Cyanide in Cassava: A Review

CHAPTER ONE
INTRODUCTION
1.1 Background to the Study
Cassava (Manihot esculenta Crantz) is an important tropical root crop providing energy
to about 500 million people (Otusola, 2011). Almost all the cassava produced is used for human
consumption and less than 5 percent is used in industries. As a food crop, cassava fits well into
the farming systems of the smallholder farmers in Nigeria because it is available all year round,
thus providing household food security. Compared to grains, cassava is more tolerant to low soil
fertility and more resistant to drought, pests and diseases. Furthermore, its roots store well in the
ground for months after maturity. Cassava is important, not just as a food crop but even more so
as a major source of cash income for producing households. As a cash crop, cassava generates
cash income for the largest number of households, in comparison with other staples, contributing
positively to poverty alleviation. The presence of cyanogenic glycosides in cassava which when
broken down through enzymatic reaction librates hydrogen cyanide poses a great concern in
cassava utilization as food and as industrial raw material.
Cyanide, is usually found in compounds. It can interact with metals and other organic
compounds. Cyanide refers to all of the cyanide compounds that can be determined as the
cyanide ion, CN. The cyanide ion is a conjugate base of a weak acid, hydrogen cyanide, which is
an extremely poisonous gas with an almond odor. Other forms of cyanide compounds are sodium
cyanide (NaCN) and potassium cyanide (KCN). Cyanide can be produced by certain organism
(e.g bacteria, fungi and algae), and equally present in plants. Cyanide ion is one of the most
rapidly working poisons. Lethal doses taken orally act in minutes, cyanide, poisons by
asphyxiation, as does carbon monoxide, but the mechanism is different. Instead of preventing the
cells from getting oxygen, cyanide interferes with oxidative enzymes, such as cytochrome
oxidize, which is vital to every cell in use of oxygen. Oxidizes are enzymes containing metal
usually iron or copper. Cyanide binds tightly to the enzyme cytochrome C and forms stable
cyanide complexes with Fe3+ ion and inactivates the enzyme system.
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Much of the cyanide in soil, water and air comes from industrial processes gold mining,
waste waters from starch industry. The major source of cyanide in water are discharges from
metal mining processes, other sources include exhaust, release from certain chemical industries,
municipal waste burning and use of pesticides containing cyanide. Underground water can be
contaminated by cyanide present in landfills. In other body, cyanide can combines with plants
foods including almonds, millet sprouts, lima beans, soy spinach, bamboo shoots and cassava
roots, cyanide occurs as part of naturally occurring sugars or other complex organic compounds.
With respect to Cyanide levels, cassava varieties are broadly divided into two groups; the
sweet cassava known for low cyanide content and the bitter cassava with its high characteristic
content of Cyanogenic Glycosides (CGs) that is highly toxic when consumed (FSANZ, 2014).
Total cyanide in cassava products exists in form of CGs (linamarin and lotaustralin),
cyanohydrin and free hydrocyanic acid (HCN). Notwithstanding the CGS, according to FAO,
(FAO, 2011) 172 million tons of cassava were produced world-wide in 2000 with Africa
accounting for 45%, Asia 28% and Latin America and the Caribbean 19%. The five main
producing countries are Nigeria, Brazil, Thailand, Congo (DRC) and Indonesia. The on-going
challenge is to ensure that the presence of these cyanogenic glycosides is minimized through
proper understanding and possibly control of factors that affect cyanogenic glycoside content of
cassava. Roots and leaves contain the highest amount of linamarin (Cereda, 2016).
1.2 Hydrogen Cyanide (HCN)
Hydrogen cyanide (HCN) was discovered by Scheele in 1982. He made it by heating
sulphuric acid with Prussian blue; hence the old name was prussic acid. HCN occurs in nature as
glycoside amygdalin in some plants, for almonds, cassava etc. Hydrogen cyanide together with
sodium cyanide and potassium cyanide are the most of cyanide likely to be found in the
environment as a result of industrial activities. Its presence could be found in air, water, soil, and
even in gaseous state (present in solution in cassava root), with a faint, bitter, almond like odour.
