Cassava (Manihot esculenta) root is long and tapered, with a firm homogeneous flesh encased in a detachable rind, about 1 mm thick, rough and brown on the outside.
When I was in high school years. My mom and dad always plant cassava in our farm and sell it for a living. I love broiled and boiled cassava without knowing that the food I was eating contain cyanide.
Cyanide is a naturally occurring chemical, generally considered to be poisonous if consumed in large amounts.Exposure to high levels of cyanide for a short time harms the brain and heart and can even cause coma and death. Workers who inhaled low levels of hydrogen cyanide over a period of years had breathing difficulties, chest pain, vomiting, blood changes, headaches, and enlargement of the thyroid gland.
Cyanides are naturally present in plants. Amounts are usually low in the edible portion but are higher in cassava. Pits and seeds of common fruits, such as soursop, apricots, apples, and peaches, may have substantial amounts of cyanide.
Cassava roots and leaves cannot be consumed raw because they contain two cyanogenic glucosides, linamarin and lotaustralin. Cassava varieties are often categorized as either sweet or bitter. The sweet cultivars can produce as little as 20 milligrams of cyanide per kilogram of fresh roots, while bitter variety may produce 50 times as more, 1 g/kg. Cassavas grown during drought are especially high in these toxins. A 40 mg of pure cassava cyanogenic glucoside is sufficient to kill a cow.
A safe processing method is to mix the cassava flour with water to form a thick paste and then let it stand in shade for five hours in a thin layer spread in tray. Through this method, 5/6 of the cyanogenic glycosides are broken down by the linamarase; the resulting hydrogen cyanide escapes to the atmosphere, making the flour safe for consumption .The traditional method used in West Africa is to peel the roots and put them into water for 3 days to ferment.
More info at ATSDR .