Cover Image
close this bookTraditional Storage of Yams and Cassave and its Improvement (GTZ)
close this folder5 Cassava
close this folder5.6 The processing of cassava roots
View the document5.6.1 The purpose of processing
View the document5.6.2 Hydrogen cyanide and its release
View the document5.6.3 The production of cassava chips

5.6.1 The purpose of processing

As stated in the preceding chapters, the storage ability of fresh cassava roots is very limited in time This can only be prolonged slightly by the use of technical processes which, in some cases, are very costly (e.g. refrigeration). In view of this it is not surprising mat processes to conserve cassava roots have been developed.

There is a great variety of such processes ranging from simple drying through to processes which have to be considered as foodstuff technology (GRACE, 1977; COCK, 1985). Traditional methods of processing which are typical for some regions, e.g. the production of gari in West Africa, have been completely mechanised during the course of time. This has contributed to relieving particularly women of work (NZOLA-MESO and HAHN, 1982).

The main purpose of processing cassava roots is to get a product which will keep and which can be stored. Numerous production processes achieve this by drying the cassava roots. A welcome side-effect of drying is the concentration of the contents which determine its value. The ability of the product to be transported is considerably improved.

In addition to conserving, processing also detoxifies the cassava. This is necessary since the bitter varieties of cassava in particular have very high concentrations of hydrogen cyanide which can lead to serious health hazards.

5.6.2 Hydrogen cyanide and its release

Cassava roots contain hydrogen cyanide (HCN) which is a very strong poison. The lethal dose for an adult is approx. 60 mg per day (HAHN, 1989). Due to the high content of HCN, an unbalanced diet containing only fresh cassava products can lead to poisoning, deficiency and deformity. These occur especially if cassava roots have not been sufficiently detoxified and if there is a protein deficiency in particular of amino acids containing sulphur. The latter promote a very effective natural body detoxification process (HAHN, 1989).

The concentrations of hydrogen cyanide in the cassava roots depend on the variety. The content can amount to only a few milligrams but also to over 300 mg per kilogram of fresh root (HAHN, 1989). HCN is also unevenly distributed within the root. There are high concentrations in the outer cell layers and in the upper part of the root (HAHN, 1989). The bitter flavour cassava roots have does not indicate the content of HCN (LANCASTER and COURSEY, 1984).

Hydrogen cyanide does not occur freely in the cassava root but is combined with linamarin and lotaustralin, two cyanoglycosides. HCN is released by means of a hydrolytic process which is activated by the enzyme linamarase (COURSEY, 1982). Hydrolysis always takes place when the enzyme comes into contact with the cyanoglycoside. The natural release of hydrogen cyanide is encouraged by the mechanical destruction of the tissue or the disintegration of the cellular structures due to storage (LANCASTER and COURSEY, 1984).

Drying, boiling, immersing in water over a longer period and fermenting also encourage the release of HCN. What is promoted here is less hydrolysis and more the release of HCN which has already been detached due to the activity of the enzymes with the glycoside.

Even when the cassava roots are properly processed a residue of hydrogen cyanide remains. The concentrations however are mostly so minute that no hazard to health will occur from eating them (HAHN, 1989).

5.6.3 The production of cassava chips

The production of cassava chips is the most simple way of obtaining a product, on the basis of cassava, which will keep and which can be stored. Cassava chips are for the purpose of self-sufficiency, as e.g. in West Africa (STABRAWA, 1991) as well as for obtaining income and foreign currency. The latter applies particularly to Thailand (COCK, 1985).

The production process always follows the same pattern and more or less shows a high degree of mechanization. Slight deviations from this lead to chips with varying quality features reflecting the regional demand and flavour preferences. The possible variations on the standard processes here, will be dispensed with at this point. Firstly, these are far too numerous, often-only of regional importance, and secondly, documentation on this is rare.

Chips are not only made from cassava but can also be produced from yams. Due to the lower content of dry matter in yams in comparison to cassava correspondingly more energy has to be used to dry them. Seen from the volume of production, cassava chips are far more significant than yam chips. As the production processes for both products are virtually identical the method of producing yam chips is not to be discussed at this point.

5.6.3.1 Preparation of the cassava roots for the production of chips

The cassava roots are peeled immediately after harvesting with the traditional cutting tools, e.g. brushwood knife (machete). The peeling, mainly carried out manually by women, requires a great deal of work. One woman can peel about 20 -25 kg roots in one hour (SADIK, 1987). The loss in weight occurring due to peeling amounts to about 30% of the fresh weight (ibid.).

Various peeling machines have been developed in West Africa. These have not been widely accepted because the purchase prices are too high and the machines cause too great a loss in peeling (ibid.).

