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CHAPTER 6 - CASHEW PROCESSING OPERATIONS


6.1 General processing

Cashew processing methods have improved considerably over the years. Difficulties in shelling cashew nuts are due to the irregular shape of the nut, the tough leathery outer shell, and the CNSL within the shell that must not be allowed to contaminate the kernel during its removal from the shell. An early method used to remove the CNSL in cashew producing countries was to burn the raw nuts for a short period in order to burn the shells and the CNSL without affecting the taste or appearance of the kernel. This was a delicate operation requiring an experienced processor to gauge the length of time required for burning. Kernels produced using this method are only suitable for either home consumption or for the local market.

The most economic features of processing are the ratio of kernels to whole nuts obtained, and the percentage of whole kernels obtained. Kernel yields usually vary between 22 and 24 percent of the total weight of raw material processed. The percentage of whole kernels at the end of processing varies between 55 and 85 percent depending upon the processing method and factory management. In general, 65 percent may be considered a satisfactory result.

The main objective of processing is to remove the valuable cashew kernel from the shell with as little damage as possible. Whole kernels command a higher price than do broken pieces. Pale, ivory coloured or white kernels are preferable to coloured or burnt ones. The CNSL has to be removed during the process, without either contaminating the cashew kernels or burning the hand of the processor. The processor must therefore, finely tune the process in order to achieve the best quality kernels.

Extraction of the kernel from the shell of the cashew nut has traditionally been a manual operation. Roasting causes brittleness of the shell and loosening of the kernel from within the shell. Soaking, increases the moisture content of the kernel, thereby reducing the risk of it being scorched during roasting and increasing its flexibility so as to make it less likely to crack. The CNSL is released when the nuts are roasted. Collection of this material in sufficient quantities can be economically advantageous. CSNL is unlikely to be collected by very small-scale processors, due to the high cost of the specialised roasting equipment required for its collection.

After the kernels are taken out of the shells, the testa (the thin skin covering the kernel) must be removed, following which the kernels are graded and packaged. The process consists of five main steps:

Each of these five steps involves a number of operations. The various processing steps differ in accordance with the scale of operation. In some cases, all steps of the process are manually carried out by small-scale processors, while various pieces of equipment are used in commercial scale processing. A general overview of the process and the various processing options is summarised in Figure 7.

Figure 7: Overview of cashew nut processing

 

Figure 7: Overview of cashew nut processingFigure 7: Overview of cashew nut processing
Figure 7: Overview of cashew nut processingFigure 7: Overview of cashew nut processing


6.2 Cleaning, sizing and conditioning

The first processing operation is the removal of foreign matter and dirt from the nuts. The nuts are collected from the ground after falling from the trees. Apples are removed along with other foreign matter. At the simplest level, the nuts can be sieved by hand using a three-quarter inch (20 mm) mesh sieve to remove dust and dirt (ITDG, 2000).

The cleaned nuts are then conditioned in preparation for removal of the shell. Conditioning increases the brittleness of the shell and thereby facilitates its removal.

6.2.1 Soaking or conditioning

The nuts are soaked in water in order to avoid scorching during the roasting operation. Conditioning is carried out in order to prepare for removal of the CSNL.

In small-scale operations, after cleaning, the nuts are placed in a large open drum (180-220 litres / 40-45 gallons). Water is poured into the drum and the nuts are allowed to stand for ten minutes prior to draining off the water through a hole in the base of the drum. The dampened nuts are then allowed to stand in order to absorb the adhering water. This soaking and conditioning operation is repeated up to three or four times until a moisture content of nine percent is attained.

