Extraction of starch from dried cassava roots

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A limited quantity of the cassava imported into Europe in the form of chips and dried sliced roots is manufactured into starch. The dried roots are cleaned, washed and grated and the starch is separated by cylindrical sieves; however, this practice is costly and the starch produced is of inferior quality for the following reasons:

(a) The brown skin, which contains chlorophyll and coagulated proteinous substances, adheres strongly to the ligneous tissues. While it is easy to remove this skin from the fresh roots, it is very difficult to remove it from the dried roots and, therefore, the starch of dried roots is always dark.

(b) The nitrogenous substances are found in a colloidal state enveloping the starch granules in the pulp slurry of the fresh roots. It is easier to separate these nitrogenous particles in the pulp slurry of fresh roots than in dried roots.


3. Baked tapioca products

The baked products for which cassava flour is the basic ingredient are known commercially as tapiocas or tapioca fancies. In Malaysia and some other areas these products are commonly known in the industry as sago products. The term probably originated with the Chinese production of sago-palm starch products. The manufacture of tapioca fancies is a logical follow-up of the production of the flour itself in the countries of origin. Separation of the processing of the flour and of the derivatives would be illogical. Many medium-size and larger factories are also equipped for the manufacture of such baked products as flakes, seeds, pearls, and grist.

These products are made from partly gelatinized cassava starch obtained by heat treatment of the moist flour in shallow pans. When heated, the wet granules gelatinize, burst, and stick together. The mass is stirred to prevent scorching. They are manufactured in the form of irregular lumps called flakes or of perfectly round beads 16 mm in diameter known as seeds and pearls (Figs. 24-26). The grist is a finer-grained product obtained by milling gelatinized lumps, and siftings and dust are residual products of the manufacture of seeds and pearls.

Preparation of wet flour

The raw material for baked products is the flour scooped up from sedimentation tanks or tables after the supernatant, or excess water, has been drained and the "yellow" flour scraped off. Clearly the use of moist starch, an intermediate stage in the processing of the Dour, is economically advantageous.

Only very white first-quality flour can be used in the manufacture. To obtain this, sulfurous acid is often added in the first sedimentation. This chemical should, however, be washed out as completely as possible by a second sedimentation in clean water; any traces of the acid left in the flour tend to spoil the quality of the end product. It is strongly advised not to use active chlorine preparations in this case, as they influence the agglomeration of the starch into pearls and other forms in an unfavourable way.

The cake of moist flour, containing about 45 percent water, is broken up by a small mill, spades or pressing it through frames strung with steel wire spaced about 10-20 cm apart, after which the lumps are rubbed through a screen of about 20 mesh/inch to produce a coarse-grained moist flour.

At this stage the flour is ready only for gelatinization and the production of flakes; to prepare pearls and seeds, the small aggregates of moist starch should be subjected to a process of building up and consolidation. which gives them the size and cohesive strength desired for the further treatment. The operation is known by the Indonesian name as the gangsor method. A portion of the moist starch is put into a long cylindrical bag of twill cloth which is held at each end by one man. Together. with a rhythmical strong jerking movement, they throw the mass of starch lumps from one end of the hag to the other (Fig. 27). After a few minutes of this treatment the irregular lumps have grown into beads of varying size and have gained in firmness. Another portion of the moist flour is added and the gangsoring is continued. the operation being repeated until the heads have grown more or less to the desired size. Depending on the skill of the worker. the size of the starch balls is fairly uniform. Curiously enough, the knack of gangsoring is achieved only by a fraction of all workers, so the operation should be classified as skilled labour.

In Malaysia the flour is fed into open, cylindrical rotating pans about 0.9 m in diameter and 1.2 m deep (Figs. 28, 29). During rotation the starch grains are forced to adhere together in the form of small particles or beads. The resulting product depends on the speed and the length of time of rotation.

After gangsoring, beads of the right size are sorted out by screening between plates with circular holes corresponding to the required dimensions (Fig. 30).

Gelatinization

In gelatinizing, starch undergoes a radical alteration in molecular arrangement, with a concomitant change in properties. From a practically insoluble product of semicrystalline structure it becomes an amorphous substance, miscible with water in any proportions at sufficiently high temperatures, giving viscous solutions which after cooling set to a semisolid elastic mass: a jelly, or gel.

This process may be brought about by the action of chemicals or by heating in an aqueous medium; only the latter case is of interest here. The onset of gelatinization is characterized by a loss of granular, structure which also promotes swelling; both processes can easily be followed under a microscope. With cassava starch, gelatinization sets in at about 60C, and the process is completed at about 80C. The point of gelatinization depends to a certain extent on granule size, the smaller granules being more resistant to swelling.

