ÐÓ°ÉÔ­´´

IT WOULD BE difficult to design a more appropriate food crop for the
tropics than the cassava root. Cassava survives and produces a reasonable
harvest even in the poor soils that characterise large parts of Africa,
Asia and Latin America. It tolerates drought and acid soils. The cassava
plant resists damage by pests and diseases and is one of the most efficient
converters of solar energy to carbohydrate. Small farmers in tropical countries
like the crop because they can propagate from stem cuttings and because
they can intercrop it with other staples, such as maize.

People eat more than 60 per cent of all cassava produced. About a third
of the harvest feeds animals and the rest is transformed into secondary
products, such as starch. People eat the cassava root in virtually all tropical
countries. The boiled leaves, which are an excellent source of protein,
are eaten in many parts of West and Central Africa and in the Amazonian
Basin. The Portuguese introduced cassava to the African continent from its
home in Latin America.

Yet despite its many advantages and the fact that world production of
cassava increased by 24 per cent between 1971 and 1981, the importance of
cassava, relative to cereals, has decreased. Aid from the rich countries
of the north has helped to promote the view that cassava is poor folks’
food. Aid to developing countries often takes the form of subsidies for
food that is not grown locally. For example, aid agencies and government
typically supply cereals grown in the Northern hemisphere to countries where
the staples are root crops, such as cassava or yams. City-dwellers may receive
a further subsidy as governments keep the price of bread artificially low;
they benefit directly at the expense of small local farmers whose crops
cannot compete. In Brazil, subsidies make bread cheaper than cassava. Without
a market for their crops, farmers leave the land for the city. In Latin
America, the production of cassava fell by 12 per cent between 1971 and
1981, largely because of the availability of highly subsidised cheap wheat.

This move towards cereals in tropical countries destabilises local agriculture
and increases dependency on aid. The agricultural pricing policies of importing
countries and the subsidies of exporting countries have further favoured
cereals. Consequently the consumption of cereals in tropical countries has
increased while the consumption of cassava and other tropical root crops
has declined. For example, in sub-Saharan Africa cereal imports amounted
to US$2 billion a year in the early 1980s. This dependency on imported food,
and the consequent neglect of traditional local crops that can give excellent
yields in tropical climates, is a disturbing trend. The picture is changing,
however, because of the uncertainties associated with long-term food aid,
and the trend towards agro-industrial development in tropical countries.

Many development projects that attempt to replace direct subsidies by
improving agricultural methods have compounded the problem because they
have not built on local traditions: inappropriate crops or damaging technologies
are introduced. For example, tractors at first improved the yields of crops
in tropical Africa, but after a few years they destroyed the fragile structure
of the topsoil, causing massive erosion (‘A green revolution for Africa’,
New ÐÓ°ÉÔ­´´, 7 May 1987). Other development projects have ignored staples
such as the roots and tubers that, in tropical countries, people grow mainly
for their own consumption. Women are often responsible for the cultivation,
harvesting, processing and marketing of these staples. Consequently, traditional
crops such as cassava have a low social status and, in the past, have attracted
little research.

There are other reasons for the neglect of cassava, however. One disadvantage
of cassava is that it is difficult to hold stocks for even a few days because
the roots deteriorate quickly after harvest. Research aimed at increasing
the productivity of cassava and finding ways to store it could support local
agriculture, and benefit people for more than just a few years.

During the past 10 years, researchers from Britain’s Overseas Development
Natural Resources Institute have addressed these problems. The ODNRI is
one of the scientific arms of Britain’s Overseas Development Administration.
Work on cassava began by examining the biochemical, physiological and pathological
characteristics of the deterioration of the root. The institute has now
developed into research on cheap ways to store roots, and to improve the
quality of fresh and processed cassava and its nutritional value. It hopes
to increase the popularity of cassava, and so ensure the future of small
farmers, by distributing the root to novel markets and finding new uses
for cassava.

Cassava is grown principally by small farmers for their own consumption
and for sale to local markets or processing plants. They avoid the problem
of rapid deterioration by harvesting a few roots at a time. Women also process
cassava soon after gathering it so that it can be stored for longer than
a few days. In West Africa, women peel, wash and grate cassava tubers to
prepare gari. They ferment the grated pulp in sacks under weights for a
few days. Heating the fermented cassava drives off water and cyanide . The
roots contain enzymes that convert certain glucosides into hydrogen cyanide,
which can poison or kill people who eat it. In tropical Brazil, women peel
the tuberous roots, then pack the cassava into a stretchy basketwork tube,
the tipiti, to extract the juice which contains most of the cyanogenic glucoside.
They roast the damp meal over a fire then use it for bread or dumplings.
These elaborate ways of preparing cassava prolong its life considerably.
Chips of dried cassava or dried and milled cassava can last even longer.
Knowing how cassava deteriorates after harvest might suggest easier methods
of preparation and more efficient ways of storage.

