COLUMBUS, Ohio – Cassava is the third-most important food source in
tropical countries, but it has one major problem: The roots and leaves
of poorly processed cassava plants contain a substance that, when eaten,
can trigger the production of cyanide.
That’s a serious problem for the 500 million people who rely on
cassava as their main source of calories, among them subsistence farmers
in Sub-Saharan Africa, said
Richard Sayre, a professor of
biology at Ohio State University. He and his colleague Dimuth
Siritunga, a postdoctoral researcher in plant biology at the university,
have created cyanogen-free cassava plants. A cyanogen is a substance
that induces cyanide production.
Their study appeared in a recent issue of the journal
Cassava is a hardy plant – it can remain in the ground for up to two
years and needs relatively little water to survive. It’s the key source
of carbohydrates for subsistence farmers in Africa. But an unprocessed
cassava plant contains potentially toxic levels of a cyanogen called
The proper processing of cassava – drying, soaking in water, rinsing
or baking – effectively reduces cassava’s linamarin content. But, said
Sayre, shortcut processing techniques, which are frequently used during
famines, can yield toxic food products.
“If we could eliminate the cyanogens in cassava, the plant wouldn’t
need to be processed before it’s eaten,” he said. “In Africa, improperly
processed cassava is a major problem. It’s associated with a number of
cyanide-related health disorders, particularly among people who are
Cyanogens in cassava plants convert to cyanide when raw
cassava is eaten or processed.
Chronic, low-level cyanide exposure is associated with the
development of goiter and with tropical ataxic neuropathy, a
nerve-damaging disorder that renders a person unsteady and
uncoordinated. Severe cyanide poisoning, particularly during famines, is
associated with outbreaks of a debilitating, irreversible paralytic
disorder called Konzo and, in some cases, death. The incidence of Konzo
and tropical ataxic neuropathy can be as high as 3 percent in some
People who get little or no protein in their diets are particularly
susceptible to cyanide poisoning, as they lack the proper amino acids
necessary to help detoxify the poison.
“But these cyanogen-free
plants represent a safer and more marketable food product as
well as a tool to determine the role of cyanogens in
protection against insect pests and crop productivity.”
Sayre and Siritunga engineered cassava plants in which the expression
of the genes responsible for linamarin synthesis was blocked. They then
analyzed the linamarin content in these plants’ leaves and roots,
finding a significant reduction of the cyanogen in leaves (by 60 to 94
percent) and in roots (by 99 percent) compared to normal cassava plants.
Cassava leaves contain more linamarin than do the plant’s roots,
Sayre said. Plant biologists believe that somehow linamarin is
transported from the leaves to the roots early in a plant’s life.
Turning off the linamarin-inducing genes in cassava leaves might reduce
the levels of linamarin in the plant’s roots.
“When leaf linamarin was reduced by 40 percent, the roots contained
less than 1 percent of the cyanogen,” Sayre said, adding that it’s
critical for the leaves of a growing cassava plant to contain a small
amount of linamarin.
Women are usually charged with processing the plant, which
leaves them susceptible to inhaling cyanide gas.
“Linamarin protects cassava from being eaten by insects or animals,”
he said. “Plants with moderate linamarin levels in their leaves and that
contain nearly no linamarin in their roots are both protected from
herbivores and contain far less of the cyanogen in their roots.”
Cyanide forms when cassava is processed. But the toxin is volatile
and is released into the air, rather than remaining in the food. Correct
processing methods ensure that the cyanogen content in cassava plants
will be within an acceptable range, Sayre said. (The
United Nations’ Food and Agriculture Organization has established
maximum recommended cyanide levels for foods.)
“In African countries, it’s mostly women and children who are
susceptible to cyanide poisoning,” Sayre said. “Women are usually
charged with processing the plant, which leaves them susceptible to
inhaling cyanide gas. Cyanide can poison a person by either inhalation
The shelf life of a cassava root is very short once it’s removed from
the stem, so there’s an urgency to get the food to market.
In processing, the cassava plants are first crushed and then
“Roots can turn to mush in less than a week,” Sayre said. “Cassava’s
fresh market time is very small, so it has to be processed immediately.”
And that’s where consumers can run into problems -- the rush to get
cassava to the market may keep some batches of cassava from being
processed properly. Even if the plant is properly processed, exposure to
the volatile cyanide can cause health problems in people charged with
processing the roots and leaves.
While cassava roots contain less than 10 percent of the linamarin
level found in cassava leaves, it’s the long-term exposure to cyanide
that presents a threat to humans.
“Linamarin is converted to cyanide when eaten,” Sayre said. “Repeated
exposure of low doses of cyanide over time can lead to health problems.
“But these cyanogen-free plants represent a safer and more marketable
food product as well as a tool to determine the role of cyanogens in
protection against insect pests and crop productivity.”
“However, preliminary studies indicate that linamarin may be
important in the transport of nitrogen from cassava leaves to its roots
in young plants,” he continued. “Plants producing varying levels of
linamarin need to be tested in field trials to determine if the
inhibition of linamarin synthesis affects plant yield.”
Written by Holly Wagner,