The cashew tree is evergreen. It grows up to 12 metres high and has a spread of 25 metres. Its
extensive root system allows it to tolerate a wide range of moisture levels and soil types,
although, commercial production is advisable only in well-drained, sandy loam or red soils.
Annual rainfall needs to be at least 889mm (35
inches) and not more than 3048mm (120
inches). Cashew trees are most frequently
found in coastal areas.
Figure 1: Cashew Fruit
The main commercial product of the cashew
tree is the nut. In the main producing areas of
East Africa and India, 95% or more of the apple
crop is not eaten, as the taste is not popular.
However, in some parts of South America and
West Africa, local inhabitants regard the apple,
rather than the nut kernel, as a delicacy. In
Brazil, the apple is used to manufacture jams,
and soft and alcoholic drinks. In Goa, in India,
it is used to distil a cashew liquor called “feni”.
The cashew fruit (figures 1 and 2) is unusual in
comparison with other tree nuts since the nut is outside the fruit. The cashew apple is an edible
false fruit, attached to the externally born nut by a stem. In its raw state, the shell of the nut is
leathery, not brittle. It contains the thick vesicant oil, CNSL, within a sponge-like interior. A
thin testa skin surrounds the kernel and keeps it separated from the inside of the shell. The
primary products of cashew nuts are the kernels which have value as confectionery nuts.
Cashew nut shell liquid (CNSL) is an important industrial raw material for resin manufacture and
the shells can be burned to provide heat for the decorticating operation.
Figure 2: Cross-section of a Cashew Fruit
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Cashew nut processing
Key: dotted boxes
Large scale production
Soaking in Water
Figure 3: Overview of cashew nut processing
Traditionally, extraction of the kernel from the shell of the cashew nut has been a manual
operation. The nut is roasted which makes the shell brittle and loosens the kernel from the
inside of the shell. By soaking the nuts in water, the moisture content of the kernel is raised,
reducing the risk of it being scorched during roasting and making it more flexible so it is less
likely to crack. The CNSL is released when the nuts are roasted. Its value makes collection in
Cashew nut processing
sufficient quantities economically advantageous. However, for very small-scale processors, this
stage is unlikely to take place due to the high cost of the special roasting equipment required for
the CNSL collection (see the section on ‘hot oil’ roasting). If the nuts are being manually
shelled, gloves need to be used or alternatively, the nuts should be tumbled in sawdust or ashes
to absorb the liquid coating which has a harmful affect on the skin.
The shell can be cracked either manually, using a hammer, or mechanically. Manually operated
blade openers (as listed in the supplier’s section) are relatively inexpensive however the more
successful mechanical methods depend on the nuts having passed through the ‘hot oil’ CNSL
extraction operation. Care must be taken not to break or split the kernel at this or subsequent
stages as whole kernels are more valuable than broken ones. Once the kernel is removed from
the shell, it is dried, the testa is peeled off and the kernel is graded. Figure 3 gives an overview
of cashew nut processing and the various choices in methods.
All raw nuts carry foreign matter, consisting of sand, stones, dried apple etc. The presence of
foreign matter in the roasting operation can be avoided by cleaning the nuts. The raw nuts can
be sieved by hand using a ¾ inch mesh sieve.
The next stage is to soak the nuts in water to avoid scorching them during the roasting
operation. This can be done by placing the nuts in a 40-45 gallon drum or vat and filling it with
water until all the nuts are covered. After being left to stand for about ten minutes, the water
should be drained off via a plug near the base of the drum. The nuts should then be left for
periods of not less than four hours in order to allow the water left on the surface of the nuts to
be absorbed. The process of covering the nuts with water, draining and standing should be
repeated with the same nuts about three times until a moisture content of 9% is reached.
Where the production output runs from 2-10 tons of nuts per day, a simple cleaning and
conditioning arrangement can be used. Two people open the sacks of harvested nuts on a stand
and clean the raw nuts as they are moved along a flat sieve, to two vats which are used for
storage until the soaking process begins. Two vats are useful because one can be emptied while
the other is being filled.
The application of heat to the nut releases the nut shell liquid and makes the shell brittle which
facilitates the extraction of the kernel when breaking the shell open. Three methods of roasting
exist: open pan, drum roasting and the ‘hot oil’ method. The latter is more suitable to mediumscale operations with associated higher equipment costs and viability of CNSL collection.
Open pan: An open, mild steel, circular dished pan of around 2 feet in diameter is
supported on a basic earth fireplace. When heated, 2-3lbs (1kg) of nuts are placed on the
pan at one time and stirred constantly. The CNSL starts to exude and then ignites. This
produces a long flame and black smoke. After approximately two minutes, the pan is
dowsed and the charred, swollen and brittle nuts are thrown out of the pan. The moisture
evaporates quickly leaving the nuts ready for shelling.
