Three patients admitted to the Accident and Emergency Unit of Lagos
University Teaching Hospital (LUTH) after eating a cassava based meal 'Gari'
died shortly after admission. The patients vomited and complained of abdominal
pain immediately after the meal. They were unconscious with renal failure and
died of cardiopulmonary arrest. The cyanide levels in the blood and urine
averaged 1.12 and 0.54 mg 1-1, respectively. Cassava contains cyanogenic
glycosides which slowly release cyanide and this may have been responsible for
the death of these patients. There is an urgent need to establish maximum
tolerable levels of cyanide in 'Gari' and other cassava food products. (Akintonwa & Tunwashe, 1992)
Nature of Cassava toxin
Cassava is fed to livestock
in the fresh or processed form. In the whole unbruised plant the cyanogenic
glucoside remains intact in the form of linamarin and lotaustralin. When the
cellular structure is disrupted, the intracellular glucoside becomes exposed to
the extracellular enzyme linamarase. Hydrocyanic acid (HCN) is then produced.
The reaction has been shown to proceed in two steps by Nartey, (1978) viz:
i.
Cyanogenic glucoside is degraded to sugar
and cyanohydrin (x - hydroxynitrile);
ii.
Cyanohydrin then dissociates to ketone and
hydrocyanic acid. Thus, for linamarin the glucoside is first hydrolysed by
linamarase to produce B-D-glucopyranose and 2 - hydroxyisolentyronotrite or
acetone - cyanohydrin, after which the latter is degraded to acetone and HCN.
Cyanohydrin produced as a result of linamarin activity is stable only under
moderately acidic condition (pH 4.0); in neutral or alkaline condition it
undergoes spontaneous hydrolysis to yield HCN (Cooke et al. 1985).
In spite of the relative
instability of cyanohydrin it coexists with intact glucoside and HCN in
differently processed cassava products. It is therefore clear that the cyanide
in cassava products exists in three forms: (i) the glucosides (linamarin and
lotaustralin), (ii) the cyanohydrin and (iii) the free hydrocyanic acid (HCN).
However, the quantitative
estimation of cyanide by various methods has produced incomparable results, and
in many cases a gross underestimation, emanating from quantification of free
HCN alone in the reports of earlier investigators. The harmonization of current
analytical and presentation methods is therefore suggested.
The
paragraphs below were reported by Joey, 2008:
On
14 January 2008, the Centre for Food Safety (CFS) advised members of the public
to avoid consuming Piranha brand crackers and snacks manufactured by Tixana
Australia Pty Ltd. The appeal was made following a warning issued by the Food
Standards Australia New Zealand (FSANZ) due to the higher-than-usual levels of
naturally occurring cyanogenic glycosides in the ingredient cassava in a batch
of exported vegetable crackers. The CFS contacted the relevant authorities and
was informed that the affected products had been exported to Hong Kong. The CFS
alerted the trade to stop selling the affected products.
What are Cyanogenic
Glycosides?
Cyanogenic
glycosides are a group of chemical compounds which occur naturally in over 2
000 plant species. There are at least 25 cyanogenic glycosides known to be
found in the edible parts of plants. Cyanogenic glycosides alone are relatively
non-toxic. However, as a result of enzymatic hydrolysis by beta-glucosidase
following maceration of plant tissues as they are eaten, or by the gut
microflora, cyanogenic glycosides are broken down to release hydrogen cyanide
which is toxic to both animals and humans. The potential toxicity of a
cyanogenic plant depends primarily on its capacity to produce hydrogen cyanide.
What are the Symptoms of
Cyanide Poisoning?
In
humans, the clinical signs of acute cyanide intoxication include rapid
respiration, drop in blood pressure, rapid pulse, dizziness, headache, stomach
pain, vomiting, diarrhoea, mental confusion, twitching and convulsions. Death
due to cyanide poisoning can occur when the cyanide level exceeds the limit an
individual is able to detoxify. The acute lethal dose of hydrogen cyanide for
humans is reported to be 0.5 to 3.5 mg per kilogram of body weight. Children
are particularly at risk because of their smaller body size.
Chronic
cyanide intoxication may lead to the development of certain conditions
including disturbance of thyroid function and neurological disorders. It tends
to affect those individuals who have regular long-term consumption of cassava
with poor nutrition status.
What is Cassava? What is it
Used for?
The
roots of cassava are rich in carbohydrates, mainly starch. According to Food
and Agriculture Organization, cassava is the third most important source of
calories in the tropics, after rice and corn. Cassava is consumed in a variety
of ways, including eaten as whole root, grated root or root chips. In addition,
it is prepared into flour which in turn can be used for cooking or production
of cassava-based products such as breads, crackers, and puddings or beverages
made with tapioca pearls. Cassava leaves are also eaten in some countries
following extensive boiling. Apart from being used as human food, cassava
products are also used as animal feed.
How should Cassava be
Processed to Render it Safe for Consumption?
Cassava
contains more than one form of cyanogenic glycosides. Different varieties of
cassava are generally classified into two main types: sweet cassava and bitter
cassava. Sweet cassava roots contain less than 50 mg per kilogram hydrogen
cyanide on fresh weight basis, whereas that of the bitter variety may contain
up to 400 mg per kilogram.
Sweet
cassava roots can generally be made safe to eat by peeling and thorough
cooking. However, bitter cassava roots require more extensive
processing. One of the traditional ways to prepare bitter cassava roots is by
first peeling and grating the roots, and then prolonged soaking of the gratings
in water to allow leaching and fermentation to take place, followed by thorough
cooking to release the volatile hydrogen cyanide gas. Cutting the roots into
small pieces, followed by soaking and boiling in water is particularly
effective in reducing the cyanide content in cassava. Whilst fresh cassava
requires traditional methods to reduce its toxicity, adequately processed
cassava flour and cassava-based products have very low cyanide contents and are
considered safe to use.
What Other Edible Plants
Contain Cyanogenic Glycosides?
Bamboo
shoot is a popular food item among Asian population. The cyanogenic glycoside
present in bamboo shoot is decomposed quickly in boiling water. Other edible
plants containing cyanogenic glycosides include kernels within the pits of some
stone fruits (e.g. bitter apricot kernels), lima beans, etc.
Advice to Consumers
1.
Buy food from
reliable suppliers.
2.
Prepare cyanogenic
plants such as cassava and bamboo shoot properly before consumption. Cyanogenic
plants should be cut into smaller pieces, soaked in water and cooked thoroughly
in boiling water.
3.
Maintain a
balanced diet to avoid excessive exposure to harmful chemicals from a small
range of food items.
Advice to the Trade
1.
Source food and
ingredients from reliable sources.
2.
Adhere to the Good
Manufacturing Practice to minimise the risk of natural toxins in food.
Bibliography
Akintonwa, A., &
Tunwashe, O. L. (1992, 1 1). Sage Journal. Retrieved 11 2019, from Human and Experimental Toxicology: https://journals.sagepub.com/doi/abs/10.1177/096032719201100107 ?journalCode=hetb
Joey KWOK (2008): Cyanide Poisoning and Cassava. Centre for Food Safety.Honkong. https://www.cfs.gov.hk/english/multimedia/multimedia_pub/multime dia_pub_fsf_19_01.html
Nartey, F. 1978. Manihot
esculenta (Cassava): Cyanogenesis ultrastructure
and seed germination. In Abstracts on Cassava, Vol. 4, Series 083C-4. C.I.A.T.
Publication, Colombia.
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