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IMPORTANCE OF QUICK PROCESSING
In the processing of cassava starch it is vital to complete the whole process within the shortest time possible, since as soon as the roots have been dug up, as well as during each of the subsequent stages of manufacture, enzymatic processes are apt to develop with a deteriorating effect on the quality of the end product. This calls for a well-organized supply of roots within relatively short distances of the processing plant and, furthermore, for an organization of the stages of processing that will minimize delays in manufacture. Thus, while simple in principle, the manufacture of a good cassava flour requires great care.
The roots are normally received from the field as soon as possible after harvest and cannot be stored for more than two days. Since the presence of woody matter or stones may seriously interfere with the rasping process by stoppage or by breaking the blades, the woody ends of the roots are chopped off with sharp knives before the subsequent processing operations.
PEELING AND WASHING
In small and medium-size mills the general practice is to remove the peel (skin and cortex) and to process only the central part of the root, which is of much softer texture. With the relatively primitive apparatus available and limited power, the processing of the whole root would entail difficulties in rasping and in removing dirt, crude fibre and cork particles, whereas comparatively little extra starch would be gained.
The structure of the root permits peeling to proceed smoothly by hand (it is often done by women and children). Work starts in the morning as soon as the roots are brought in; as it must be finished as quickly as possible, numerous hands are needed. The roots are cut longitudinally and transversely to a depth corresponding to the thickness of the peel, which can then be easily removed. Any dirt remaining on the smooth surface of the core of the root can now be washed off without any trouble and the peeled roots deposited in cement basins where they remain immersed in river water until taken out for rasping (Fig. 9). Frequent treading by foot cleans any loosely adhering dirt from the roots.
In the larger factories, whole roots are generally processed. The washing here serves to remove the outer skin of the root as well as the adhering dirt. Provided the root is sufficiently ripe, skin removal may proceed without the use of brushes. Only the outer skin or corky layer is removed, as it is profitable to recover the starch from the cortex. The inner part of the peel represents about 815 percent of the weight of the whole root.
The mechanical washer (Fig. 10) is a perforated cylindrical tank which is immersed in water. A spiral brush propels the roots while they are subjected to vigorous scrubbing in order to remove all dirt. A centrifugal pump is fitted to one end of the machine and connected to a series of jets arranged along the carrying side of the brush. These jets produce a countercurrent to the flow of the roots, ensuring that they receive an efficient washing.
Another efficient washer is a rotary drum with an interior pipe which sprays water on the roots. The drum is either wooden or perforated metal, about 3 to 4 m long and 1 m in diameter, with horizontal openings; it is mounted inside a concrete tank. In some, rotating paddles are fitted along the axis. Washing is done by the action of water sprayed, assisted by the abrasion of the roots both against one another and against the sides of the cylinder or the paddles.
The roots are hand-fed from one end and when they come out at the other they are clean and partially peeled, the action being continuous. Dirty water and skin are periodically drained out through a small opening in the concrete tank.
Some trials in Brazil have attempted the complete peeling of roots for the production of a white starch, and also have used copper, brass or bronze equipment instead of iron, which in contact with wet starch may lead to the production of ferrocyanide (the result of a reaction between iron and hydrocyanic acid), which gives the starch a bluish colour.
In modern factories the roots are pre-washed by soaking in water to separate the coarse dirt and then passed through a combined unit for washing and peeling as described above.
RASPING OR PULPING
It is necessary to rupture all cell walls in order to release the starch granules. This can be done by biochemical or mechanical action. The biochemical method, an old one, allows the roots to ferment to a certain stage; then they are pounded to a pulp and the starch is washed from the pulp with water. This method does not give complete yields and the quality of the resulting starch is inferior. Mechanical action is carried out by slicing the roots and then rasping, grating or crushing them, which tears the flesh into a fine pulp.
By pressing the roots against a swiftly moving surface provided with sharp protrusions, the cell walls are torn up and the whole of the root is turned into a mass in which the greater part, but not all, of the starch granules is released. The percentage of starch set free is called the rasping effect. Its value after one rasping may vary between 70 and 90 percent: the efficiency of the rasping operation therefore determines to a large extent the overall yield of starch in the processing. It is difficult to remove all the starch, even with efficient rasping devices, in a single operation. Therefore, the pulp is sometimes subjected to a second rasping process after screening. The rasping is carried out in different ways with varying efficiency.
Hand and mechanical rasping
On very small holdings in some cassava-growing regions the roots are still rasped by hand on bamboo mats. Where daily production amounts to several hundred kilograms of flour, simple mechanical implements are used.
A simple but effective grater is obtained by perforating a sheet of galvanized iron with a nail and then clamping it around a wheel with the sharp protruding rims of the nail openings turned outward. The wheel may be driven by hand, but it is often driven by foot like a tricycle, the worker pressing the roots from above onto the rasping surface: or the rasping surface is attached to one side of a rotating disk equipped with a crank transmission, which is driven by foot. The pulp is collected in baskets or wooden containers to be carried to the sieves.
Hydraulic raspers
Larger water-powered raspers can be used where running water is available. The waterwheel is rotated by a flywheel and driving belts to a pulley on the shaft of the rasping drum. The drum. 20-30 cm in diameter. is either attached to a primitive wooden construction or fitted into a "rasping table." The operator. seated at the table, presses the roots against the drum. The grated mass is forced through a narrow slit between the drum and the shelf before it drops into the trough, whence it is carried to the sieves.
The rasping devices described above are made of perforated inplate. Though inexpensive, they are relatively inefficient as the rasping plate must often be replaced on account of rapid wear.