On a slightly larger scale, in the processing of 2-10 tonnes per day for example, a simple cleaning and conditioning system can be set up. This consists of three main parts:

The sacks of harvested nuts on the stand are generally opened by two people, who clean the nuts as they pass over the grill and into one of the vats (Figure 8). Water is then sprayed on to the nuts contained in the vat. The water trickles down through the nuts, while excess water is drained through a hole situated at the bottom of the tank. Spraying is stopped when drainage of excess water begins, and the surface water which adheres to the nuts is allowed time to be absorbed by the nuts. The spraying treatment is repeated at three-hourly intervals until the required moisture condition (9 percent) of the raw nuts is met.

All nuts conditioned in this way are further processed in the same batch and the vat is completely emptied prior to the further addition of nuts. If nuts are left in the vat when new ones are added, the moisture content of the remaining nuts will be too high for processing. After the vat is emptied it must be thoroughly cleaned to remove all traces of dirt.


Figure 8: Medium-scale cleaning

 Figure 8: Medium-scale cleaning


The platform grill and vat can be locally made from a variety of materials. The platform must however be sturdy enough to withstand the impact of the many bags of nuts being dumped on to it. The grill must also be sturdy since the full weight of the nuts resting on it at any one time may increase to as much as 100-150 kg. The vat can be constructed from either concrete blocks or from bricks, rendered on the inside with cement to give a smooth finish. Drains contained within the vat should be small enough to prevent the nuts from flowing out with the water. The drains should be kept free so as to to allow the water to drain away freely. The vat can be fitted with a small closure that can be lifted to allow the nuts to flow out for subsequent processing.

A vat with internal dimensions of 3.0 x 2.4 x 2.1 metres will hold about 10 tonnes of cashew nuts.

6.2.2 Large-scale cleaning and conditioning

Specially designed equipment for cleaning and conditioning operations has been developed for large scale cleaning operations. The equipment basically consists of a feed hopper into which the raw cashew nuts are delivered. The nuts flow out of the hopper through a cylindrical cleaning trommel. The cleaning trommel consists of two concentric cylinders made of mild steel rods built on rings of flats. The inner cylinder is made of 13 mm (0.5 inches) rods spaced at 33 mm (1.25 inch) centres, about 260 mm (10 inches) in diameter and 2 metres (6 feet) long, mounted on a central shaft. The outer cylinder consists of 7 mm (0.25 inches) rods spaced at 13 mm (0.5 inches) centres, about 75 mm (30 inches) in diameter and mounted on the same shaft. The cylinders are lined up at the feed end and the inner cylinder projects 375 mm (15 inches) beyond the discharge end.

The cylinders rotate and the shaft is mounted at a slight angle in order to ensure that the material passes through it during rotation. The cashew nuts are fed into the inner cylinder. Large pieces of foreign matter are retained in the inner cylinder and removed later at the discharge end. The nuts and small pieces of foreign matter pass through the inner cylinder to the outer cylinder where the nuts are retained and the dirt and debris falls through, on to the floor below. The clean nuts are discharged into a chain bucket elevator hopper.

The buckets on the chain elevator are drilled with drain holes. They pass through a water bath at the bottom of the chain. The nuts fall into the bucket elevator while it is on the downward leg of travel, passing through the water bath and then draining as they are taken on the upward leg of the chain.

At the top of the elevator they are discharged into a mild steel silo with a conical bottom. They are discharged from the silo via a hole in the apex of the inverted cone. Excess water drains through this hole. The capacity of the silo is sufficient for one day’s processing in the roasting plant. There is a second silo into which the nuts from the first silo can be transferred by a belt bucket elevator.

Additional water can be added to the silos as required. The quantity of water to be added is determined from experience. Conditioned nuts leave the second silo for the roasting plant. When this happens, nuts from the first silo are transferred to the second. When silo one is empty, more nuts are loaded onto the receiving hopper to re-fill the first silo.

An alternative method of cleaning and conditioning employs the use of a shaking sieve whilst blowing air to remove all lightweight debris. The nuts are washed and then pass over sizing grills which separate them into three different grades. The nuts fall into the relevant bins where they can be further conditioned if necessary (FAO, 1969).

