ITDG 
Interdev Agroalimentaire  

Small-scale cashew nut processing

	Type de fiche	Techniques/Technologies
	Mise en œuvre	ITDG(Royaume Uni)
	
	Zone géographique	Global; 
	Mots clés	SECHAGE; GRILLAGE; ANACARDE; EGRENAGE

	Echelle d’action	Régional (transnational); Groupe
	Public(s) cible(s)	Small and medium-scale food processors

Contexte
The cashew nut is the main commercial product of the cashew (Anacardium occidentale) tree, which is an evergreen tree that grows well in coastal regions. The main producing regions are east Africa, India and Brazil.

The cashew fruit is unusual in comparison to other tree nuts since the nut is outside the fruit. The cashew apple is an edible false fruit, attached to the externally borne nut by a stem.  The nut is the commercially important part of the fruit. In its raw state, the shell of the nut is leathery, not brittle. It contains a thick corrosive liquid called cashew nut shell liquid (CNSL). CNSL is an important industrial material for resin manufacture. The shells can be burned to provide heat for the decorticating operation. 

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.  

Problématique / Objectifs
  The CNSL is released when the nuts are roasted.  If it is collected in sufficient quantities, it has real economic value.  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 either be cracked manually using a hammer, or mechanically.  Manually operated openers are relatively cheap, 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 any stage of processing as whole kernels are more valuable than broken ones.  
  With the hot oil roasting method, care must be taken not to heat the tank to over 200 C because at this point the CNSL polymerises.  The temperature can be maintained by continuous firing.  The heating tank should be emptied and cleaned after each day's roasting.  The life of a tank made of a 3mm thick mild steel plate should exceed one and a half years and can be constructed locally with welding facilities.
  It is important that the drying capacity is higher than the shelling capacity in case of periods of heavy rainfall. The kernels will absorb moisture very quickly and require longer to dry.  Sun drying, where the kernels are spread out in thin layers under strong sunshine is possible, however artificial drying is necessary for medium or larger-scale producers.  Artificial driers are cleaner and more controllable and produce a higher quality product.
  Drying usually takes six hours, at a temperature of around 70 C.  A uniform temperature throughout the drier is essential to avoid under drying or scorching.

Description/Mise en oeuvre
Cashew nuts are processed according to the following method;
  Cleaning
  Soaking
  Roasting
  Shelling
  Separation
  Pre-grading
  Drying
  Peeling
  Grading
  Rehumidification
  Packaging

A process flow sheet is available (see the link)

Cleaning
Nuts are cleaned to remove sand, stones, dried apple and other debris. The raw nuts can be sieved by hand using a 20mm mesh sieve.

Soaking
Nuts are soaked in water to avoid scorching during the roasting operation.  Nuts are placed in a large vat of water (150-200 litre capacity) so they are all covered and left to stand for about 10 minutes. The water is drained off via a plug near the base of the drum and the damp nuts left to stand for about 4 hours or more so that all the surface water is absorbed. The soaking, draining and standing routine is repeated 3 times until a moisture content of 9% is reached.

Roasting
Heating the nut releases the cashew nut shell liquid (CNSL) and makes the shell brittle, which facilitates the extraction of the kernel when the shell is broken open.  There are three methods of roasting; open pan, drum roasting and the hot oil method.  The latter is more suitable to medium-scale operations as it has higher equipment costs.

.	Open pan: An open, mild steel, circular dished pan of around 50-60cm in diameter is used. This is supported on a basic earth fireplace. Around 1kg nuts are placed onto the heated pan and stirred constantly.  The CNSL will start to exude and then ignites, which produces a long flame and black smoke.  After approximately 2 minutes, the pan is dowsed in water and the nuts removed. The moisture evaporates quickly and the nuts are ready for shelling.

.	Drum roasting: This is an adaptation of the open pan method. The nuts are heated inside a rotating drum, which gives a more uniform roasting. The drum is tilted slightly so that the nuts pass through the drum as it is rotating. They emerge at the lower end roasted and ready for shelling.  The roasted nuts are dowsed with a continuous spray of water.

	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 185-190 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 depending on the size of the nuts.  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.  