In the manufacture of baked products, the treatment is kept at a moderate temperature so as to cause gelatinization only in the surface layer of the lumps of moist starch. The product obtained therefore consists of agglomerations of practically raw starch enclosed by a thin layer of the tough and coherent gelatinized form.

For flakes, gelatinization is performed in shallow pans about 60-90 cm in diameter and 20-25 cm deep, having the profile of spherical segments, which are placed in holes on a brick oven and heated on a moderate fire. In order to prevent burning the starch, and perhaps also as an aid in achieving an end product of the desired lustre, the pans are wiped beforehand with a towel soaked in an edible oil or fat. Shorea (tenkawang fat) or Bassia (illipe fat), having properties approaching those of cocoa butter, seem to be preferable for the purpose, but groundout oil is used as well. Furthermore, it is necessary to rake the mass continuously with large forks, both to prevent burning and to ensure uniform gelatinization. From time to time a sample of the flakes is tested for toughness until proper consistency is attained.

The hand-baking process can also be applied in the manufacture of pearls and seeds, but rather irregularly shaped beads are obtained, inferior in colour and in other qualities.

Better mechanical methods for obtaining a first-rate product have long been known. In one of these, gelatinization is performed with the direct application of steam. The starch beads are poured onto plates in a rather thick layer, the plates forming a conveyor belt which is slowly drawn through a tunnel charged with steam. In this way, uniform gelatinization is ensured.

A device widely used in Indonesia (Java), which combines the advantages of several other methods, consists of a hollow cylinder revolving on rollers and driven by motor via a suitable transmission, all resting on a foundation which at the same time serves as a hearth with fire-holes. Flanges on the rollers hold the revolving drum, which is inclined at an angle of about 10. The raw beads are poured into a gutter at the higher end of the drum at such a rate that they spread into a single layer by the time they reach the hotter parts of the inner surface of the drum. The width of this flow of beads need not exceed 15 cm if the drum has a diameter of 80 cm and revolves at 8-10 rotations per minute. A suitable length for the drum is 4 m. In rolling down, each bead covers the same long, screw-shaped path across the inner drum surface, and in the hotter regions a gradual gelatinization sets in, the rotation of the drum preventing overheating. By the rolling movement, moreover, the surface of each bead is uniformly gelatinized and at the same time becomes perfectly spherical in form.

Drying

The gelatinization process in the hand-worked flakes changes the moisture content of the product by no more than a few percent, and the same applies to the steam-treated pearls and seeds. In the drum described above, drying sets in parallel with the gelatinization and may be promoted by ventilating the drum, but the removal of water here is also incomplete.

Thus, in general, a final drying after gelatinization is necessary in order to bring down the moisture content to the desired level of about 12 percent. Drying in this case is best accomplished in chamber driers of the circulating type. For instance, in a chamber drier for pearls and seeds the initial temperature should not exceed 40"C lest further gelatinization and bursting of the beads set in. Toward the end of the treatment the temperature may be raised to 60-70C. With efficient exhausters, drying may be completed in 1 1/2-2 hours. Normally, from 16 tons of moist starch, 10 tons of the dried product are obtained.


4. Cassava products for animal feeding

Cassava products have long been used for animal feeding. Large quantities of cassava roots and cassava waste are utilized in the cassava-producing countries for this purpose. Imports of dried cassava roots and meal into European markets for the supply of the compound feed industry are also increasing.

Chips

This is the most common form in which dried cassava roots are marketed and most exporting countries produce them. The chips are dried irregular slices of roots which vary in size but should not exceed 5 cm in length, so that they can be stored in silos. They are produced extensively in Thailand, Malaysia, Indonesia and some parts of Africa.

PROCESSING CASSAVA C HIPS

The present method of processing chips in Thailand, Malaysia and some other countries is very simple, consisting in mechanically slicing the cassava roots and then sun drying the slices. The recovery rate of chips from roots is about 20-40 percent. However, the products are considered inferior in quality by some quality-conscious feedstuff manufacturers, although many others consider them satisfactory.

Preparation of the roots

When the roots are not sorted, peeled and washed, the chips are usually brown in colour and have a high content of fibre sand and foreign objects as well as hydrocyanic acid. Trimming, peeling and washing the roots in a similar manner as for the processing of cassava flour are recommended in order to produce white chips of superior quality.