Engine-driven raspers
Engine-driven raspers are more economical when production rises above a certain level - say, for the handling of 10 tons of fresh roots a day. The most current model is the Jahn rasper. The machine has a rotor of hardwood or drawn steel tube, 50 cm in diameter, with a number of grooves milled longitudinally to take the rasping blades or saws. The number of saw teeth on the blades varies from 10 to 12 per centimetre according to need. The blades are spaced 6-7 mm apart on the rotor.
In simpler versions, the rotor is fitted into a housing in such a way that the rasping surface forms part of the back wall of the receptacle for the roots. Facing the rasping surface, a block or board is inserted which is movable by a lever and turns on an axis near the upper rim of the compartment. By manipulating this buffer the roots are pressed onto the rasping surface, which moves downward in the hopper, and the mass is propelled through a slit in the bottom of the hopper. It is advisable to give the inner surface of the buffer the form of a circular segment corresponding to the section of the rotor exposed so that, at its extreme position inward, the distance between rotor and block is only a few millimetres. This, however, is generally possible only in the all-steel raspers to be described later.
In many medium-size factories, water is run into the hopper during rasping, in order to facilitate crushing and removal of pulp. The drawback of this practice, however, is that relatively large fragments of the roots escape crushing; hence it is not to be recommended from the point of view of effectiveness. It is never applied in well-equipped factories.
In a rasper of the type used in larger factories, the housing is equipped with adjustable breasts with sharp steel edges for the control of rasping fineness. More recent constructions provide for the return to the rasping surfaces of those pieces of the roots which were thrown out sideways. The pulp has to pass a screen-plate with sharp-edged holes or slits, during which it is homogenized to a certain degree and, in fact, undergoes a secondary crushing.
Power consumption during rasping
To obtain the maximum rasping effect, the power supply should be accurately attuned to the constructional details of the rasper, i.e., to the distance between the surface of the rotor and the breasts in the housing.
The energy required to tear up the roots is derived from the momentum of the rotor, a certain minimum of kinetic energy being necessary to obtain any rasping effect. Above a certain rotor speed, however, it is to be expected that no considerable further increase in the rasping effect will be obtained. There is thus an optimum speed for the rasper in conformity with the need for a high rasping effect on the one hand and with the economy of power supply on the other. In this connection it should be remembered that only the linear velocity of the rasping surface counts. In practice it has been found that a rasper of the usual dimensions - a diameter of 40-50 cm and a length of 30-50 cm for the rotor - should be driven at 1000 rotations per minute, corresponding to a linear velocity of the rasping surface of about 25 m per second. The power of the engine required to drive a single rasper of this type is 20-30 hp. In most cases diesel engines are used.
Variations in resistance of the roots to rasping
At or near optimum speed of the rasper, both rasping effect and energy required per I 000 kg of fresh roots still depend a good deal on the kind of roots being treated. Comparative results obtained for six different varieties of cassava, peeled and unpeeled, with an electrically driven experimental rasper reproducing as far as possible the form and working conditions in medium-size factories are given in Table 3. As is to be expected, high rasping effects involve a lower use of energy. The influence of peeling the root is important in both respects.
Secondary rasping or grinding
In view of these results it is no wonder that the rasping effect differs widely in different factories. In modern factories, it may be estimated that an effect of about 85 percent is attained at the first rasping; at these production levels, however, it is economical to submit the pulp to a second crushing process, either in a second rasper or in special mills where the pulp is ground between stones. These mills, however, do not seem to have found much favour with cassava manufacturers.
In a secondary rasper, the indentation of the saw blades should be somewhat finer, about 10 per centimetre (25 to 27 teeth per inch) as compared with about 810 per centimetre (19 to 26 teeth per inch) for the primary rasper. The overall rasping effect is raised to over 90 percent by the secondary rasper.
The differences in output under different rasping techniques are shown by the following figures: in one medium-size factory, using a single Jahn rasper, the capacity was at most three tons of roots per hour; a larger factory, working with primary and secondary raspers, achieved more than double this amount per hour per primary and secondary rasper unit.
SCREENING
In separating the pulp from the free starch a liberal amount of water must be added to the pulp as it is delivered by the rasper, and the resulting suspension stirred vigorously before screening. Mixing with water can be carried out more or less separately from screening, but more often the two operations are combined in "wet screening" - that is' the mass is rinsed with the excess water on a screen which is in continuous motion.
Hand screening
In the smallest mills, screening is done by hand. The rasped root mass is put in batches on a cloth fastened on four poles and hanging like a bag above the drain leading directly to the sedimentation tanks. Spring water or purified river water is run in from a pipe above the bag, and the pulp is vigorously stirred with both hands. Sometimes bamboo basketwork is used to support the screening cloth. The pulp under processing still contains appreciable amounts of starch and therefore has a certain value (e.g., as a cattle fodder): in the small mills it is pressed out by hand, and the lumps obtained are dried on racks in a well-ventilated place.
The rotating screen
A simple form of rotating screen consists of a conical frame of hardwood, fixed on a hollow, horizontal axis, at least 3 m long, covered with ordinary cloth or phosphor-bronze gauze. Phosphor-bronze is often preferred for its durability, but its use necessitates frequent brushing in order to remove clogging pulp particles. The crude pulp is fed into the cone at the narrow end and by the rotation of the screen, at approximately 50 revolutions per minute, slowly moves down to the other end, whence it is conveyed to the pulp tanks. In the meantime, water is sprayed on it under pressure (e.g., 6 atmospheres) from a number of openings in the hollow shaft. Thus, by the time the pulp reaches the lower end of the cone, it is more or less completely washed out. The rotation screen has the advantage of preventing the plugging of the meshes of the sieve with gummy materials ( they tend to agglutinate with the fibre as the screen rotates). The flour milk is caught in a cemented basin stretching out below the screen over its whole length, and from there runs along channels into sedimentation tanks or flour tables.
