Is Cassava Poisonous?

It can be if it is not prepared properly. CDC's MMWR says cassava, an edible tuberous root often made into flour, contains cyanogenic glycosides that can cause fatal cyanide poisoning if not properly detoxified by soaking, drying and scraping before eating. An outbreak report in the WHO Western Pacific Region's surveillance journal says cassava roots with less than 50 micrograms of hydrogen cyanide per gram of fresh weight are considered sweet, and roots above that level are considered bitter.

Processing is what matters. The WHO journal report says sweet cassava roots can be made safe to eat by peeling and thorough cooking, while for bitter cassava one traditional way to effectively reduce cyanide is peeling, grating, soaking for 18 to 24 hours, squeezing and thorough cooking. CDC's MMWR says even sweet cultivars still require detoxification, described as peeling the tubers, soaking them in water for 4 to 6 days, sun-drying or roasting, then scraping and grinding into flour. The Codex Alimentarius code of practice says only cassava known to be low in cyanide, the sweet type, should be used for direct cooking and eating, because cyanogenic glycosides are heat stable.

Poisoning can be sudden or chronic. CDC's MMWR reported that in September 2017 an outbreak of suspected cyanide poisoning with 98 cases and two deaths occurred in western Uganda, and says repeated exposure to nonlethal amounts through a monotonous cassava-based diet can lead to paralytic diseases such as tropical ataxic neuropathy and konzo, which causes sudden, irreversible spastic paralysis. A 2018 review in Brain Research Bulletin says konzo is associated with chronic reliance on improperly processed bitter cassava as the main source of food.