A collaborative project between the International Center for Tropical
Agriculture (CIAT) in Colombia and the ODNRI has begun to unravel the two
processes by which cassava roots perish. Cassava begins to rot within three
days, often after 24 hours of harvesting depending on the damage a root
has suffered. Microorganisms begin to promote the deterioration of the roots
usually only after the cells of the root have been damaged. June Rickard,
of the ODNRI, and Chris Wheatley, working first for Wye College, University
of London (funded by the Overseas Development Administration), and later
at the CIAT, showed that the root begins to deteriorate as the activity
of oxidising enzymes in its cells increases. The enzymes generate phenols,
such as catechins and leucoanthocyanidins, that later polymerise to form
tannins. The first sign of damage shows up within 24 hours of harvesting
as bright blue fluorescence under ultraviolet light as the phenolic coumarin
and scopoletin accumulate. Tannins then colour the white root tissue blue
or brown, a condition known as vascular streaking. Damaged cassava cooks
poorly and often tastes unpleasant; tannins may also bind to proteins in
the gut, preventing their absorption. Storing the roots at low temperatures,
keeping them in a humid atmosphere and excluding oxygen slow the deterioration.

Cassava can, however, cure itself: high temperatures and humidity promote
the healing of wounds in the roots naturally. In these conditions, fatty
substances move into the walls of the cells known as the parenchyma, forming
a cork layer that resists decay and is waterproof. A layer of rapidly growing
cells (similar to those in a root tip) forms beneath the wound. This minimises
the loss of water that leads to vascular streaking.

Studies at the CIAT showed that the rate of decay can vary, depending
on the variety of cassava and the environment in which it is growing. Plants
whose roots are slow to rot contain less starch than those that decay rapidly,
but they are not as valuable as a crop. So breeding for resistance to physiological
deterioration will probably prevent an increase in the amount of starch
in cassava. Instead, researchers need to look for ways to preserve the crop
after harvesting.

Pruning the plant two or three weeks before harvesting causes the root
to shrivel a little, but makes it keep better. Unfortunately, cassava treated
in this way does not taste as good as that from unpruned plants; it takes
longer to cook and has a different texture. Yet it might be a useful approach
for animal feed.

Whatever treatment is used to preserve the root, about five days after
harvest damage by microorganisms begins. Most of the early work to isolate
the agents of decay in cassava concentrated on the later stages of this
process. Bob Booth and Robert Noon, researchers from the ODNRI working at
the CIAT, identified various species of moulds and bacteria, but consistently
failed to isolate any specific microorganism from the advancing margin of
rot in the cassava root. A variety of parasitic microorganisms (saprophytes)
hasten secondary decay.

The ODNRI-CIAT project also investigated other ways to store cassava.
Subsistence farmers commonly leave cassava in the ground until it is needed.
However, this leaves the land underused, while the cassava often become
tougher and less marketable. The risk of crop loss from pests and diseases
increases. The success of other storage methods in pits, field clamps or
in boxes with moist sawdust depended on achieving the curing conditions
that slow down decay. First, researchers picked the best roots to store
because digging up cassava sometimes damages a root so badly that curing
is ineffective. Bob Booth, for the ODNRI and the CIAT (now with the United
Nation’s Food and Agricultural Organisation as coordinator of its programme
to prevent food losses), reported that he had stored cassava for several
weeks in field clamps, similar to those used for storing potatoes. Clamps
are difficult to manage especially where seasonal variations in climates
cause the requirements for ventilation and drainage to alter after the clamp
has been sealed. Nor do cassava farmers and traders in Latin America need
clamps if they sell only small amounts of cassava quickly. Storage in boxes
proved too expensive for small farmers unless the cassava is to be exported,
in which case farmers get a price high enough to make the boxes affordable.

