Cassava (tapioca, manioc) agriculture, cyanogens and rising African production

Cassava is the third most important food source in the tropics and the staple food of tropical Africa. It originated in Brazil and perhaps also in Central America and was introduced into Africa many hundreds of years ago. Its production over 24 African countries has increased threefold from 1980 to 2005 (compared with a 42% increase for Indonesia) and the population has doubled compared with a 1.5 times increase worldwide (Nhassico et al., 2008). Cassava is a hardy, drought resistant crop that gives acceptable yields on low-fertility soils and is of great importance for subsistence farmers throughout the tropics. In some countries such as Thailand it is also grown in plantations to produce starch and pellets for animal feed, much of which is exported to developed countries.

Cassava roots are very starchy and their protein content is less than that of other tropical root crops such as yam, sweet potato, and taro. Cassava contains the cyanogenic glucoside linamarin and a small amount of lotaustralin (methyl linamarin) which are broken down to cyanohydrins and glucose, catalysed by the enzyme linamarase also present in the root. The cyanohydrins readily break down to hydrogen cyanide in neutral or alkaline conditions.

The cassava plant has a large amount of linamarin in the leaves and in the skin of the root. The inner part of the root called the parenchyma is either cooked and eaten or is processed into cassava products such as flour and gari (see Processing of cassava roots reduces cyanide compounds and promotes storage). Sweet cassava has a small amount of linamarin in the parenchyma and bitter cassava has a much larger amount. The bitter taste is mainly due to linamarin but there are other bitter compounds in the root and also sour constituents, which confuse the taste buds. Different cassava varieties have very different amounts of cyanide in the parenchyma ranging from very low, 1 mg HCN equivalents / kg fresh root (1 ppm) to very high (1550 ppm). In Nigeria one variety is called "chop and die". Young cassava leaves are a good source of greens with a high protein and vitamin content and are used in many countries, but must be processed to remove cyanide before eating.