Processing of Cassava Roots Reduces Cyanide Compounds and Promotes Storage
Cassava roots are the most perishable of the major root crops and deteriorate in air at ambient temperature in 3-4 days. In subsistence agriculture the plants are left in the ground until needed for food or for processing. The twin problems of perishability and the poisonous nature of the cyanide present in cassava roots have been partly overcome by the development of a large number of traditional forms of processing in different parts of the world. In East Africa cassava flour is preferred, whilst in West Africa a roasted product called gari is produced most commonly. In South and Central America there are many different cassava products. In the South Pacific, where the introduced cassava varieties are virtually all low cyanide, the roots after peeling are boiled and eaten.
Since there are many traditional methods used around the world, we will limit discussion to the major methods used in Africa. In East Africa the simplest method is "sun drying". This involves peeling the roots followed by drying in the sun whole roots, or large pieces cut longitudinally, The brittle, dry material is then pounded in a wooden pestle and mortar and sieved to remove fibrous material, which produces white flour. The amount of cyanide retained as linamarin is quite high at 25-33%. " Heap fermentation" is more work intensive and time consuming. It involves peeling and cutting the roots and leaving them in a small heap for 3-5 days during which some fermentation takes place with liberation of hydrogen cyanide. The roots are then sun dried, pounded and sieved as already described. Flour produced by heap fermentation is slightly dark coloured and the retention of cyanide is 12.5-16.5% (Cardoso, et al. 2005). In times of drought when the total cyanide content of roots are high, heap fermentation is preferred over sun drying (Ernesto et al., 2002a).
The amount of reaction between the enzyme linamarase and linamarin (both present in the root) with liberation of hydrogen cyanide is increased greatly by grating or crushing to give small pieces of cassava root. In the production of gari in West Africa, the grated cassava is stored in a bag for 2-3 days. The excess water is then squeezed out in a press, the damp product is dried and hydrogen cyanide removed by roasting in a metal dish over a wood fire. The retention of cyanide in gari or farinha is only 1.8-2.4 % (Cardoso et al. 2005) .
In Southern, Eastern and Central Africa where cassava flour is made by sun drying and heap fermentation the retention of cyanide is about ten times that of gari and farinha. The situation is made much worse in years of low rainfall which are a normal feature of the climate, because of increased total cyanide content of roots as a result of water stress. In Mozambique we have made a successful trial with rural women of a new wetting method, to reduce greatly the cyanide content of cassava flour just before it is used in cooking (see Simple wetting method to remove cyanide from flour).