and boiled for 2-3 hours until they are soft.
The skin is then peeled, cut into slices, sun or smoke dried until they are readily break between the fingers.
Is then packed and stored.
The chips is crushed into small grits, boiled and mixed with copious quantities of leafy vegetables and palm oil to form a meal.
PRODUCTION OF COCOYAM FLOUR:
To produce cocoyam flour, cleaned and peeled corms and cormels are chipped, parboiled or blanched, sun-dried, ground, sieved to flour and stored.
The coarse particles (mostly fibre) are fed to live stock. The flour is usually made into a smooth thick paste in hot water and eaten with rich vegetable soup.
3. SWEET POTATOES:
They can be boiled and mashed or baked, or processed into intermediate moisture as well as low moisture content fried chips.
The tubers contain free sugar as well as starch in a readily digestible form.
Holding these potatoes in cold storage (10-14°c) even for a few days causes accumulation of reducing sugars whose caramelization results in dark brown fried chips that are sugary.
Sweet potatoes can be de-sugared by holding the tubers at 40°c and 95% relative humidity prior to processing.
This treatment gives chips with moderate brown colour and less sweetness which are preferred by consumers.
4. IRISH POTATOES:
Irish potatoes can be dehydrated to make non instant potato chips.
Frozen French fries, a popular product from irish potatoes can be produced by cutting into finger length slices (1cm x 1cm), washed and peeled potatoes which are then individually quick frozen and packed. Frozen potato slices are deep-fat fried before consumption.
Boiled, peeled and mashed irish potatoes can also be dried to produce mashed potatoes.
To produce instant mashed potato, the cooked mashed potato is first partially dehydrated to about 50% moisture content, left to cool for 2-2.5 hours, coarse sieved and then dried to 3.5-4.8% moisture content in cabinet or tunnel dryers.
The dried product can be reconsituted for use in water heated to about 95°c to which 0.5% salt has been added (1:3.5-4.0 w/w powder: water ratio).
Reconstitution in boiling water results in an undesirable pasty product.
5. CASSAVA:
Cassava is a major source of carbohydrate for the peoples of West and Central Africa. The protein is low in quantity and also poor in quality.
The tuber is used widely as feed for livestock.
In the diet, cassava is always combined with other nutritious foodstuffs which enhance their nutritional value.
For example, cooked garri and fufu are always served with a rich vegetable soup in Nigeria. Boiled cassava can be taken with roasted groundnut, coconut, fish or meat.
Traditional cassava salad also provides very good combinations of highly nutritious food stuffs.
Gari, cassava flour, cassava fufu, abacha and farinha de mandioca are the major cassava-based products used for human food in the tropics.
REFERENCES:
Pomeranz, Y. and Meloan, C. E. (1980). Food Analysis: Theory and practical. Revised ed. AVI publishing Co. Westport C. T.
Potter, N. N. (1980). Food Science. Third Ed. AVI publishing Co. Westport C. T.
Pyke M. (1981). Food Science and Technology. Fourth Ed. John Murray, London.
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