It is a potential metabolic poison present in some food crops and other plants. Hydrogen cyanide
is a small molecule composed of a carbon, hydrogen and nitrogen atom joined together by a
stable triple bond. This poison is best known for its inhibition of many enzymes that are
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important in animal metabolism. Enzymes are proteins that act as catalyst in biochemical
reaction.
It could be made to act as an anti-herb ivory compound to discourage plant consumers
(pests). Most often, it attaches itself to other molecules in the form of cyanogenic glycosides. In
example of one such compound is amygdalin (from stems of cherry, apricot etc). In this form,
cyanide is non-toxic to the plant, only in the breakdown of cyanogenic glycosides, during animal
consumption or digestion, is hydrogen cyanide released. For example, cows feeding on some
species of grasses containing cyanogenic glycosides became ill as they chew on the grass, in this
fashion, it is hypothesized that cyanide in non lethal does effectively deters herbivory.
Some cyanide containing plants are listed below (plants and relative cyanide level):
 Cassava (+ + + +)
 Lima beans (+ + +)
 Sorghum (+ +)
 Millet (+ +)
 Bamboo Shoots (+ +)
 Sweet Potatoes (+)
 Maize (+)
1.3 Cyanide in Plants
The cyanogenic glycosides are a group of nitrile-containing plant secondary compounds
that yield cyanide (cyanogenesis) following their enzymatic breakdown. The functions of
cyanogenic glycosides remain to be determined in many plants; however, in some plants they
have been implicated as herbivore deterrents and as transportable forms of reduced nitrogen
(McMahon et al., 2015). It is estimated that between 3,000 and 12,000 plant species produce
and sequester cyanogenic glycosides. The major edible plants in which cyanogenic glycosides
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occur are almonds, sorghum, cassava, lima beans, stone fruits and bamboo shoots (Poulton,
2010). In certain sapindaceous seeds, HCN may arise during cyanolipid hydrolysis. More
frequently, HCN production in higher plants results from the catabolism of cyanogenic
glycosides. The approximately 75 documented cyanogenic glycosides are all O-β-glycosidic
derivatives of ahydroxynitriles. Depending on their precursor amino acid, they may be aromatic,
aliphatic, or cyclopentenoid in nature. Most are cyanogenic monosaccharides in which the
unstable cyanohydrin moiety is stabilized by glycosidic linkage to a single sugar residue.
Alternatively, in the cyanogenic disaccharides [e.g. (R)-amygdalin, (R)-vicianin, and
linustatin] or trisaccharides (e.g. xeranthin), two or three sugar moieties, respectively, are
involved in such stabilization. Sulfated, malonylated, and acylated derivatives of cyanogenic
glycosides are also known. are also known. Cyanogenesis is not exclusive to those plant species
accumulating cyanolipids and cyanogenic glycosides. All higher plants probably form low levels
of HCN as a coproduct of ethylene biosynthesis. This might explain why even 'acyanogenic'
plants contain significant levels of the cyanide detoxifying enzyme β-cyanoalanine synthase.
Cyanogenesis is also known in animals, but is restricted to the arthropods, notably to certain
centipedes, millipedes, and insects. In fungi and bacteria, HCN may originate via oxidative
decarboxylation of glycine.
A cyanogenic food of particular economic importance is cassava (Manihot esculenta),
which is also known by the names manioc, yuca and tapioca. Cassava is by far the most
important cyanogenic food crop for humans and is an important source of dietary energy in
tropical regions. The predominant cyanoglycoside in cassava is linamarin. It is present in leaves
and tubers, both of which are eaten. Linamarin is also present in beans of the lima or butter type.
Amygdalin is the cyanogenic glycoside responsible for the toxicity of the seeds of many species
of Rosaceae, such as bitter almonds, peaches and apricots. Sweet almonds are low in amygdalin
as a result of breeding processes. Their use in marzipan is common but the preparation procedure
should eliminate most of the cyanide. Cyanogen levels can vary widely with cultivar, climatic
conditions, plant part and degree of processing.