The screen is mounted close to the rasper and at a somewhat lower level in order to ease the flow of the crude pulp. The washed-out pulp discharged at the lower end of the screen is carried off by some form of conveyor to basins outside the factory. Since its dry matter still consists mostly of starch, this byproduct after drying and pulverizing is marketed as a fodder.
A more elaborate type of rotating screen is equipped with two sets of brushes, one set being arranged to convey the fibre along to the discharge, the other acting as beaters, which at the same time keep the screens clear to allow the starch milk to flow away readily. Both sets of brushes are adjustable, so that excessive wear on the bristles can be taken up and the maximum life obtained from them. The screens are carried in aluminium frames which are removable for changing covers. Up to now a single rotating screen is most generally used in factories of medium capacity. In larger factories, for economic reasons, the starch must be extracted from the whole root as thoroughly as possible with the minimum amount of water. This often implies a more intricate arrangement of the operation of rasping and screening as well as more efficient screening devices.
The shaking screen
In large factories the rotating screen is replaced by the shaking screen. It consists of a slightly inclined, horizontal frame, 4 m in length and covered with gauze, which is put into a lengthwise shaking motion in short strokes by means of an eccentric rod. The fresh pulp, after being mixed with water in distribution tanks, is conducted by pipes to the higher end of the screen; during screening, the pulp remaining on top of the screen is slowly pushed downward by the shaking motion.
It is advantageous to let the suspensions pass a series of shaking screens of increasing fineness (80-, 150-, and 260-mesh), the first one retaining the coarse pulp, the others the fine particles. The pulp remaining on the first of these screens is often subjected to a second rasping or milling operation and then returned to the screening station.
Another means of increasing efficiency is to perform the combined operations of screening and washing the pulp in two stages. In the first stage' the pulp is vigorously stirred with water in a washer provided with coarse screens at the bottom and with paddles in order to obtain thorough mixing during the transport of the pulp toward the end of the trough. In the second stage, the crude flour milk from these washers is conducted to a shaking screen below, which retains the rest of the fine pulp. The operation is twice repeated with the pulp thus obtained in similar washer-and-screen units, which may be arranged in a battery.
The complete separation of free starch from pulp is achieved here by the countercurrent principle. In the third (lower) washer, the pulp from a first rasping is washed out with flour milk from the second washer-and-screen unit. The pulp from this first treatment passes a secondary rasper, whence it is conveyed to the washer, where it is rinsed with starch milk from the first (upper) washer-and-screen unit. Finally, the pulp is conveyed to the upper washer where fresh water is run in.
Efficient rising of the pulp on the screens is promoted by inserting one or more shallow transverse channels in the surface of the screen, where the strong whirling movements caused by the shaking of the screen effectively loosen the starch granules from the pulp.
Jet extractors
An efficient machine for the separation of starch from cellulose fibre is the jet extractor, or the continuous perforated-basket centrifuge. The starch-pulp slurry is put in a conical basket and centrifugal action separates the starch dispersion from the fibrous pulp. Jets of water sprayed on the pulp as it travels the length of the cone assure complete recovery of the starch.
The Dorr-Olivier DSM screen
Another type of modern equipment used in the starch industry for the complete separation and washing of fibre is the Dorr-Oliver inclined DSM screen, which consists of a stationary screen housing equipped with a con cave wedge bar-type screen (see Fig. 11). The suspension to be screened is fed tangentially either by gravity or under pressure into the screen-plate and flows in a direction perpendicular to the bars. Each bar of the screen surface slices off a layer of liquid of a thickness approximately one fourth the slot width. Different types of screens, with slot widths ranging from 50 up to 3 mm, are used in the starch industry.
After rasping, the starch-pulp slurry flows down the DSM screen by gravity and the pulp and starch are separated. As many as four screens are operated in series to assure that the starch dispersion is completely separated from the pulp. The pulp from one screen is discharged into a basin, redispersed with dilution water, and pumped to the succeeding screen.
SETTLING AND PURIFICATION OF STARCH
The term "settlings' as used here includes the whole series of operations for separating the pure starch from soluble contaminants. The quality of the flour produced depends to a great extent on the proper performance of these operations, which comprise settling in successive tanks, settling on flour tables, and the action of modern separators. Each operation can be used alone or carried out in different combinations. They all result in a more or less concentrated suspension of starch in pure water.
Duration of settling process and quality of product
As has already been stated, the entire processing of cassava must be completed within as short a time as possible. This is particularly true of the separation of the free starch from its suspension in the so-called fruit-water - the watery part of crude starch milk - because of the very rapid chemical changes in this solution (the formation of very stable complexes between starch and proteins, fatty material and so on). As it is almost impossible to separate the pure starch from these complexes, the value of the flour for many purposes is seriously lowered by those processes.
At a later stage - the fruit-water being rather rich in sugars and other nutrients - microorganisms start to develop and eventually lead to a vigorous fermentation. Alcohols and organic acids are produced, among which butyric acid is particularly noticeable on account of its odour. These biochemical changes exert a negative influence on the quality of the flour similar to the foregoing physicochemical ones. It is all but impossible to prevent the formation of this acid in the processing of cassava; traces of it are discernible even in very good brands of the finished flour. Indeed small rural mills can often be located by the smell of butyric acid.
As a consequence of the necessity for speed, the technique of settling has developed rationally from the simple settling tank to the settling table, with a considerable reduction in the time of contact between starch and fruit water. In modern processing methods the whole period between rasping and drying is reduced to about one hour.
Before the different methods of settling are discussed in detail. a few of the fundamental facts on sedimentation should be discussed.
Settling and granule size
Besides this. however. other factors. such as the pH of the medium and its content of protein and other colloidal matter. through the corresponding changes in colloidal state. have some influence on the rate of sedimentation and especial on the consistency of the settled flour.