Another way to keep cassava fresh is to pack it in plastic bags. A group
at the CIAT led by James Cock, including Rupert Best, Carlos Lozano and
Chris Wheatley, found that the high humidity and temperature within the
polyethylene bag promotes root curing. Roots can also be treated with a
fungicide, thiabendazole, to inhibit secondary damage by microorganisms.
Thiabendazole is an approved fungicide for use in food products: residue
levels in cassava are less than 1 part per million. (Permitted levels in
potato are up to 5 parts per million.) This is an economic and safe use
of an agrochemical. The procedure is simple, requires little capital or
labour so farmers readily accept it. Treated cassava stores well for two
to three weeks. Preliminary studies suggest that increasing sugar content
in roots determines this limit. Field tests with Colombian farmers show
that roots should be treated and packed within a few hours of harvest and
that damage to roots should be minimised.

Future projects involving the CIAT and the ODNRI will concentrate on
adapting the storage treatment to the requirements of different cassava-producing
regions in Latin America. It solves some of the problems of getting cassava
to city markets in good condition. In Colombia, economists estimate that
the benefit to the urban population would be more than US$20 million a year
because cassava stored in polythene bags and treated with fungicide is cheaper
because of reduced losses and of better quality. This technique is also
being used commercially in Paraguay, Ecuador and Panama.

There remains, however, the problem of toxicity. Cassava roots are starchy,
and so a good source of carbohydrate, but they contain little protein. In
times of famine, cassava is often the only food available. If roots are
damaged or prepared carelessly, they will have a high content of hydrogen
cyanide. Cassava foods may also contain residual cyanide that can cause
nutritional problems. Some lines of cassava contain less cyanogenic glucoside
than others, but researchers have failed to locate a cassava strain that
is free of cyanide. The quantity of cyanide in commercially grown cassava
is rarely sufficient to cause acute poisoning, but there are fears over
the long-term effects of eating even minute portions of cyanogenic glucoside.
Researchers first had to determine how much of this people were exposed
to. The methods were tedious, irreproducible and inaccurate. Rod Cooke,
of the ODNRI, developed an enzymic assay for the cyanide in cassava that
obviates many of these problems. This has been used in field trials run
by the CIAT, the International Institute of Tropical Agriculture in Nigeria,
the Food Technology Research Centre (CITA) in Costa Rica and the National
Institute of Technology and Standards (INTN), Paraguay. Other centres in
Africa, India and the Far East have also adopted it.

Using this assay, a picture emerges of how effective traditional methods
of removing cyanoegnic glucoside are. In some cases the breakdown of the
glucoside is the slowest step, in others the hydrolysis of the nonvolatile
cyanohydrin to volatile hydrogen cyanide is the limiting step in the removal
of cyanide.

An extended study in Zaire by A. M. Ermans and colleagues from the Hopitale
St Pierre, Brussels, pointed to a link between eating cassava and the development
of goitre and related neurological syndromes where dietary iodine is in
short supply. Goitre was most common in areas where people ate large amounts
of cassava. The human metabolic route for cyanide detoxification increases
the requirment for amino acids containing sulphur (methionine). Furthermore,
one of the products of detoxification, thiocyanate, inhibits the absorption
of iodine. People who have sufficient iodine in their diet did not show
signs of goitre.

Other factors also complicate the picture we have of cassava. For example,
dried and processed cassava contain tannins. Tannins in other foods, such
as sorghum, make it harder to digest proteins and to absorb iron and methionine.
So people eating large amounts of cassava may fail to absorb exactly the
amino acids they need to protect themselves against its toxic effects. June
Rickard and Nigel Poulter at the ODNRI, working with researchers led by
John Blanshard from the University of Nottingham, hope to find out whether
tannins in cassava products have similar effects.

Hans Rosling of the International Child Health Unit, Uppsala, Sweden,
and his colleagues in Mozambique are investigating the association between
eating inadequately processed cassava at times of famine and a failure of
the nervous system that controls muscles, spastic paraparesis (which is
known throughout Central Africa as konzo). There appears to be a link if
diets contain high levels of cyanide from eating poorly prepared cassava
and are low in protein. During recent years, this disease has crippled several
thousand women and children in Zaire, Mozambique and Tanzania when food
is short in areas where cassava grows. Rosling points out that the toxic
effects of cassava are slight compared with the other public health problems
in tropical countries, and to cassava’s importance as a staple food. In
most cases where people have been poisoned, the only alternative for the
families concerned was starvation.