Drum roasting: The idea of continually feeding the nuts into a rotating drum over a fire
developed from the pan method. A slight horizontal slope in the mounting ensures the
movement of the nuts through the drum. The drum is pierced so that the flames touch the
nuts and the smoke is controlled by a hood and chimney arrangement. The nuts are dowsed
using a continuous spray.
Cashew nut processing
This process was further modified by using the heat from the burning CNSL being
harnessed to roast the nuts some more. The roaster consists of a contained helical screw
which moves the burning nuts at a controlled rate. The design was a distinct improvement,
with little fuel being consumed and there being greater control on the roasting time.
Figure 4: Diagram of a Drum Roaster Fired from a Furnace below
‘Hot oil’ method: The principle employed in this method is that oil-bearing substances i.e.
the shells, when immersed in the same oil at high temperature, will lose their oil, thus
increasing the volume of the oil in the tank. For this method, conditioning becomes
important. The equipment consists of a tank of CNSL heated to a temperature of 185190°C by a furnace underneath and a wire basket used to hold the nuts for immersion into
the tank. The depth of the basket must be sufficient so that the rim remains well above the
oil during the roasting. Immersion time can range from 1½ to 4 minutes. About 50% of
the liquid is extracted from the nuts. Draining trays are needed at the end of the tank for
the roasted nuts to dry and the residue oil can be returned to the tank. Caution must be
taken not to heat the tank to over 200°C because at this point polymerization of the CNSL
takes place. The temperature can be maintained by continuous firing. The tank should be
emptied and cleaned after each day’s roasting. The life of a tank made of an eighth inch
thick mild steel plate should exceed one and a half years and can be constructed locally
with welding facilities.
The objective of shelling is to produce clean, whole kernels free of cracks. In India, this
operation has always been done manually. Other countries have difficulty in competing with the
great skill and the low wages of the Indian workers. Therefore, India has enjoyed a virtual
monopoly of cashew processing for a long time. Manual shelling is still relevant to the smallscale processor, although a close look at the mechanical option is advisable in all cases.
In the manual shelling process, the nuts are placed on a flat stone and cracked with a wooden
mallet. As mentioned above, because of the residue CNSL wood ash for covering the shells or
gloves are required. An average sheller can open one nut in about six seconds or ten nuts per
minute. In an eight-hour working day, this amounts to about 4,800 nuts or about 5kg of
kernels. At an extraction rate of 24%, this quantity corresponds with about 21kg of raw nuts per
Cashew nut processing
day or about 7 tons per year. However, experienced shellers in India can produce around half as
much again, with a quality of 90% whole kernels.
The most successful mechanical shellers work on nuts which have previously passed through the
‘hot oil’ process and is detailed under the paragraph ‘centrifugal shellers’.
A semi-mechanised process that has been used predominantly in Brazil uses a pair of knives,
each shaped in the contour of half a nut. When the knives come together by means of a foot
operated lever, they cut through the shell all around the nut, leaving the kernel untouched. Two
people work at each table; the first cuts the nuts and the second person opens them and
separates the kernel from the shell. Daily production is about 15kg of kernels per team.
The first mechanised shelling system, Oltremare, is also based on two nut-shaped knives. The
nuts are brought to the knives on a chain, each nut in the same position to fit between the
knives. The nuts are pushed between the knives and cut. The chain itself has to be fed
manually. After coming together, the knives make a twisting movement, thus separating the
shell halves. The disadvantages of this method are that nuts smaller than 18mm cannot be
processed and output is reduced because not all the spaces on the chain can be filled which can
count for as much as 10% of the production volume.
The shelling machines of the Cashco system are also chain fed but the nuts are automatically
placed in the right position. The shelling device has two knives that cut the sides of the nut and
a pin that is wedged into the stalk end of the nut separates the shell halves. The advantage of
this system is a fully mechanised operation with an output of about 75% whole kernel quality.
Nuts smaller than 15mm cannot be processed.
Centrifugal shellers use a system which is simple and enables a continuous flow. A rotary
paddle projects the shells against the solid casing and the impact cracks open the shell without
breaking the kernel. All sizes of nuts can be processed by this method however, it is necessary
to grade the nuts into four or so group ranges because a different rotary speed is used for the
various size groups. The percentage of whole kernels produced is around 75%. By preparing
the shells with grooves and weakening the strength of them before the operation begins, the
percentage can be increased. The speed of the rotor can thus be turned down and the risk of
damaging the kernels is reduced.
After shelling, shell pieces and kernels are separated and the unshelled nuts are returned to the
shelling operation. Usually blowers and shakers are used to separate the lighter shell pieces
from the kernels. The greatest problem is to recover small pieces of kernel sticking to the shell.
This is usually done manually from a conveyor belt used to carry all the sorted semi-shelled nuts.
Pre-grading can be done before or after drying the kernels and may greatly reduce the final
grading work. For large-scale processors pre-grading can be done mechanically, separating
mainly the whole from the broken kernels and sometimes separating the different size groups of
By Sue Azam Ali, Published by Practical Action on 02/02/01