The diameter of cassava starch granules ranges between 4 and 24 microns: thus. a gradation according to granule size has to he expected in successively deposited layers of sediment. This gradation will sharpen with the length of the path of sedimentation from the initially mixed suspension. Therefore. during tank sedimentation the lower layers will contain granules of a wide range of sizes, settled during the first stages of filling the tanks with the crude starch milk. The gradation mentioned will only be noticeable if the tanks are filled entirely. The size distribution found experimentally in a sediment of 30-cm thickness after 24 hours of settling hears out these expectations. as shown in Table 4.
It is seen that gradation by particle size sets in above one third the height of sediment. The standard deviation in the top layer is relatively large. It can be seen that this layer contains many granules of all sizes. which under the microscope have a corroded appearance. Moreover. one finds a certain proportion of fine cellulosic debris. precipitated proteinous and other organic matter.
In sedimentation on flour tables a corresponding gradation may he expected. in this case with respect to the distance from the head of the table.
Centrifugal separators, however, will produce a uniform mixture of granules of all sizes which occur in the starting material.
Settling in tanks
This is the oldest method, and, indeed, tanks are the obvious means at low production levels in small rural mills. In very small mills, wooden barrels or troughs serve the purpose, but as soon as the production reaches several hundred kilograms of flour per day it is usual to construct tanks of cemented brickwork sunk into the ground. Rectangular and round tanks or basins are used in the Far East for the settling of starch as well as for the washing and the purification of the settled starch, as shown in Figure 12. The dimensions and the number of tanks are determined by level of production and convenience of handling (e.g., 2 x 4 m in surface dimensions and 0.5 to I m in depth).
It is essential that the flour does not remain in contact with cement or masonry any longer than necessary, as this has a notably deteriorating influence on the quality of the flour. Therefore. the bottom of the tanks is covered with wood of a kind which is proof against the prolonged action of the slurry and does not react on the flour. A wooden skirting, moreover, is fitted on the walls to a height of, say, 10-15 cm so that the whole mass of flour contained in a tank full of starch milk will settle against a wooden surface. The lining may be of tiles rather than wood. Holes provided with stoppers are fitted into the walls, preferably at different heights, to let off the supernatant, or excess, liquid after settling, one hole just above the floor being used for the purpose of cleaning the tank between settlings.
During this process a number of tanks are usually filled in succession, the flow of starch milk being conducted to the next tank, after the previous one has been filled up, by means of checks placed in the channels. Settling takes at least six hours; thus, after rasping which is carried out early in the morning, the supernatant liquid is let off in the afternoon. However, rasping is often carried out late in the morning, and in that case the flour is left to settle overnight, up to 20 hours or more. Though settling is more complete in this case, the action of enzymes and microorganisms may also have progressed.
The fruit-water is now let off by removing the stoppers from the holes, beginning with the upper ones, thus reducing turbulence as far as possible. Notwithstanding this, in drawing off the last of the supernatant liquid, appreciable amounts of the lighter starch fractions in the upper layers of the sediment go with it; as in general, the drain waters are not processed in these small mills' they constitute a loss, which together with the starch originally left in suspension may be estimated at 5-10 percent of the flour produced.
The upper layer of sedimented flour, which has a yellowish green tint, contains many impurities and is generally scraped off and rejected. The remaining moist flour is then stirred up with water and left to settle again. In most cases, two settlings suffice to obtain a reasonably clean flour. In larger factories producing flours for special purposes, settling may be repeated several times with or without the addition of chemicals.
In large factories of medium size a great step forward in the settling operation has been taken through the replacement of the settling tanks by flour tables or basins. Because of the space it occupies, a table is generally practical only in medium-size and larger factories.
The flour table is a shallow channel, some 50 m long, about 30 cm deep and of a width varying with the amount of starch to be worked up daily. The bottom is covered with wood or tiles, as described in the previous section, and in principle should be horizontal, though it is sometimes given a slight inclination, say of 1 cm per metre.
The flour milk enters at one end, preferably from a compartment of the table itself, occupying over one half metre of its length and separated by a silt about 20 cm high, which ensures a uniform overflow over the whole width of the table. The liquid drawn off at the end of the table should be substantially free of starch and is thus rejected.
In settling on a table, the sedimentation path of the starch granules, which is vertical in the case of settling in tanks, will be drawn out into oblique lines on account of the horizontal movement of the slurry. The longer the time needed by any particle to pass from its position in the suspension down to the bottom, the further its ultimate place in the sediment will be from the head of the table. The stratification obtained in settling tanks is therefore in the present case partly converted into a differentiation as to granule size over the length of the table. Hence, floating or very slowly deposited fibre and dirt particles, including protein, will be removed at the end of the table, and the flour settling on the higher parts will contain a larger proportion of the larger starch granules and very little of the protein and other contaminants. Thus, the sediment on these parts of the table constitutes, generally, a better grade of flour, the rest being worked up as a lower grade. As the time of settling for all particles is much shorter than in settling tanks, the contact of the starch with the fruit-water is likewise considerably reduced.
As settling is most copious at the upper end of the table and slowly falls off toward the far end, the sediment soon shows the effect of an inclination of the table itself. As the working proceeds, the movement of the slurry is therefore accelerated on the upper end, tending to accentuate the difference in quality.
The flour table acts most efficiently if filled to maximum capacity. Some factories which must work up different quantities from day to day have flour tables of varying width - say, 2, 3 and 4 m.
The advantages of the flour table over the settling tank may be summed up as follows:
1. The time of contact of the flour with the fruit-water is shortened.
2. The starch settled on different parts of the table is differentiated according to purity and granule size, thus enabling the manufacturer to produce simultaneously and without extra cost at least two brands of different quality.
3. Losses of fine starch are far less because the sedimentation path is much shorter and drainage proceeds at a minimum rate.
