Processing and production factors
Starch processing operations
Step 1 Importance of quick process
In the processing of cassava starch, it is vital to complete
the whole process within the shortest time possible. As soon
as the roots have been dug up, and during each of the subsequent
stages of manufacture, enzymatic processes have a deteriorating
effect on the quality of the end product. This are calls for
a well-organized supply of roots within relatively short distances
of the processing plant and, furthermore, for organization of
the stages of processing that will minimize delays in manufacture.
Thus, while simple in principle, the manufacture of 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.
Step 2 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 removing
dirt, crude fiber 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.
Frequent treading by foot cleans any loosely adhering dirt from
the roots.
In 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 8.5 percent of the weight of the whole
root.
The mechanical washer 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 onto 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 color.
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.
Step 3 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.
3.1 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, with 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.
3.2 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 be replaced often on account of rapid wear.
3.3 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 centimeter 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 millimeters. This, however, is generally
possible only in the all-steel raspers to be described later.
In many medium-sized 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.
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