Shelling
The objective of shelling is to produce clean, whole kernels free of cracks. Manual shelling is relevant to the small-scale processor, although a close look at the mechanical option is advisable in all cases.

Manual shelling
The nuts are placed on a flat stone and cracked with a wooden mallet. As mentioned above, because of the residue CNSL, the processor should wear gloves or use wood ash to prevent burns to the skin. 

Mechanical
The most successful mechanical shellers work on nuts that have been roasted by the hot oil method. 

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 that 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. 

Separation
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
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 whole kernels.

Drying 
The shelled kernel is covered with a testa that needs to be removed. The kernels are dried to facilitate removal of the testa.  Drying also protects the kernel from pest and fungus attack at this vulnerable stage.  The moisture content is approximately 6% before drying and 3% after.  

It is in the dried condition the kernels are most vulnerable, being both brittle and susceptible to insect infestation. Therefore, at this stage, they must be handled with care and moved to the next stage of peeling as quickly as possible.

Peeling
The testa is removed manually by gentle rubbing with the fingers. Bamboo knives can be used to remove pieces that are more firmly attached. Care should be taken not to scratch, damage or break the kernels. One person can peel about 10-12kg of kernels per day.

Grading
The grading operation is important as it is the last opportunity for quality control on the kernels. With the exception of a few grading aids, all grading is done by hand.  For large operations looking towards export markets, it is necessary to grade the kernels to an international level.

Rehumidification
Before the kernels are packed it is necessary to ensure that their moisture content rises from 3% to around 5%.  This is to make the kernels less fragile, thus reducing the risk of breakage during transport.  In humid climates, the kernels may absorb enough moisture during peeling and grading to make a further rehumidification process unnecessary.

Packing
The normal packaging for export of kernels is in air-tight tins of 12.5kg (25lbs) in weight.  The packing needs to be impermeable as cashew kernels are subject to rancidity and go stale very quickly.

After filling and weighing, the cap should be soldered on in preparation for the ‘vita pack’ process. This consists of removing all air from the tin and substituting this with carbon dioxide (CO2). 

Some processors do not have vacuum pumps and displace the air in the tin by feeding in carbon dioxide through a small hole in the bottom of one side of the tin.  The carbon dioxide valve is turned off when all the air has been replaced.  The holes in the tin are then sealed, with the hole at the bottom of the side of the tin being done first, and the one on the top last.

Résultats/Eléments chiffrés
An average sheller can open one nut in about six seconds or ten nuts per minute.  In an 8 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 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
Enseignements
  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.

  Mechanisation of peeling is not very successful. Different methods are used, including air-blasting, suction, a freezing operation and a system of rubber rollers.  The operation has a low efficiency due to the difficulty of removing the testa and the amount of breakages can be as high as 30%.

  The advantages of packing cashew kernels in carbon dioxide are twofold.  Firstly, carbon dioxide is an inert gas and will not support life.  Any infestation that may have been present is therefore arrested. Secondly, carbon dioxide is soluble in cashew oil and goes into solution as soon as the seals are made.  In a short space of time, it can be seen that a decrease in pressure takes place as the carbon dioxide goes into solution and the sides, top and bottom are drawn inwards.  Thus the kernels are held tight in the tin, preventing movement and breakage during transport.  Carbon dioxide, being a heavy gas causes the upward displacement of air and will remain in the tins after the filling process.  Some large-scale machines will operate on six tins at a time, creating a vacuum in each and then filling with carbon dioxide.

  Cashews are prone to infestation with various insects and pests.  The most important defence against infestation of any type is cleanliness and is essential in the rooms used for drying, peeling, grading, conditioning, and packaging.  Floors and walls must be sound and free from cracks.  They should be kept white-washed regularly.  Some processors have filled the corners and places where the wall meets the floor with a curved filling so that the room can be properly swept, all corners having been eliminated.

  Speed of operations between drying and packaging must be stressed as this reduces the critical period when attacks may occur to a minimum.  The equipment used must also be thoroughly cleaned on a regular basis as insects may breed in hidden crevices and gaps.

Auteur : Sue Azam Ali
Saisie le : 14/01/2002
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