Slicing or shredding

The roots are shredded in a special machine, which is usually made locally. The machine consists of a rotating notched cutting disk or knife blades mounted on a wooden frame equipped with a hopper as shown in Figure 31. The cassava roots are cut into thin slices and pieces as they pass through the machine.

Drying

Sun drying is used mostly where the sliced roots are spread out on drying areas, or concrete floors of various dimensions. Experiments in Madagascar showed that the concentration of chips during drying should not exceed 10-15 kg/m2, the required drying area space being about 250 m2 for each ton per day of dried roots produced.

To produce good quality chips the roots must be sliced and dried as quickly as possible after harvest. The chips should be turned periodically in the drying period, usually two or three sunny days, until the moisture content reaches 1315 percent. The chips are considered dry when they are easily broken but too hard to be crumbled by hand. The thickness of the slices also has an effect on the quality of chips. Thick slices may appear dry on the surface when their internal moisture content is still high.

When rain threatens during the drying process, the chips are collected by hand or by a tractor into piles under a small roof. Interrupted sun drying affects the quality of the finished chips and pellets. When the semidried chips are wet again by rain, they become soggy and upon completion of drying lose their firm texture. In rainy regions, where continuous sun drying is difficult, some form of artificial heat drying is required.

Broken roots

Similar to chips in appearance, but generally thicker and longer, they are often 12-15 cm long and can jam the mechanism of handling equipment. They are produced mainly in Africa where local processors prefer to produce longer roots because of the domestic demand mainly for products suitable for human consumption, as cassava is part of the staple diet. Once processed into chips the product becomes inedible, and the producer wants to conserve the local market.

Pellets

The pellets are obtained from dried and broken roots by grinding and hardening into a cylindrical shape. The cylinders are about 2-3 cm long and about 0.4-0.8 cm in diameter and are uniform in appearance and texture.

The production of pelleted chips has recently been increasing as they meet a ready demand on the European markets. They have the following advantages over chips: quality is more uniform; they occupy 25-30 percent less space than chips, thus reducing the cost of transport and storage; handling charges for loading and unloading are also cheaper; they usually reach their destination sound and undamaged, while a great part of a cargo of sliced chips is damaged in long-distance shipment because of sweating and heating.

Pellets are produced by feeding dried chips into the pelleting machine, after which they are screened and bagged for export. The powdered chips which fall down during pelleting are re-pressed into pellets and the process is repeated. There is usually about 2-3 percent loss of weight during the process.

Meal

This product is the powdered residue of the chips and roots after processing to extract edible starch. It is generally inferior in quality to chips, pellets and broken roots, has a lower starch content and usually contains more sand. The use of cassava meal in the European Economic Community has declined with a shift to the other cassava products during the last few years. However, there will remain some demand for this product, especially by smallscale farmers who produce their own feedstuffs. since it does not require grinding and thus can be readily mixed with other ingredients.

Residual pulp

During the processing of cassava flour, the residual pulp which is separated from the starch in the screening process is used as an animal feed. It is usually utilized wet (75-80 percent moisture content) in the neighbourhood of the processing factory but is sometimes sun dried before it is sold. This product is considered a by-product of the cassava starch industry and represents about 10 percent by weight of the cassava roots.

The approximate analysis of this product (dry matter) is as follows:

  Percent
Protein 5.3
Starch 56.0
Fat 0.1
Ash 2.7
Fibre 35.9
TOTAL 100.0

5. Cassava starch factories

The profitability of any cassava factory depends primarily on the following conditions:

(a) year-round availability of cassava roots of the desired quality in sufficient quantity;
(b) presence of abundant water with the needed qualities;
(c) reliable power supply;
(d) transportation facilities both for the roots and the end products;
(e) availability of capital and labour.

Small and medium-size factories are more frequently found in rural regions with a rather dense agrarian population, numerous streams, and at least one highway to a not too distant commercial centre.

Power

In the small and medium-size factories the only processes consuming a considerable amount of energy are rasping of the roots and, where a bolting installation is present, crushing of the crude dry flour. At the lowest production levels the manufacture can, therefore, be effected entirely by hand; the larger rural mills, however, have recourse to running water as the chief source of power

A rasper and eventually a rotating screen can be driven by a simple waterwheel about I m in diameter, constructed of hardwood and revolving on an iron shaft. The mill is set up preferably near a riverside or brook. At some point upstream, water is led off into a channel of suitable size. The amount of water running in the channel before reaching the waterwheel is regulated by the operation of lock gates.

