Cassava processing and dietary cyanide exposure in Tanzania

Abstract

Mlingi, N.L.V. 1995. Cassava Processing and Dietary Cyanide Exposure in Tanzania. Acta Universitatis Upsaliensis, Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine 571.69 pp. Uppsala. ISBN 91-554-3603-X. The relationship between dietary cyanide (CN) exposure from the cyanogenic glucoside linamarin in cassava and the methods used to process this impor­ tant root crop were studied in Tanzania. An outbreak of acute intoxications in southern Tanzania coincided with a drought in 1988. The affected population attributed intoxications to short-cuts made in the sun-drying of cassava roots. Processing experiments showed that these short-cuts yielded flour with high residual levels of cyanohydrin, a linamarin metabolite that is transformed to CN in the gut. A causal role of CN in the intoxications was supported by hundred-fold higher mean urinary levels of the main metabolite thiocyanate (SCN) in affected (1120 pmol/1) compared to non-aftected (7 pmol/1) population groups. An epidemic of the paralytic disease konzo, that has been attributed to CN exposure from cassava, occurred during a drought in northern Tanzania in 1985. Qualitative and quantitative interviews revealed that the estab­ lished way of heap-fermenting crushed cassava roots was shortened during the drought due to food shortage and intensive trade. Experiments showed that this chain of events resulted in high levels of cyanohydrin in flour. Low protein intake may have enhanced CN toxicity due to low supply of sulphur for CN to SCN conversion. Studies of 217 women from an iodine deficient area in western Tanzania showed that the total goitre rate of 73 % could be explained by iodine deficien­ cy as verified by a median urinary iodine of 3.6 pg/dl. Although 98% ate cassava roots daily their mean urinary SCN was only 128 pmol/1. Use of mechanical milling was associated with low SCN, probably because milling ensures complete drying of roots, which removes cyanohydrin. A mean urinary SCN of 36 pmol/1 indicated low CN exposure in 193 schoolchildren studied in Dar es Salaam. They consumed cassava roots without prior processing that effectively removes cyanogens. The low CN exposure can be explained by the use of non-bitter varieties with low linamarin levels and low consumption frequency and that ingested linamarin is partly excreted in the urine without releasing cyanide in the body.