The roots peeled are men washed. If the chips are obtained from bitter cassava varieties, the roots frequently are kept in water after peeling. This causes hydrogen cyanide to be released, reducing the danger of poisoning (JAKUBCZYK, 1982). For a sufficient release of hydrogen cyanide, the roots should be soaked for 2 - 4 days (JOSEPH, undated). A good release of hydrogen cyanide is attained if the roots are cut into pieces prior to soaking. These are men soaked in water for 15 minutes and then boiled for 2 minutes (JAKUBCZYK, 1982).

Another method of preparation is to briefly boil the freshly peeled roots in water. Then they are halved lengthwise and soaked in water for 1 - 2 days. The water should be changed once to twice during this time (ONWUEME, 1978).

Which process is preferred, particularly regarding the release of hydrogen cyanide, has not yet been sufficiently investigated.

The cassava roots prepared in this way are cut into pieces for drying. How the roots are split up and how large the pieces are, vary from region to region and depend on the relevant eating. The size of the pieces of root is also influenced by climatic drying conditions. Thus the pieces are mostly larger in the dry northern parts of Ghana man those in the south of the country (KWAKU, 1991).

In some cases the cutting of the roots has also been mechanised. The machines used for this chip the roots into small pieces which dry correspondingly well (COCK, 1985).

5.6.3.2 Drying the cassava chips

To store well, the chips have to be dried to a moisture content of about 12% (COCK, 1985). Completely dried chips are white and break easily without crumbling (INGRAM and HAMPHRIES, 1972). Drying is frequently inadequate when the chips are to be sold directly after they have been dried (INGRAM and HAMPHRIES, 1972). Pricing which is oriented to the weight of the product, can be manipulated in favour of the seller by increased moisture content.

The prepared chips are spread out on all sorts of supports to dry. They are laid out on the roofs of houses, the edges of roads or in yards. No special constructions developed for chip drying are known of in West Africa. Chips laid out to dry are often soiled by rain, sand and animal excrement which leads to losses in quality due to hygiene (JAKUBCZYK, 1982).

The energy from the sun and wind are mainly used to dry the chips. High energy costs normally make the use of external energy (wood and fossil fuels) to dry the chips unviable. The drying process however, is often supported by wood fires and the use of heat from stoves (CHINSMAN and FIAGAN, 1987). The smoke emitted is said to act as an insecticide. But smoke also leads to discolouring and changes in the flavour of the chips which is not always desired.

The duration of drying depends on the size of the chips and on climatic conditions. Under optimum conditions. the chips can be completely dried within 2 days by using the energy from the sun and the wind (COCK 1985 )However the drying period is mostly much longer and frequently takes between two and three weeks (INGRAM and HAMPHRIES, 1972).

During the long drying period the chips often become mouldy and ferment. This makes the originally white chips discoloured and also changes their flavour. The Ada, an ethnic group native to Ghana, want this qualitative change to take place during drying (NICOL 1991). In the opinion of the Ada, the fungus settling on the chips is evidence of a low content of hydrogen cyanide. Consequently, they believe that chips infested by mould are quite suitable for human consumption (ibid.). Mould as an indicator for the non-toxicity of chips has not yet been proven scientifically.

Chips are often briefly boiled in water (parboiled) after drying and men dried again. This makes the chips harder and is to improve their storage ability and reduce their susceptibility to infestation by pests. Investigation however show varying results (STABRAWA, 1991;INGRAM and HUMPHRIES, 1972).

5.6.3.3 The storage of cassava chips

The demands cassava chips have on storage conditions are similar to those of cereals (COURSEY, 1982). Cassava chips are hygroscopic and tend to draw moisture which promotes the formation of mould and thus early deterioration.

Many stored product insects which cause damage to cereals also infest cassava chips (cf. Chapter 5.6.3.4). Consequently, storage structures should on the one hand provide some protection from reabsorbing moisture, but should also avoid infestation by pest insects. This must be qualified by saying that cassava chips are often infested by pest insects during the drying process. For this reason, as already mentioned in Chapter 5.6.3.2, the drying process is of particular importance in the storage of chips.

In contrast to the yam tubers for which specific storage systems have been developed, cassava chips are kept in stores which are also used to store cereals and grain legumes (STABRAWA, 1991). Thus, cassava chips are stored in baskets, in wooden containers, in sacks or in bulk in storage rooms as well as in various traditional storage systems intended for cereals (INGRAM and HAMPHRIES, 1991). Frequently varying storage systems are used side by side which can serve to fulfill the varying storage requirements (STABRAWA, 1991).

Of great importance in the selection of certain storage systems are the availability of various building materials, the existence of certain artisanal knowledge, capital and labour. In contrast, cultural customs and traditions play only a minor role (COMPTON, 1991). In many areas, there are however skill close associations between the structural features of storage systems and certain ethnic groups. These are normally a result of artisanal traditions and experience being passed down within certain groups. This experience is also freely passed on to members of other groups and used by these ('bid.), indicating some openness regarding technical storage innovations.