Above a certain production level, depending on various factors, the energy consumed by the rasper, rotating screen, disintegrators (in bolting installations) and accessory equipment (such as pumps) is such that it is more advantageous to employ a diesel engine or an electric motor. In modern factories located near cities, power for industrial purposes can usually be purchased from the local power station at reduced rates. In the factory, a small stand-by engine generator is recommended for use in the event of power failure.

Water

Apart from its use as a source of power, the availability of ample pure water is of the utmost importance in processing cassava flour. During the greater part of the process, the starch granules are in contact with water which, besides the soluble constituents of the roots, contains all the substances originating from the water added in wet-screening of the pulp and in sedimentation. The deleterious effect of crude suspended matter in the water used (turbidity from clay, etc.) will be obvious. Moreover, starch in its natural state acts as a moderately strong absorbent of electrolytes and colloidal matter in solution. As a result, any ions in the water, even if present in small concentrations, are apt to be accumulated in the granules, thus influencing the outward appearance and the physicochemical properties of the flour.

Iron ions have a particularly bad effect in this respect because, apart from being strongly adsorbed, they tend to fix hydrocyanic acid, a normal component of cassava, in the form of dark-coloured compounds. In larger factories specializing in the production of first-rate flours, therefore, even the use of iron in piping and other equipment should be avoided where contact with the flour milk is possible.

Smaller factories, as a rule, will resort to spring water for processing, on account of its greater purity as compared with river water. Not infrequently, pure water is obtained from springs in the neighbourhood of rivers or artesian wells, so that both kinds of water can be used together. River water, crude or cleared in a sedimentation tank, is used for washing the peeled roots; and spring water or artesian water, which needs little filtration, is used in contact with the flour in processing. If only river water is available, it may be used after sufficient purification.

The daily consumption of water for processing required by small rural mills is no more than a few cubic metres. A simple pit in which river water is left standing may suffice to obtain pure water.

An improved system of water purification is illustrated in Figures 32 and 33. River water enters one of the two cement cisterns communicating at the base. The water in the second tank rises slowly through a bed of filtering material - for instance, some sprigs covered with a layer of pebbles. A very efficient filtering bed is obtained from a material available in most tropical regions - the fibre from the leaf-sheath of the sugar palm (Arenga saccharifera), in Java called injuk or indjuk. On account of its peculiar texture this material is not easily clogged and retains most suspended matter. However, in filtering, ordinary sand may serve equally well.

WATER PURIFICATION INSTALLATION FOR CASSAVA PROCESSING (Plan)

WATER PURIFICATION INSTALLATION FOR CASSAVA PROCESSING Vertical cross section.

A small-scale factory using sedimentation tanks for its flour milk consumes about 1.5 m3 of water for 100 kg of fresh roots; hence, the tank should be able to provide the necessary water for a production level of about 2 tons of dry flour per day.

Purification may be aided by chemical means - for example, by the addition of a little aluminium sulfate (alum) - but this is not a widespread practice. A particularly useful and economical aid for cleansing the process water is the cultivation of certain floating water-plants in the purification tanks, principally the following tropical species: Eichornia crassipes, Utricularia spp., Salvinia auriculata and S. cacculata. Suspended clay and other material collect on the hairy roots of these plants. Application is not restricted to small-scale factories; large factories often have their vast purification tanks covered with this kind of vegetation. The ion exchange process is applied in some modern factories to reduce the mineral content of water used for the purification of starch as well as for steam boilers.

A suitable outlet must exist also for the resulting waste water of the factory. Very often this waste water is not allowed to drain off into the public sewage system without purification.

Types of factories

Cassava processing can perhaps best be outlined in the form of the layouts of or other data on existing factories belonging to each of the three production levels (small, medium, and large) which have been taken as a basis for classification.

LAYOUT OF A SMALL MILL

In such mills the work is performed entirely with simple hand-driven tools or at the most a waterwheel as a source of power. These factories, as a rule, do not produce more than 200 kg of crude, unbolted flour a day; if run by a family, the daily production is generally not more than 100-120 kg.

A small factory with a daily output of about 200 kg of dry flour functions as follows. Water is drawn off from a brook, dammed up for the purpose, by the channel leading to a waterwheel. The rotation of the wheel is transmitted via the flywheel and a belt to the rasper which is mounted in the rasping table with seating bench. The roots are peeled and dumped into the basin, where they are washed with clean water from a feed pipe, after which they are transferred to the rasper. The pulp obtained by rasping is transferred to the washing basins, where it is washed thoroughly with spring water or purified river water. The flour milk runs into the settling tanks.