The amount of cyanide left in cassava varies with the method of preparation.
Introducing new varieties without regard for local ways of growing and processing
roots and the local diet may cause problems. Good varieties of cassava can
sell themselves, however. In Nigeria, researchers tested improved varieties
of cassava by growing them in the poorest soil, without protection from
pesticides. Local farmers quickly adopted the best cultivars and sold cuttings
to neighbours as soon as their own fields had been planted.

Little research has been done, however, on why people prefer different
varieties of cassava. So the IITA and the CIAT with the Rockefeller Foundation
began a study of how cassava was grown and used in sub-Saharan Africa. The
ODNRI helped to design surveys of how cassava is processed and sold. The
CIAT and ODNRI are also investigating why some lines of cassava have higher
yields yet to do not appeal to consumers, and how factors such as cooking
time, taste and starch content depend on the age at which a plant is harvested,
its environment while it grows and how the root is processed.

Alan Reilly, Andrew Westby and David Twiddy, microbiologists from the
ODNRI, are examining the role of microorganisms in the detoxification of
cassava when making gari and fufu, traditional African foods. Gari is a
fermented granular cassava product, which is partly gelatinised, and is
prepared in most of West Africa. Fufu is a creamy-white fermented paste
eaten in Nigeria and Zaire. The microbiologists showed that two stages of
processing eliminate most of the cyanide: grating raw roots and roasting
the final product. The lactic acid bacteria, which predominate during fermentation,
develop the flavour and extend the food’s shelf-life but play little part
in the breakdown of the cyanogenic glucosides.

The future for cassava may, however, lie with its secondary products:
cassava chips make a nutritious animal feed; ground cassava can partially
substitute for wheat in bread; and cassava can be fermented to make alcohol
and single-cell protein. During the past 20 years, for example, Thailand
has built up its exports of sun-dried cassava to 7 million tonnes a year.
Shipped as compressed pellets, mainly to EEC countries, it is added to compound
animal feeds. Many Latin American countries are producing cassava chips
for their animal feed industries. In Ecuador, finely milled dried cassava
acts as a nutritious additive that also helps to stick shrimp food together.

Starch and starch derivatives, such as dextrins, glucose and high-fructose
syrup are the main products of the cassava agro-industry. Cottage industry
production of starch dominates South America, Asia and Africa but larger
scale operations in Thailand, Brazil, the Philippines and Indonesia produce
8 per cent of the world’s starch. Starch is probably the most familiar cassava
product outside the tropics: as tapioca, rice-sized beads of cassava starch,
it has probably been on most schoolchildren’s plates.

* * *

Safe cooking – the chemistry of preparing cassava

THE SOURCE of cyanide (-C≡N) in cassava are compounds called cyanogenic
glucosides, such as linamarin and lotaus tralin. These consist of a glucose
ring combined with a cyanohydrin, which has a cyanide group and hydroxyl
group attached to the same carbon atom. When the cassava tissue gets damaged
by, for example, grating it to a pulp, it liberates an enzyme called linamarase.
This catalyses the reaction of a cyanoglucoside with water to release the
glucose and the cyanohydrin, which can then decompose reversibly into a
ketone, such as acetone (CH₃COCH₃) and free hydrogen
cyanide (HCN), depending on the conditions. An enzyme called hydroxynitrile
lyase can catalyse this reaction.

Once the hydrogen cyanide is freed, it is easy to remove by heating
(drying and roasting the cassava) because it is a gas. The cyanoglucosides
and cyanohydrin are more difficult to get rid of, although cyanohydrin decomposes
quite quickly at a neutral pH. Under acidic conditions, however, this reaction
slows down dramatically.

Studies at the Overseas Development of Natural Resources Institute,
Kent, show that the key factor determining how much cyanide is left in cassava
foods depends on its preparation. In products based on whole roots or pieces
of cassava, the rate at which the cyanogenic glucosides break up controls
the amount of cyanide present (reaction a). When the cassava is milled or
pounded to a pulp, the conversion of cyanohydrins to hydrogen cyanide may
become the limiting factor (reaction b).

Homogenised cassava in water rapidly becomes acidic because the bacteria
associated with fermentation produce lactic acid. The acidic conditions
then slow down reaction b and stabilise the amount of cyanide present.

Rodney Cooke is former head of the food plant department of the Overseas
Development of Natural Resources Institute (ODNRI), and has recently become
a deputy director. James Cock leads the cassava programme at the International
Center for Tropical Agriculture, Cali, Colombia. This article represents
the personal views of the authors.