Control of Konzo and Kits to Determine Cassava Cyanide and Urinary Thiocyanate

Mild method to remove cyanogens from cassava leaves

Cassava, the staple food of tropical Africa has a starchy root and leaves that are rich in protein, vitamins and minerals. Cassava leaves may be harvested throughout the year and are used particularly by the Congolese population of Central Africa and in Liberia, Sierra Leone and Guinea, and there is moderate use in other tropical African countries. By contrast, cassava leaves are not used at all in the South Pacific, because of their high levels of toxic cyanogens and the ready availability of other greens. Cassava leaves contain large amounts of cyanogenic glucosides, which are broken down by the enzyme linamarase to produce cyanohydrins that are further decomposed by another enzyme hydroxynitrile lyase also present in the leaves, to give hydrogen cyanide (HCN) and a ketone.
The supply of protein is inadequate for poor village people living in the savanna and subsisting on a monotonous diet of bitter cassava roots, but is improved somewhat by consumption of cassava leaves as the main subsidiary food. The leaves are usually pounded for 10-15 min, which allows the enzymes to come in contact and catalyse hydrolysis of the cyanogens, followed by boiling in water for 10-120 min to remove all cyanogens, but boiling pounded leaves for 30 min was found to reduce the protein content by 58% and the amino acid methionine content by 71%. This large loss of protein and methionine due to boiling pounded cassava leaves in water means that less than one half of the original protein and methionine would be available to detoxify ingested cyanide to thiocyanate. Furthermore there are large losses of vitamin C, thiamin, riboflavin and nicotinic acid due to boiling.

We have developed a mild method to remove cyanogens from cassava leaves that involves three consecutive steps (1) pounding, (2) standing for 2 h in the sun or 5 h in the shade in the tropics and (3) washing three times in water (Bradbury and Denton, 2014). We used four cassava cultivars and the mean residual total cyanide content after steps 1, 2 and 3 was 28%, 12% and 1% respectively. The pounded cassava leaves retain their bright green colour and texture. The traditional method for removing cyanogens from pounded cassava leaves is by boiling in water which removes all cyanogens in 10 minutes. However boiling causes the pounded leaves to become dull green in colour and there is considerable losses of vitamins, protein and methionine, which are already in short supply in the diet of poor village people in tropical Africa.
It is hoped that this mild method of removing cyanogens from cassava leaves, may be a useful alternative to boiling pounded leaves in water, which would save on fuel for cooking and much more importantly would improve the nutritional status of the cassava eating population of tropical Africa.