Perhaps Hans Rosling’s most significant discovery in his field of research was his explanation of konzo, an illness with lower-limb paralysis as its main symptom. He showed that the illness arose in regions of severe malnutrition and was caused by a monotonous diet consisting of poorly prepared cassava.
Here, Linley Chiwona-Karltun, who did her doctoral research under Hans Rosling’s supervision, explains aspects of the crop that are crucial for understanding its effects.
Cassava has many other names, including manioc. It is a drought-tolerant root crop, which grows well in poor soils and is the main source of carbohydrates for people living in sub-Saharan Africa. It is native to South America, where it was first cultivated. Portuguese explorers introduced the plant into West Africa in the sixteenth century and by the twentieth century it had spread widely.
The starchy cassava roots store more carbohydrate per hectare than any other crop. The leaves can be boiled and provide an important source of proteins, vitamins, and minerals.
Cassava is classified as either “sweet-cool” or “bitter”; the bitter variety is poisonous, the sweet one less so, and edible without prior cooking. Bitter cassava contains cyanogenic glucosides in much higher concentration than the sweet type, and these substances become further concentrated in drought conditions. The plants also contain an enzyme that can break down the glucosides and eventually generate free hydrogen cyanide. Bitter cassava must be carefully processed before eating: the enzyme must have time to do its job. Afterward, the hydrogen cyanide must be washed away. In practice, the roots are soaked, grated or fermented, and then either dried in the sun or roasted before cooking. The detoxification process can take between three and fourteen days.
In dry areas such as the Mozambican plateau near Nacala, lack of water makes drying in the sun the only possible method. That form of processing takes several weeks.
At times when cassava is the most important source of energy, the growers normally prefer to cultivate the toxic varieties, because of their bigger yields and high tolerance to poor soils and little or no rain. The poisons are protective against crop theft by monkeys and men. The growers are usually women, who have learned to distinguish between plants with low and high toxicity, and tend to surround patches of sweet cassava with rows of the bitter type.
During field studies in Malawi, Hans and I studied these women’s knowledge about the different cassava varieties. When we asked them just how bitter and poisonous they were, the woman would show us by pointing at the root to indicate just how much of it you could eat without becoming ill. We were later able to prove in the laboratory that they knew exactly what they were talking about.
Linley Chiwona-Karltun