by Bunmi Ogah
Cassava has been a major staple food crop in Nigeria for more centuries. Cassava roots provide an important source of energy-rich food for millions of people. However, one of their nutritional shortcomings is their potential toxicity due to the presence of two cyanogenic glucosides: linamarin and lotaustralin.
Linamarin is a cyanogenic glucoside found in the leaves and roots of plants such as cassava and lima beans, with those of cassava reaching up to 80% of the total glucoside. In cassava, hydrogen cyanide is produced, alongside acetone and glucose, when linamarase, an endogenous enzyme present in cassava, break the β-linkage that occur in linamarin under high pressure and temperature.
Hydrogen cyanide is the chemical responsible for tissue hypoxia, a condition in which tissue cells experience inadequate utilization of oxygen. Chronic exposure to hydrogen cyanide may cause neurological, respiratory, cardiovascular and thyroid defects. Onset of symptoms depends on dose and duration of exposure. The lethal dose of cyanide is 1mg/kg of live weight. Should the cyanide content be high enough to exceed such a dose, the cassava product is regarded as toxic. Toxicity in higher animals results from the combination of cyanide with iron (II) ion (Fe+2) which accounts for the formation of cyanohemoglobin.
However, animals have a detoxification mechanism to avoid death when the cyanide released is slow. This mechanism, depending on the pH, is present in swines i.e. pigs (monograstic with pH 3.0 in the stomach), but not in bovine such as cow, ox and most ruminant animals (polygrastic with pH 7.0 in the stomach). In humans, linamarin can be broken down by linamarase found in bacteria that reside in the intestinal tract resulting in release of hydrogen cyanide.
Thus, humans can readily neutralize about 10mg of cyanide by a reversible reaction with methemoglobin fraction in the red blood cells. Rodanase can further convert majority of the cyanide to less toxic thiocyanate, which is then excreted in the urine. Values from 15 to 400ppm (mg CN–/kg of fresh weight) of hydrogen cyanide in cassava roots have been mentioned in literature.
Generally, different varieties of cassava have been classified into two main types: sweet cassava and bitter cassava. Sweet cassava roots contain less than 50mg/kg hydrogen cyanide on fresh weight basis, whereas that of bitter variety may contain up to 400mg/kg. Sweet cassava roots can generally be made safe to eat by peeling and thorough cooking.
However, bitter cassava roots require more extensive processing to reduce the cyanide content. Many traditional methods have been developed in various parts of the world for preparing cassava for human consumption and feed. These involve peeling, grating the roots, prolonged soaking of gratings in water to allow leaching and fermentation to take place, followed by thorough cooking to release the volatile hydrogen cyanide gas.
Food products made from cassava include garri (toasted cassava tubers), porridge-like fufu, and cassava flour. While fresh cassava requires traditional methods to reduce its toxicity, adequate processed cassava flour and cassava-based products have very low cyanide contents and are considered safe to use. Recent research efforts have developed a transgenic cassava plant that stably regulates linamarin production via RNA interference.