After settling, the fruit liquor is let off through a drain, joining the wash water from the roots and the water from the channel on its way to the river. The moist flour is conveniently dried near the factory on racks in the open, and the packing of the crude dry flour and other related work are performed in a small shed. The waste pulp is worked up in factories like this one; it is dried in the sun and sold to a bolting factory together with the crude flour.

LAYOUT OF A MEDIUM-SIZE FACTORY

In these factories the installation of an electromotor or diesel engine of about 20 hp raises the production capacity to a level of about 5 tons a day, principally on account of more efficient rasping. The other operations also change somewhat in character as compared with the small-milL methods, but they are the same in principle and little skilled labour is needed. The power supply mentioned above is sufficient to drive one mechanical rasper. In many instances, however, the factory includes a bolting installation, which, in general, is driven alternately with the rasper, and the factory produces an assortment of finished flours: in this case a somewhat larger power supply (at least 25 hp) is necessary. Factories of this kind are very suitable for rural areas where unskilled labour is comparatively cheap but technical equipment and skill are difficult to procure.

Both small mills and medium-size factories. in general, have to buy their roots from landowners in their neighbourhood. On account of many economic and social factors the supply often lacks stability and continuity. Consequently, the possibility of production planning is slight, and this is perhaps the most important factor limiting the size and output of such factories. In areas where farmers or farmer organizations have more advanced ideas, where they are commercially minded and combine in rural industrial enterprises to process their own agricultural product, the supply of roots can be organized to the great economic benefit of all concerned.

Figures 34 and 35 show the main elements of a typical medium-size factory with a capacity of 2-3 tons of dry flour per day. The arrangement and dimensions, given m centImetres, are those recommended by an expert with long practical experience. Figure 34 shows vertical sections of the arrangement through the axis of the rotating screen (above) and perpendicular to this axis (below). The peeled roots are stored in basin A, washed in basin B, and transferred from the latter basin to the rasper (C), mounted on a rasping table (L in Fig. 35). A 20-hp diesel engine (H) coupled to the pump (G), which supplies water from the well (F), drives both the rasper (at 800 rev/min) and the rotating screen (at 120 rev/min) via the transmission gear (1). The flour milk passes from the rotating screen (D) to the flour table (E), which has a slope of about I percent.

FIGURE 34. MAIN ELEMENTS OF A TYPICAL MEDIUM-SIZE CASSAVA-PROCESSING FACTORY(Vertical sections).

FIGURE 35. MAIN ELEMENTS OF A TYPICAL MEDIUM-SIZE CASSAVA-PROCESSING FACTORY (Horizontal sections).

LARGE OR ESTATE FACTORIES

By starting with a sufficiently large investment of capital, it is possible to overcome the limitations mentioned above and reach at once production of the order of 40 tons of dry flour a day. Manufacture at this level presupposes that a continuous sale is secured with the dextrin industry, one of the industries using cassava starch. Supplying cassava for specific industrial purposes, however, in turn places definite demands on the flour mills, which can be summed up as the demand for a regular supply of an assortment of flour of specific and constant quality. Clearly, this demand will be met only when the factory can rely on adequate raw material - roots - from its own extensive plantations where a selected strain of cassava is grown. On this level only, appropriate machinery for purification and more elaborate techniques are coming into their own, to save labour, minimize losses, and so process more economically.

Division into three classes of factories is, of course, arbitrary: medium-size factories may have fairly modern machinery, such as centrifuges for the preliminary drying of the flour, whereas a much larger factory may be limited to rather out-of-date methods of drying. Still, as a rule, each operation in processing the flour is carried out in a form characteristic of the particular class in the above classification of factories.

The processing operations in different types of factories are illustrated in the following flow sheets and diagrams. Figure 36 shows an example of the operations used in a small to medium-size cassava starch factory in Malaysia. The equipment and methods of manufacture are mostly old-fashioned. Figure 37 shows an example of the processing operations in a medium- to large-size factory in Thailand. Most of the equipment is modern and the production is mostly prepared for export. Figure 38 shows a diagram of the operations of a large factory with modern equipment proposed for Nigeria.

FIGURE 36. Flow diagram of operations in an old-fashioned small to medium-size processing factory.

FIGURE 37. Flow diagram of operations in a modern medium to farce processing factory.

FlGURE 38. Flow diagram of operations proposed for a large modern processing factory.


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