Cassava is the most important food in the humid and semiarid tropics of Africa, and it is third in importance, after rice and maize, in the entire tropics. But eating it can be a health hazard because the roots contain the glucosides linamarin and lotaustralin, which, when hydrolysed (for example by cooking) release hydrogen cyanidea potent poison.
Various laborious techniques will reduce or eliminate the cyanide in cassava before it is eaten. These are not always 100% effective, and poor processing is often related to other problems such as famine and drought. In addition, women performing the task in African villages may become ill from repeated exposure to cyanide vapour. At least a few people are thought to die each year from cassava cyanide poisoning, and many more suffer from serious disorders produced by regular sublethal doses.
The cassava breeding program of the International Institute of Tropical Agriculture (IITA) aims to reduce the initial hazard of cassava toxicity by selecting and breeding low cyanide cultivars. ACIAR is involved through an AIDAB/ACIAR/CGIAR Special Purpose Grant, and has commissioned the School of Life Sciences at the Australian National University (ANU) to collaborate in the research. The ANU team has already studied the chemistry and nutritional importance of cassava in the Pacific, as part of a broader ACIAR study on tropical root crops (project no. 8204).
The Australian researchers will collaborate with IITA in Nigeria and elsewhere because of the Institute's cassava mandate and its linkages to many cassava-growing countries. They will also undertake collaborative projects with the Philippines, which has requested ACIAR assistance in evaluation of its cassava germplasm (in particular, its significant collection at Visayas State College of Agriculture (ViSCA), which may contain useful, low-cyanide varieties); with Indonesia, which has recently expressed interest in participating; and with Pacific countries that the earlier ACIAR project identified as possessing low-cyanide varieties. The Australian scientists will also work on a low-cyanide cassava recently found in a collection of the University of Queensland.
The project entails five broad lines of research:
. Development of a simple screening method for cyanide content to replace the present unreliable picrate leaf test. (This work, suitable for both leaves and tubers, will be carried out at ANU.)
. Examination of the relationship between various environmental factors and the cyanide content of cassava. (Field trials will be conducted by IITA in Nigeria in four different climatic regions. The ANU will also conduct growth chamber experiments to investigate the effects of various watering regimes and fertiliser applications.)
. Screening of existing cultivar collections, using the new method, to identify cultivars with low to very low cyanide content. (The screening of collections in Australia and the South Pacific will be done at ANU. Low-cyanide germplasm will be transferred to IITA via CIAT by open pollinated seeds and possibly also by virus-free, tissue-cultured plantlets. Parallel to this, ViSCA staff will screen the extensive ViSCA collection, and screening of Indonesian cassava germplasm will be undertaken by scientists of the Bogor Research Institute for Food Crops.)
. Identification of the 'bitter principle' of cassava to determine whether this is correlated with linamarin content. (The bitterness/linamarin correlation and separation of the bitter compounds will be done at IITA. The purification and structure elucidation of the bitter principle will be done at ANU.)
. Development of a simple method to determine the beta-carotene (or vitamin A) content of yellow cassava. (A simple column chromatography procedure will be devised at ANU to separate the complex carotenoids. The amount of beta-carotene present will then be determined by absorption spectrophotometry.)
By reducing the cyanide hazard of cassava, the project has the potential to save lives and improve the health of many people, especially in Africa, but also in Asia and elsewhere. The work on beta-carotene should help to reduce blindness and other disorders resulting from vitamin A deficiency, especially among African children. The total number of people likely to benefit worldwide is very large. The cyanide research will bring little direct benefit to Australia other than academic interest and goodwill, but the rapid assay for beta-carotene could help Australian food chemists in any work requiring such estimation.