In areas of the world where cyanogenic plants such as cassava and lima beans comprise
the major item of the diet, chronic cyanide poisoning and associated pathological conditions still
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exist (Poulton, 2009). It is highly desirable that the toxicity of cyanogenic plants to humans and
livestock be reduced. This is achievable by: (a) selective breeding to produce low-cyanogen
varieties, as was accomplished for almonds, (b) screening of natural populations for low-
cyanogen varieties, (c) mutagenesis of protoplasts or cell cultures with subsequent regeneration
of plants having desired mutant genotypes, or (d) genetic engineering.
1.4 Roles of Cyanogenic Glycosides in Plants
A common feature of cyanophoric plants is that cyanogenic glycoside hydrolysis occurs
at a significant rate only after their tissues have been disrupted by herbivores, fungal attack, or
mechanical means. Although other explanations are possible, it is generally assumed that the
glycosides and their catabolic enzymes are separated in the intact plant by compartmentation at
either tissue or subcellular levels (Poulton, 2009). These possibilities have been extensively
tested in a single organism, namely the leaves of 6-day old light-grown sorghum seedlings.
Somewhat unexpectedly, the authors demonstrated that the substrate and its catabolic enzymes
were localized within different tissues. The cyanogenic glycoside dhurrin was sequestered in the
vacuoles of epidermal cells, whereas the 3- glycosidase and hydroxynitrile lyase were present
almost entirely in the underlying mesophyll cells. These two enzymes were located in the
chloroplasts and cytosol, respectively. It therefore seems likely that the large-scale hydrolysis of
dhurrin, which probably provides a defense mechanism against herbivores by liberating HCN,
occurs only after tissue disruption allows the mixing of contents of different tissues.
Available evidence from other plant species, however, favors compartmentation of
components of the 'cyanide bomb' at the subcellular level. In cassava, cells throughout the entire
root cross-section possess both cyanogens (principally linamarin) and linamarase (Kojima et al.,
2013). As in sorghum, highest glycoside levels are found in outer cell layers, again suggesting
the involvement of cyanogens in defense against herbivores or pathogens, but the subcellular
localizations of linamarin and linamarase remain unknown. In Phaseolus lunatus, the low
recoveries of linamarin, linamarase, and hydroxynitrile lyase in leaf mesophyll protoplasts
pointed to other tissues, perhaps the epidermis, as the principal site for these components.
Although these data cannot unequivocally distinguish between an epidermal or mesophyll
location, it seems certain that the P. lunatus linamarase is apoplastic. Leaf discs hydrolyzed
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externally supplied linamarin, and about one-third of the total linamarase activity was extractable
by multiple infiltrations of the leaves. The T. repens linamarase was detected by
immunocytofluorescence in cell walls, especially those of the epidermis, and in the cuticle. More
recently, protoplast isolation and tissue filtration experiments with Hevea endosperm showed
that linamarin and the hydroxynitrile lyase were intracellular but that linamarase occurred both
intra- and extracellularly. The apoplastic distribution of most linamarases contrasts with the
intracellular location of sorghum dhurrinase, a fact perhaps related to the nonglycoprotein
character of the latter (Poulton, 2009).
The physiological importance of cyanogenic compounds in plant metabolism is currently
receiving renewed interest. As with other secondary products, cyanogenics were originally
viewed as excretory substances, but their turnover (seasonal and even diurnal) argues strongly
against this hypothesis. Given the well documented toxicity of HCN, a role in plant protection
against herbivores, pathogens, and competitors is appealing. Much evidence, indeed, favors a
defence function for cyanogenics against certain animals including insects.
1.5 Statement of the Problem
Since high concentration of hydrogen cyanide is fatal to human and other life species
especially when consumed and is lethal to the body system or dangerous to health, the problem
lies on how to reduce this high concentration to a certain limit or how its concentration could be
eliminated.
1.6 Aim and Objectives of the Study
The aim of the study is to review hydrogen cyanide content in cassava. The specific
objectives are;
 To determine the effect of HCN consumption in the body.
 To examine the process of reducing HCN content in cassava before consumption.
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