Influence of chemicals on settling and the proprieters of the product
It may therefore be surmised that' apart from the rate of settling, the right consistency of sediment is important in achieving an efficient separation of the starch from the fruit-water. The starch losses incurred with draining in settling tanks will decrease as the starch settles to a firm cake, and even the efficiency of tabling will depend partly on the compactness of the sediment.
Pure starch settles in clean water to a compact mass of peculiar mechanical properties. If suddenly broken up (e.g., with a scoop). it crumbles like a brittle substance: but as soon as the forces causing deformation relax' it loses all form and spreads out like a thick syrup (melting. as it were, on the scoop). This phenomenon. termed dilatancy is explained by imagining the granules in the sediment, when at rest piled up on one another in the most space-saving manner' whereas any disturbance of this array by external forces results in an increase of the interstitial volume accompanied by a " drying up" of the cake. The same factors may give the dry flour its "crunchy" property.
The volume of the sediment and its compactness depend very much on the presence of impurities, such as fibre, which tend to result in a softer sediment. Apart from this, it has been found that the composition and the reaction of the ambient solution have an important influence on settling. An acid reaction promotes rapid settling and a compact sediment; an alkaline reaction has the opposite effect.
As in many medium-size factories chemicals are added for various purposes before settling, it seems worth while to review in some detail the effect of the substances most often applied, both on the consistency of the sediment and on the properties of the product.
It should, however' be emphasized that there is little sense in adding chemical aids where the basic conditions for the production of a high-quality flour are not fulfilled. in particular if clean working is not put first and foremost. On this condition there is no doubt that a flour of prime quality can be produced without the use of any chemicals. Moreover. because of the danger of misapplication. these additions are not to be recommended without the expert supervision as a rule available only in large factories.
Sulfuric acid In many instances this acid, which is added as an aid to sedimentation. results in a product of enhanced whiteness. The effect on sedimentation is noticeable at concentrations above 0.001 ml of the concentrated acid (specific gravity 1.84) per litre of starch of 2" Brix. (Degrees Brix are about proportional to the grams of flour per litre.) Addition of ten times the quantity causes very rapid sedimentation, but a rather soft sediment is obtained. The effect of this chemical in lowering the viscosity of the product is already appreciable at very small concentrations. Up to about 0.001 ml per litre of starch milk there is a slight increase in viscosity; at higher concentrations a marked decrease. The latter effect is a disadvantage in most applications of the flour; however, it is less so in the manufacture of baked products as whiteness is all-important.
Great care should be taken in adding this chemical, which should only be used in diluted form, prepared beforehand, and thereafter removed by one or more subsequent settlings in pure water.
Alum (aluminium sulfate). The presence of alum in the starch milk may be the consequence of the addition of a surplus of this chemical in the purifcation of the water used. It has a favourable effect on sedimentation, and also enhances the viscosity of the flour, an addition of 0.1 g per lire of starch milk of 2" Brix resulting in an increase of about 50 percent in viscosity.
Sulfur dioxide (sulfurous acid). The addition of sulfur dioxide is a common practice in the manufacture of most grain starches (e.g.' maize starch). It probably helps to separate the starch from the other substances to which it is more or less firmly bound in its protoplasmic state. Furthermore, it keeps bacterial and enzymatic action within bounds. Sulfur dioxide also acts as a bleaching agent, although the white colour thus obtained soon deteriorates. The acid produces a lowering of the viscosity of the product, especially after prolonged action, but e single settling from water containing the usual concentration of 0.3 to 0.4 g per litre followed by settling in pure water has no serious effect.
It is questionable whether the use of sulfur dioxide is advantageous in the processing of root starches and in particular of cassava. In any case, the acid should be applied with great caution and thereafter carefully washed out by subsequent settlings in pure water.
Chlorine. The addition of active chlorine in its different forms (the element itself, chloride of lime or one of the various commercial hypochlorites) considerably augments the viscosity of the product' provided the concentration is kept low - about I mg per hire of starch milk. At that concentration it acts favourably on sedimentation, while its disinfecting and bleaching properties are also very marked, the sediment obtained being compact and white. Higher concentrations of about 50 mg per litre result in a very soft and discoloured sediment and a product of very low viscosity. These properties of active chlorine preparations make them the very best means of obtaining an end product of better quality.
Sedimentation processes
Up to the Second World War, the sedimentation processes used in large cassava factories usually consisted of refinement in tanks and on tables. Separation by centrifuging, though practiced in the starch industries using potato and maize as raw material, does not seem to have found wide application with cassava during this period. Since then, more efficient centrifugal processes for the separation and cleaning of starch in general have been devised. Although originally designed for the processing of potato and maize starch, both machines and centrifuges of a more conventional type are now beginning to be applied in the cassava industry, and it may be expected that at the higher production levels they will soon supersede other methods.
Sedimentation on one flour table is not usually sufficient to effect a complete separation of pure starch from slurry. One obvious defect of both tables and sedimentation tanks is that they do not separate contaminating particles heavier than starch (sand, clay). In the large factories, when producing a high-grade flour the product is collected, after a first tabling, in containers with conical bottoms, where it is stirred moderately with fresh water. Heavy particles settle in the lower part of these stirring tanks and can be discharged from time to time from a tap in the bottom. The flour milk obtained is then pumped to a second table or set of parallel tables, where settling takes place. To prevent any reaction between the flour milk and the wall material, these channels may be coated with a resistant material such as aluminium.
The action of this second tabling operation is different when the table is inclined. Apart from settling in the channels, the more rapid motion of the liquid subjects the underlying sediment of flour to silting - that is, the starch granules and other particles, even after settling, are carried along with the stream to be deposited farther on.