In Togo, there are three traditional types of storage in particular which are preferred for storing cereals but also for cassava chips.


Fig 16: "Kpeou", a traditional storage system for cassava chips (Source: LAMBONI, undated)

The "kpeou" is a storage structure which consists of mud or often of the material from termite mounds. It is shaped like a water jug and is often divided into several chambers (Fig. 16). The store often reaches a height of over 2 metres. The upper edge of the "kpeou" has an opening for filling and entering which can be firmly closed. The "kpeou" is relatively expensive to erect but has a service life of 20 - 30 years. In Togo the "kpeou" is the only closed storage system. As there is no method of ventilation due to the way of building, the produce which is to be stored must be dried optimally (chips should not have a moisture content greater than 12%).


Fig. 17: "Katchalla", a traditional storage system for cassava chips (Source: LAMBONI, undated)

The "katchalla" is made of wood and straw. It looks like a cone which is upside down and is stabilized by wooden supports (Fig. 17). The "katchalla" has an opening at the peak of the cone which is closed by a conical roof. The storage system is not airtight, but has some ventilation.

The "tonneau" can be compared to a large barrel and is erected on a low platform. The "tonneau" consists of a wooden frame in which mats are stretched. The "tonneau" is open at the upper edge and is closed by a conical roof (Fig. 18). It is often constructed to a height of more than 2 meters. This system is also open and allows air exchange between the stored produce and the atmosphere.


Fig 18: "Tonneau", a traditional storage system for cassava chips (Source: LAMBONI, undated)

According to the studies by COMPTON ( 1991 ) and STABRAWA (1991), about 60% of cassava chips are stored in traditional storage systems (34% "kpeou" and 26% "katchalla) in the central region of Togo. The remaining 40% are kept in varying types of storage of which storage in sacks and as bulk produce in storage rooms are the most significant.

The average storage duration for cassava chips amounts to 7 months, but can extend to over one year (STABRAWA, 1991). Other sources state a storage duration of 3 - 6 months for sun-dried and of up to 12 months for "parboiled" cassava chips before serious mould begins (INGRAM and HAMPHRIES, 1972).

The duration of storage is influenced by a large number of factors which can vary greatly from region to region. In addition to natural influences, the duration of storage is also affected by socio-economic factors. In Togo, for example, the chips which are intended for sale are stored for 7 - 8 months in order to take advantage of price fluctuations due to quantities in supply. Chips serving self-sufficiency purposes are stored up to a period of 12 months, i.e. until the new harvest is brought in (STABRAWA, 1991).

5.6.3.4 Losses in storage due to pest insects

Stored product insects cause high losses in the storage of cassava chips. These pests infest not only cassava chips but also other foods which are stored under tropical conditions (HODGES et al., 1985). According to LAMBONI (undated), Prostephanus truncatus (Horn), Dinoderus minutus and Tribolium sp. are among the most significant pest insects in the storage of cassava chips among small farmholders in Togo.

Prostephanus truncatus (Horn) which did not appear as a pest in Togo until the beginning of the eighties, can be easily confused with Dinoderus which also causes damage to stores of cassava chips (STABRAWA, 1991). The losses caused by Prostephanus truncatus (Horn) can be very high. HODGES et al. (1985) determined weight losses of up to 50% for unfermented and up to 70% for fermented chips after a storage period of 4 months which were ascribed to this storage pest.

The differences in the amounts of loss are caused by the varying density of the two types of chips. Unfermented chips are denser making it more difficult for the grain borer to penetrate them man fermented chips. The production of unfermented chips cannot be recommended as protection from infestation by Prostephanus as these are also subject to serious infestation (HODGES et al., 1985).

To quantify the storage losses for cassava chips which are caused by insects is very difficult as firstly, suitable methods for an estimation of the losses do not exist. The NRI has been endeavouring to find a basis for a solution to this for some time now. Secondly, the farmers evaluate the losses of cassava chips due to insects in a different way than for e.g. maize. The badly damaged chips and the flour from boring are mostly still used for human consumption, the insects being sieved out beforehand (STABRAWA, 1991). The farmers consider the worse plasticity of the cassava paste made from this to be a considerable disadvantage of this insect infestation in comparison to that made out of uninfested chips (COMPTON, 1991). Since only a third of the paste mixture consists of cassava chip flour, the negative effect of the insect on the consistency of the paste is limited (ibid.).

Insects often infest the chips during drying (cf. Chapter 5.6.3.2). They can also not infest the stored produce until it is put in storage. Since farmers consider the losses caused by the insects only as partial losses, practically no traditional preventive measures have been developed. In particularly the high losses caused by Prostephanus truncatus (Horn) have led to isolated farmers making use of chemical products for storage protection (COMPTON, 1991). The selection of insecticides is made at random and depends only on market supply. So far, the effect of these products and the formation of possible residues which could constitute a health hazard have not been investigated. For this reason, no insecticides, dosages or application methods can be recommended here.

