Cassava roots are the most perishable of the major root crops and deteriorate in air at ambient temperature in a few 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 cyanogens present in cassava roots have been partly overcome by 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 sweet (low cyanide), the roots after peeling are boiled and eaten.
There are many traditional processing methods used around the world, but 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 sun drying of intact 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" 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 increases 2-4 times, heap fermentation is preferred by women processors over sun drying (Ernesto et al., 2002a). In Central Africa where there is sufficient water, whole peeled roots are steeped in water for 3 days and the softened tissue is sun dried, pounded and sieved to produce a low cyanide flour. However sometimes roots are immersed for only 1-2 days and converted into flour which still retains large amounts of cyanogens.
The amount of reaction between linamarin and the enzyme linamarase to produce cyanohydrins which break down to liberate 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 is only 1.8-2.4 % (Cardoso et al. 2005), but is still sufficient to cause TAN amongst some poor people in Nigeria. We have developed a simple method to reduce by about one half the total cyanide content of gari (Bradbury and Denton, 2010b).
In 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 drought years which are a normal feature of the climate, when water stress on the cassava plant causes a large increase of the cyanide content of the roots. We have developed a simple wetting method for removing cyanide from flour and have used this to control konzo in many villages in DRC (see Control of Konzo).