The drag exerted by the stream on a body at the bottom increases with its dimensions. Therefore, the more voluminous fibre particles, which on account of their specific gravity might settle on the higher parts of a table, will be swept down to the lower end by silting. Silting thus supplements the purification obtained by the flour table; in addition' it tends to homogenize the settled starch mass.
Concentration of flour milk in all sedimentation processes has definite limits. In particular, during tabling the suspension should contain no more than 25-30 g of starch per litre. Higher concentrations will result in an undesirable lengthening of the sedimentation time. In silting. higher concentrations, up to 250 g per litre, are allowed.
The supervision of the concentration is best carried out by measuring the density of the slurry with hydrometers. It is usually expressed in degrees Brix, a standard taken over from the sugar industry (grams of sucrose per litre at 17ºC The relation between the latter quantity. specific gravity, and hydrometer readings according to Brix and Baumé for different starch suspensions at room temperature.
Lamellators
The principle of cutting down the settling distance of the starch granules, as achieved by the use of a flour table. is followed also in the construction of lamellators: oblique plates (lamellae) of glass or metal (for cassava only copper can he utilized) are fitted radially into the upper part of conical tanks, the lower part being provided with a stirring device and a tap. The flour milk enters the centre of the upper part and from there flows radially and at low velocity through the spaces between the lamellae and over the outer rim of the cone.
The path of free sedimentation of the starch granules is limited here to the vertical distance between two adjacent plates, which amounts to a few centimetres only, after which they roll down more rapidly along the surface of the plates into the lower part of the cone. The larger granules will thus sink in the central part of the apparatus and will collect at the bottom of the cone; the finer grains will settle at the periphery and collect on the conical wall. As small granules slip along an inclined surface faster than large ones, clogging on the walls is minimized. Clogging of the flour in the spaces between the lamellae is prevented by their radial arrangement, each interspace widening in the direction of flow of the suspension.
Centrifugal methods
A rapid separation of starch grains from fruit liquor and the elimination of the impurities in colloidal suspension are attained by centrifuging, with consequent improvement in quality of the finished product. Centrifuging cannot, however, replace entirely the gravity settling operation: after centrifuging? the starch still has to be freed from any remaining solid impurities by settling in tanks or on tables.
One of the current conventional types of centrifugal separator consists mainly of a horizontal imperforate drum or bowl (Figs. 13, 14) with a continuous spiral-ribbon starch remover or scraping device inside (A, D). The drum rotates in a frame with bearings at both ends. Over a gearbox, the drum and the scraper are driven at slightly different speeds by a direct-coupled motor. The starch milk enters the slightly conical drum at the narrow end (B) and passes to the other end. where the liquid outlet (E) is located. On its way through the howl. the milk throws off starch grains and other solid matter' which concentrate at the periphery. Here the concentrate is taken up by the scraper and brought counter-current to the narrow end. where it is discarged (c) with the addition of fresh watter The purest starch is made by using liberal amounts of soft water. Hard water (high in lime content) has been known to leave calcium oxalate in the finished product.
FIGURE 14. Longitudinal section of a centrifugal separator
The rapid displacement of fruit liquor by fresh water has been brought to a certain degree of perfection in machines known as concentrators.
The current type of concentrator illustrated in Figures 15 and 16 consists of a separator bow with a double wall which turns on a hollow spindle (i). The starch slurry is fed through the inlet (a and b) into the inner howl (C) where it is pressed by centrifugal force onto the inner wall. which is fitted with a number of nozzles of special design. At the same time. water is pumped by a centrifugal pump (K) along the hollow spindle into the water chamber between the inner shell anti the outer howl wall. This wall is provided with similar nozzles located just opposite those in the inner shell. The fresh water from the water chamber enters the nozzles in the inner shell. thus intensively washing the starch coming out of these openings. and the diluted truit-water leaves the apparatus through .1 after passing a set of separating disks and a paring device serving to quench excessive frothing.
FIGURE 15. Cross section of a starch concentrator
The starch' together with fresh water, is pressed through the outer nozzles and leaves the apparatus through e as a concentrated suspension in substantially clean water.
The capacity of the separator depends primarily on the size of the starch granules: the throughput capacity will be lower for fine-grained starches. In fact' some loss of starch with the separated fruit-water is inevitable' because very small starch granules will escape sedimentation under any circumstances. It is claimed that such losses are smaller than in other centrifugal processes. Moreover. the separator consumes less power and its operation is less sensitive to variations in the starch concentration of the treated starch milk' which otherwise often results in clogging. Separators of this type are easy to install and do not need foundations: in operation' however. they require expert supervision.
The action of the separators (concentrators) is completed by rapid batchwise settlings in bowl centrifuges or in purifiers where the purest starch is the first to settle in a thick layer on the bowl wall' followed by strata of starch mixed with FINE fibre ("grey starch"), the fruit-water forming the inner layer. In the older types of centrifuges the operation is discontinued after a few minutes' the water is let off, and the grey starch is removed by washing. The purified starch is then stirred up with fresh water and either drawn off for dewatering or subjected to a second centrifuging.
The newest purifiers have the advantage of performing the above operations while the bowl is in motion. As shown in Figure 17, the bowl or drum turns on a rigid axis and is furnished with a winged feeding chamber. The bowl is fed through a tube (1) The pivot arm (2) carries an agitator (3) and a knife (4) which skims off the fruit-water and scrapes off the grey starch, both of which are discharged through an opening in the bottom of the bowl. The skimmer (5) for purified starch milk is mounted on another pivot. These tools are operated hydraulically and make about a quarter turn from one extreme position to the other.
The operation of a purifier is usually linked directly with that of the above concentrators, the crude starch milk being first washed and concentrated to 16-18°Bé by two concentrators in series.
