Cassava is one of the most tropical of all food crops, originating near the equator in the Amazon region of Brazil and in southern Mexico. It is a perennial shrub that grows up to 4 m (12") tall. One of the world's ten most important foods, it is grown primarily for its starchy roots which are a staple for nearly one billion people. Cassava is very closely associated with extreme poverty because it is grown and eaten mainly by people with few economic resources in Africa, Asia and Latin America.
It will survive long droughts by shedding its leaves and going dormant until rains return. It is also a plant capable of growing in soil that is very acidic, low in nutrients and high in toxic aluminum compounds. These growing conditions are increasingly common in the degraded agricultural ecosystems of the tropics. Its ability to produce food under such harsh conditions make cassava "the poor man's friend."
In many cassava growing regions the leaves are eaten as well as the roots. The value of both the leaves and the roots is limited by the presence of hydrocyanic acid (HCN), a common plant toxin. One would be tempted to steer clear of cassava leaves altogether to avoid any toxicity problems, except that the plant has several important attributes as a leaf crop. In addition to growing under difficult conditions, cassava leaves are rich in the protein, vitamins, and minerals that are needed to balance the carbohydrate heavy diet of many tropical cultures. By planting the cassava stem cuttings closer together and selecting leafy varieties, the yield of cassava leaves can be astronomical. Cultivation of cassava is increasing rapidly as productive new varieties are created and international markets for cassava products are developed.
Since people are already eating cassava leaves, the question is not whether to eat them but rather how to eat them. Encouraging the use of low HCN varieties is critical to this effort. A grinding technique that ruptures cell walls, will dramatically reduce the total amount of HCN in the leaves as it disperses into the air during drying. Making products, such as pasta, that will be cooked after drying, will further reduce the HCN content of the leaf powder and will make cassava an even better friend of the poor.
A true multipurpose crop with edible leaves and edible roots that are becoming an important international animal feed and source for industrial starch
- It adapts to poor soils on which most other crops fail.
- It resists drought, except at planting time, and it resists locust damage, making it an excellent famine crop.
- Cassava leaves are an important source of micronutrients and protein in many areas where malnutrition is severe. They are located where they are most needed.
- Cassava is relatively inexpensive to produce and requires very little weeding when planted in optimal plant populations.
- There is no critical planting date, provided there is enough moisture at planting.
- Very easily propagated by stem cuttings that, unlike the propagation material with potatoes, yams, sweet potatoes are not used for food.
- Leaf yields are already high and could be greatly increased with a few changes in growing techniques.
- Cassava leaves and roots contain cyanogenic glucosides that can be converted to poisonous hydrocyanic acid (HCN) by enzymes in the plant. This can cause either acute or chronic poisoning, especially in people who have a diet extremely low in protein. Avoiding this involves taking special measures in variety selection and processing.
- Polyphenols in cassava leaves can bind with some of the proteins making them less bioavailable.
- Cassava leaves on dry weight basis are c. 15% protein; 15% fiber and 8% ash.
- Cassava can be grown in three ways; for the roots; for leaves and roots or for the leaves. The greatest total food production is usually from the second strategy. A study in Liberia suggested that beginning to collect leaves at four months and harvesting some of the leaves every two months afterward could produce over 1 ton of leaves per hectare without affecting yield of roots. A study from Zaire showed that careful leaf harvest can actually increase tuber yield. Clearly more thorough and more locally practical research should help in identifying optimal strategies for combined leaf and root harvest.
- HCN is a fairly common toxin in food. Cassava, Lima beans, and sprouted sorghum are foods that have caused HCN poisonings. Acute HCN poisoning is quite rare. The minimum lethal dose is estimated at 0.53.5 mg per kg of body weight. So a child weighing 20 kg would need to consume between 10 and 70 mg of HCN. Ten grams of a low HCN variety dried cassava leaf would contain something like .08 mg .
- Chronic toxicity has been reported mainly where there is a great dependence on cassava and a very low protein intake. Damage to the nervous system and especially the optic nerve can be caused by chronic exposure to HCN. Chronic HCN poisoning also appears to be quite rare. Low consumption of proteins, especially sulfur bearing amino acids, cigarette smoking, and air pollution all intensify the body's negative reaction to HCN.
- Contrary to popular opinion, there was very little correlation between the HCN level of the root and leaf.
- The most important factor in reducing HCN in dried cassava leaf meal, other than selecting low HCN varieties, is grinding the leaves up before drying. This resulted in leaf powders with roughly 50% of the HCN of those made from sliced leaves and only 33% the HCN of those made from whole leaves. The reduction of HCN by drying is dramatic. For example, the variety Cigana had 737 parts per million (ppm) in its fresh leaves and only 34 ppm (or less than 5% as much) in the dried leaf powder.
- Glucosides can be removed or detoxified by washing the crop and throwing away the water. This is much more effective if the leaves are put into room temperature water then brought to a boil, rather than being added to boiling water. The toxic compounds are very water-soluble. Fermentation, drying and high temperatures also reduce the glucoside levels. When cassava leaves are eaten as a potherb is it best to boil them for at least 15 30 minutes and then rinse them with clean water.
- In Zaire there is a preference for eating leaves infected with the cassava mosaic virus. These leaves have lower HCN levels.
- There are numerous local varieties that are normally propagated from stem cuttings. Seeds are available from B & T Seeds in England.
Cnidoscolus chayamansa and C. aconitifolius (chaya); Manihot esculenta (cassava); Sauropus androgynus (katuk)