Cassava processing, consumption and dietary cyanide exposure

Abstract

Cassava is a major component of the diet of more than half a billion people in the tropics. Cassava is an important crop in the farming system in the tropics because it can be intercropped with many other crops, and yields well in poor soils. Cassava roots contain cyanogenic glucosides that release cyanide when hydrolysed during processing or when consumed. Several methods of processing cassava roots have been developed to reduce the content of cyanogenic glucosides and produce food products that have longer shelf life than cassava roots. Residual amounts of cyanogenic glucosides and products of their hydrolysis often remain in cassava foods after processing.

Consumption of cyanogenic glucosides and products of their hydrolysis in gari, the most widely eaten cassava food product in Nigeria, has been linked with the development of tropical ataxic neuropathy (TAN). TAN was reported from several communities in southwestern Nigeria in the 1960s and 1970s, but anecdotal reports suggested that the occurrence of TAN diminished in the early 1980s. A recent study, however reported high prevalence of TAN in Ososa, a Nigerian community designated endemic for TAN in the 1960s.

Recent findings of continued occurrence of TAN in Ososa motivated the studies in this thesis. The studies were conducted to investigate if method of processing cassava into gari, the intake of gari, and exposure to cyanide from intake of cyanogens in gari differs in communities where TAN is endemic and communities where TAN has not been reported in southwestern Nigeria.

The method of processing cassava into gari in TAN endemic communities differs from the method of processing cassava into gari in non-TAN endemic communities. Gari produced in TAN endemic communities contain higher levels of residual cyanohydrin compared with gari produced in non-TAN endemic communities. Food processing experiments showed that the higher residual cyanohydrin content of gari produced in TAN endemic communities was due to the effect of continuous dewatering of grated cassava mash during fermentation and the duration of fermentation. Intake of gari and exposure to cyanide were also higher in TAN endemic compared with non-TAN endemic communities.

Although the role of dietary cyanide in the causation of TAN is not established, measures to reduce exposure to cyanide should be promoted. Short-term storage of gari before consumption, and further processing of gari into eba by adding boiling water, will contribute to reduction of exposure to cyanide in gari eating populations.