5.6.3.5 Storage losses due to mould

Mould frequently infests the cassava chips during the drying stage. However, mould also forms if the chips again become moist in storage (INGRAM and HUMPHRIES, 1972). Not only one variety of fungus but several occur on the chips simultaneously. It teas not yet finally been determined which metabolites form the various varieties of fungus, or whether mycotoxins are possibly among these.

The formation of mould cannot be basically seen as a loss in quality or a cause of loss. Some ethnic groups appreciate infestation of the chips by mould and even speak of improvements in the flavour here (cf. Chapter 5.6.3.2). In Burundi, a Belgian company attempted to improve the nourishing qualities of cassava chips by directed mould infestation (JOSEPH, 1986). Disregarding the regionally varying preferences for particular flavouring, mould on chips mostly leads to distinct losses in value. This applies particularly if the chips are intended for sale. For these reasons the only recommendation at this time can be to avoid the formation of mould on chips during production.

5.6.3.6 Measures to improve the production and storage of cassava chips

The storage ability of cassava chips is strongly influenced by the drying process. Drying which takes too long, promotes insect infestation leading to extensive storage losses. If the chips are only insufficiently dried and still have mote than 12% moisture content, the danger of mould will exist. Mould also forms when the hydroscopic chips are not sufficiently protected from the moisture in the atmosphere and re-absorb moisture during storage.

Improvements to the production and storage of cassava chips have thus to begin at the drying stage. At the same time, a storage has to be practiced which not only has to provide protection from the penetration of insects, but also against re-moisturising of the stored products.

Peeling the cassava roots requires a great deal of labour. The mechanisation processes devised so far are more for peeling large quantities (e.g. for gari production) man for use on small farms. A technology which saves labour and hardly causes any extra costs, which substantially improves the labour productivity of peeling and thus seems predestined for introduction to the a.m. target group is the peeling knife developed by the IITA. In Togo at least, this knife is not widespread and should thus be put to practical tests as a measure of improving the labour productivity. A direct contribution to relieving the woman of labour could be made here since the peeling of the roots is her responsibility.

The drying process can be shortened by increasing the surface area of the chips in relation to their volume. The larger chips which are often spread out to dry in many regions of Africa have to be reduced in size to improve their drying properties. The principle to be followed here is: the smaller the chips, the faster drying takes place.

Before measures can be recommended, the reasons for the size of the chips must be investigated. If mere are reasons for this which stem from work management, it must be investigated whether a technology can be introduced to increase labour productivity. In this respect, the microeconomic viability has to be analysed just as the acceptance of the procedure by the population concerned. Examples of mechanisation for chip production using slicers can be seen in work by COCK (1985) amongst others.

In the past, only the rays of the sun were normally used for drying the chips. These are extensively reflected by the white chips and are partly lost for the drying process. As experiments have shown, drying can be substantially improved if wind energy is also used in addition to the energy from the sun (COURSEY, 1982).

For this purpose the chips are laid out to dry on a wooden frame covered with wire mesh. The frame can be any size but should be chosen so mat it can be easily handled. This is the case when it is has an approximate size of 1.5 x 1 m. The wire used to stretch over the frame should be fine enough to prevent the chips falling through the mesh. This wire can be substituted by any locally available materials which can be permeated by air.

The wooden frames are set up at a definite angle so that the rays of the sun fall on the chips and so that the natural movement of the wind constantly aerates these (cf. Fig. 19). In this way, cassava chips can be optimally dried within 2 days (COCK, 1985).

In addition to this mobile frames offer further advantages. If unexpected rain showers occur, they can be cleared away with the chips which prevents with during the drying process and thus a reduction in quality. There are also hygienic benefits of using the frames since the chips no longer come into contact with the dirt from the streets or the yard as is usual in traditional drying processes.

Storage structures where chips are traditionally stored do not always provide sufficient protection from pest insects or with Of the traditional storage structures used in Togo, the "kpeou" (cf. 5.6.3.3) seems to be suitable for the storage of cassava chips. However, the storing features of this structure must be investigated in more detail. Apart from this traditional system, other containers can be used to store cassava chips under some circumstances. Literature mentions e.g. plastic sacks. Plastic barrels and used oil barrels also seem suitable for storing cassava chips. The storage properties of these must be initially investigated before any recommendation for storage in these containers can be made.

There are no proven results on processes of chemical storage protection for cassava chips. From Togo, it is known, and this definitely also applies to other countries as well, mat the farmers use chemical insecticides for cassava chips at random when pest infestation occurs. Since considerable health hazards can occur when treated chips are consumed, investigations should be carried out to define recommendations on products and on application which will men allow storage protection without any risk to health