Cassava is one of the world's most important food crops, providing quality carbohydrates in tropical regions where grains and potatoes grow poorly if at all. The roots, also known as manioc or yucca, can be stored for long periods in the ground as a hedge against famine. The leaves can also be eaten as a nutritious green vegetable, when others are unavailable. There are two major types of cassava, though they are used similarly.
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One of the two major varieties of cassava is referred to as "sweet" cassava. This is not because it is higher in sugars than other varieties, but because it is less poisonous. Cassava contains large quantities of cyanide compounds, which must be processed out of the tubers before they can be safely eaten. The sweet variety of cassava has fewer of these compounds, and does not require as much processing. Sweet varieties also produce higher yields.
Bitter cassava is very similar in cultivation and general appearance to sweet cassava, but produces much higher quantities of cyanide compounds. Sweet cassava my contain as few as 40 parts per million, while bitter cassava varieties can range as high as 490 parts per million. Any quantity of cyanogens over 50 parts per million is considered to be hazardous. In unsettled regions, some farmers deliberately switch to bitter cassava as a deterrent to crop theft.
The presence of cyanide in cassava constitutes a clear threat to health, unless these compounds are removed before the cassava is consumed. Unprocessed cassava is toxic enough to cause death, but insufficiently processed cassava will also cause mortality over a period of time, especially when quality protein is absent from the local diet. When the body breaks down residual cyanide in cassava it produces thiocyanate, a compound known to affect thyroid function. This causes goiter in areas where cassava is the staple food.
Processing Cassava to Remove Cyanide
There are several methods of removing the cyanide from cassava. Simple drying reduces the level of cyanide, though this may not be adequate to make it safe for consumption. Soaking the roots in water first, to leach out cyanide, produces a safer starch. So does fermenting the roots, either whole, shredded or in pieces, before drying. Roasting the tubers, or boiling them in multiple changes of water, will also reduce the cyanide content to manageable levels.