The starch dispersion is washed with large quantities of water in a series of wooden tubs, settling tanks or basins as described before or in refiners and separators. The crude starch is transferred by hand or in baskets from the settling tanks into the washing tubs or basins, where the starch is agitated vigorously with clear water and then allowed to settle for 6-12 hours. This process is repeated several times until the starch is thoroughly purified. During settling, the starch sediment is sometimes covered with cloth to absorb the excessive moisture. However, in modern factories, two types of equipment are used for the purification of starch:
1. The Merco centrifugal separator, which is based on the well-known cream-separation principle. The separator features an integral re turn-flow principle which ensures a continuous and uniform output of solid products by recycling a portion of the underflow back into the rotor. This creates a flushing action and permits the use of a rotor nozzle size sufficiently large to prevent clogging.
FIGURE 17 Horizontal section of a current type of purifier
2. The Starcosa channel separator, which involves the nonturbulent flow of starch dispersion over dividing plates for the purpose of separating the heavier fine fibres from the lighter starch water dispersion.
Preliminary drying by centrifugation
At higher production levels in larger factories, technical and economic reasons have led to the adoption of a system in which concentrated slurries of pure starch are concentrated or thickened by mechanical means to a moisture content of 35 40 percent before drying by evaporation. Mechanical dewatering is generally performed in dewatering centrifuges. although continuously working vacuum filters are also used. especially in combination with modern tunnel driers.
The centrifuges for this purpose are of the basket type, as shown in Figure 18, equipped with a perforated bowl lined with a filter of cloth, small-mesh wire netting or the like. The starch is fed by batches as a slurry of 23ºBé; during centrifuging, the water is removed through the filter and the starch settles on the bowl wall in the form of a cylindrical cake. Some fine fibre and dirt always cover the inner surface of the cake and are scraped off before discharging the batch. Cassava starch, like other fine-grained starches, has properties allowing it to form a very firm sediment in the bowl, which is difficult to clear by hand or even with a mechanical clearing device. The most useful form of centrifuge is thus equipped with an exchangeable set-in which permits removal of the whole batch of starch after centrifuging. Vertical positions on the ground plate of the set-in facilitate the discharge.
In general, centrifugal drying, which brings down the moisture content to about 40 percent, is linked up with some form of evaporation drying in a continuous process. While a great variety of such driers are available in commerce only a few which are especially suited to the drying of starch will be described here.
DRYING
The removal of free water from the starch sediment obtained in settling tanks and on flour tables or from the concentrated slurries produced by separators and purifiers can be partly accomplished by mechanical means (e.g., centrifugation). The final drying. however. must always be performed by evaporation, either in the open air (sun drying) or in ovens. In modern factories, oven drying is always combined with mechanical drying, the whole operation, as in all other phases of the process, being conducted so as to take the least possible time.
Sun drying
As the sun is the cheapest source of heat, all small mills and many medium-size factories resort to this kind of drying despite the problems and the risk of contamination involved. The flour cake left after draining in the sedimentation tank or on the flour table is scooped up and after crumbling (sometimes with the aid of coarse matting or a wire screen) is spread out on basketwork trays about I m in diameter. Each tray is covered with as much of the wet product as contains some 0.5 kg of dry starch. The trays may be placed on the ground itself, but preferably should be laid on racks 1 m above the ground (Fig. 19). In this way, besides direct radiation, the heat reflected from the ground aids drying while the circulation of air is ensured on both sides of the layer of flour.
It is preferable to begin the drying process soon after sunrise so that in fair weather and a dry atmosphere it can be completed in one day. Often, however, this does not suffice, and before sundown the trays are stacked up on the factory premises. During the night, evaporation continues slowly, aided by the retained sun warmth, and is completed the next day in the open air. In the course of drying, a number of workers continually crumble the lumps of starch on the trays to speed up the drying. The crude flour is considered sufficiently dry when the remaining lumps are too hard to be crumbled by hand. The moisture content is then between 15 and 20 percent.
An important advantage of sun drying is the bleaching action of the ultraviolet rays. At the same time, however, a certain chemical degradation sets in, ultimately having an unfavourable influence on the quality of the product. Besides, contamination by dust cannot be entirely avoided during sun drying, especially on windy days; a lowered whiteness and the occurrence of "specks" will result. Finally, the baskets have to be cleansed regularly with a solution of bleaching powder, in order to prevent contamination by microorganisms. Even then, the baskets are subject to rapid wear and have to be replaced frequently.
Given sufficient space and the necessary number of baskets (about 5 000), a daily output of 2 tons of dry flour may be realized with sun drying. Of course, in cases like this, the manufacture would gain very much in efficiency and stability if drying were accelerated and concentrated in a smaller space by the use of ovens. At medium production levels ovens are rarely used, however, because both the installation and use of ovens, apart from the initial expense and the cost of fuel, require some engineering knowledge. Until now a completely satisfactory solution of the drying problem for medium-size factories has not been found. Rather primitive oven driers are used here and there, whereas factories with a somewhat higher daily output employ chamber and drum driers. The latter two types, applied in the manufacture of baked tapioca products, are described below.
Drying ovens
The simplest type of oven consists of a firing tunnel of brickwork covered with galvanized iron or copper plates on which the moist flour is spread in a thin layer. Firing should be moderate, so as to keep the temperature of the plates well below the gelatinization point of the starch, and the flour should be frequently raked up. The space above the oven should be vigorously ventilated. In Malaysia and other parts of the Far East, ovens called "drying yards," about 30-40 m long and 3-5 m wide are used for the drying of cassava starch. Enough wood is burned in the tunnel to heat the cement surface to the required temperature (see Fig. 20). The number of drying yards ranges from two to five, depending on the size of the factory and the kinds of products.
FIGURE 21. Sectional view of a chamber drier
Chamber drier
The chamber drier consists of a number of adjoining compartments with insulated walls, each one equipped with heating, ventilating and control devices. The wet material is placed on the trays. which are either directly introduced into the chamber drier or loaded on a trolley that is pushed into the drier. The process can be rendered more economic in this kind of drier by a system of air circulation. In a model drier, the air current produced by the screw fan is warmed by a heating element and moves across the material to be dried, giving off heat while taking up water vapour from the moist flour (Fig. 21). Through the adjustable slot a small part of the circulated air is discharged from the chamber, and at the same time a corresponding quantity of fresh air is drawn in from the outside, whereas the main air current recommences the cycle as described above. A considerable reduction of the drying time can be obtained by the insertion of air-guiding surfaces which effect an equalization of the air speed over all the trays.
Although drying in this apparatus takes relatively much time and labour, it is easy to handle and for that reason suitable for medium-size factories producing limited quantities of flour.
Drum driers
Probably the simplest arrangement for the continuous drying of flour is a horizontal or inclined revolving drum, heated from the outside, into which the moist flour is fed at one end. During transport inside the drum, which may be accomplished by various mechanical means, the product gives off its moisture to a stream of ventilating air (Fig. 22). In applying direct fire or steam the usual precautions against overheating have to be taken.
Bell driers
These represent an efficient form of continuous drier, combining a high capacity with a simple construction, which does not necessitate supervision by skilled workers. Here the starch is carried along on a series of conveyor belts, one on top of the other, in a stream of hot air. Moist starch is shed onto the top belt and conveyed over the whole length of the construction; at the end it drops on to the belt below, which is driven in the opposite direction, and so on. With each drop from a belt to the one below, the starch is turned over and ventilated. Heaters such as steam pipes are fixed between the belts and effect a rapid evaporation, the water vapour being removed by the upward draft.
Tunnel driers
The manufacture of a uniform product of definite moisture content is best ensured with modern tunnel driers in which the moist starch is carried on a conveyor belt through a tunnel divided into compartments forming drying zones. The circulating air is kept at a definite temperature, and moisture content in each zone is automatically controlled by conditioning devices. The flour is sucked up by vacuum from a concentrated slurry on a revolving cloth sieve with a cake-scoring device and a spring discharge. The starch cake, containing 40 percent water on the wet basis, is discharged in small broken strips directly to the travelling bed of the drier, where it encounters gradually changing drying conditions. The flour is discharged at the other end of the tunnel with a moisture content of about 17 percent, in the form of very loose agglomerations which are easily crumbled and bolted. The drier, 2.5 m wide and 10 m long and divided into four zones, has a capacity of 15 tons of dry flour per 24 hours.
Pneumatic driers
Another type of drier is the pneumatic flash drier. The starch cake is led from the basket centrifuge by a warm conveyor to a pneumatic drier, where the final moisture content is reduced to 10-13 percent. Drying is effected by hot air produced by a set of oil burners working on the atomized burning principle and compressed air. The required quantities of fresh air are sucked into the hot air generator through an air filter and heated to about 150°C. During the drying process the starch is pneumatically conveyed from the bottom to the top of the drier and then deflected downward.
Starch particles which are not quite dry are returned to the drying unit located at the bottom, while the dry starch is separated in the cyclone from the conveying air and led through a rotary pocket seal into a starch powder sifter.
FINISHING AND PACKAGING
Bolting
Crude dry cassava flour consists for the greater part of hard lumps of starch. As it is useless for most purposes in this form, it has to be subjected to a pulverizing process followed by dry-screening. The latter operations are often referred to as bolting.
As a bolting installation is remunerative only where production is relatively high, smaller enterprises do not as a rule install their own equipment for the purpose. Often, however, a number of small mills deliver their crude flour to a central bolting factory, which at the same time may function as a trading concern for the finished product. In these central installations bolting is carried out as in medium-size and larger cassava factories, while at the same time definite "brands" are composed by mixing.
At the medium production levels it pays to have simple bolting machines, in which the flour is crushed between rolls. The apparatus, if necessary, can be driven by hand.
Roller bolting The crude flour is shed into a hopper placed above a pair of rollers turning in opposite directions at the same speed. The agglomerations of starch are broken up by the action of the rollers, but fibre and other tough particles are left intact. The crushed flour is subsequently received in a conical rotary screen of the same construction used for wet-screening and described previously. Here the small lumps which have escaped crushing, fibre and other foreign particles are separated from the starch by a screening gauze 100 to 200 mesh/inch. The dry pulp discharged is fed back into the hopper once more. The rolls and the revolving screen are coupled to the same motor, which in an emergency may be replaced by a hand-driven crank.
Roller bolting is a relatively slow process and has therefore been superseded by disintegrator bolting. Recently, however, the particular advantage of the roller process - its relatively mild crushing action - has been combined with a greater speed of working in new machinery with a system of grooved rollers.
Bolting by disintegrators At present. most medium and all larger factories are equipped with beater disintegrators for the bolting process. In the disintegration action not only the starch lumps hut also here and other foreign material are pulverized and forced through screen plates of 100 mesh/inch or finer. as desired. If the starting material was of questionable purity. the resulting flour may contain appreciable amounts of nonstarch material. which cannot be separated easily. The power consumption of these disintegrators is between 10 and 20 hp depending on the amount of flour that needs to be disintegrated per hour: the working speed is some 1 200 rev/mint
The arrangement of the equipment is similar to that for roller bolting. As much starch is strewn during the operation. the disintegrator and rotating screen are housed in wooden chambers provided with windows for the discharge of the bolted flour which can be closed by shutters or a thick cloth when in operation.
Storing and packaging
The finished starch should he stored in a dry place. preferably on a board floor or in bins. where it can he mixed in order to obtain a uniform lot.
Before storing, the starch is sifted to assure lump-free uniform particles. It is usually packaged in gunny sacks for shipment, but multiwall paper bags are becoming more popular.