All the conditioning operations must be done in a closed environment. They all require a certain amount of time and an experienced operator.


6.3 Roasting and centrifuging

Following conditioning, the nuts must be prepared for the removal of shells. The application of heat to the nut releases the CNSL and makes the shell brittle, thus facilitating extraction of the kernel when breaking the shell open. Three methods of roasting are used: open pan roasting (as described in Chapter 5), drum roasting and roasting via the hot oil method. The latter is best suited to medium-scale operations because of the associated higher equipment costs and viability of CNSL collection. The roasted cashew nuts may be centrifuged to remove any adhering surface liquid from the nut.

At the start of the cashew industry in India, open pan roasting was the method used by all processors. The only advantage of the method was its low cost. The fumes and large amounts of black smoke given off during this process made it a very unpleasant operation. Particular care and attention were required in order to ensure that the kernels were not lost or ruined. The process also suffered from the disadvantage that the by-product CNSL was lost.

Other cleaner methods of roasting were therefore developed.

6.3.1 Drum roasting

An improvement on the open pan roaster was the development of a drum roaster, within which the cashew nuts are roasted. The drum is tilted at an angle over the fire and rotated during heating to prevent the nuts from burning (Figure 9). During rotation, nuts pass through the cylinder and out of the opposite end of the drum. The duration of the roasting process can be regulated by changing the speed of rotation of the drum. The cylinder is covered in a hood connected to a chimney which draws the black smoke upward into the atmosphere and makes it less unpleasant for the operator (FAO, 1969).

Figure 9: Diagram of a drum roaster fired from a furnace below

Figure 9: Diagram of a drum roaster fired from a furnace below


Many of the drums were originally manually rotated, but were later fitted with a power drive. Although this method was an improvement over the original method, it was imperfect in that kernels were lost due to overheating and burning and the CNSL was lost.

6.3.2 The hot oil method

An increased demand for CNSL in the mid 1930s, led to a major change in cashew nut processing. The ‘hot oil’ method was developed and was widely adopted.

The principle of this method is that oil bearing substances, when treated in the same or similar oil at a high temperature, give up their oil constituents to the bulk, thereby increasing the volume of the bulk. When cashew nuts are submerged in a bath of hot CNSL, the CNSL within the shell is therefore extracted, resulting in an increase in the volume of the bath liquid.

Conditions for successful operation of the ‘hot oil’ method are described in Box 6.1.


Box 6.1. Critical conditions for the ‘hot oil’ method of CNSL extraction.

Source: FAO, 1969.

Cashew nut shell liquid can be heated to a temperature of 200°C. On attaining a temperature of 100°C, moisture present is boiled off. At a temperature of 150°C decarboxylation occurs, and loosely adhering carbon dioxide molecules are given off, resulting in frothing at the oil surface. Further to emission of the carbon dioxide content of the oil, the liquid can be further heated with no frothing to 200°C. At temperatures higher than this polymerisation commences, which must be avoided as it gives poor results. A temperature range of 185 to 190°C is ideal for roasting, since it is sufficiently hot to be exceed the decarboxylation temperature.

During the hot oil process, cold raw nuts are submerged into the hot liquid, thus constantly lowering its temperature. Heat must therefore be constantly applied in order to maintain the temperature. A critical volume ratio of hot liquid to the volume of nuts being treated at any one time, must be maintained. This varies from 30:1 to 50:1 in accordance with the type of plant being used. A minimum critical volume of liquid must be maintained from the time roasting commences until the entire quantity of raw nuts has been treated.

The cashew nuts must be retained in the hot oil for approximately 1½ minutes, following which they are removed. Liquid is constantly being removed from the tank, through its adherance to roasted nuts and to the vessel in which the nuts are contained. During the process, oil extracted from the nuts replenishes that lost by adherence to the nuts. If necessary, the roasted nuts can be allowed to drain so as to allow oil to flow back into the tank.

Simple hot oil process

The simplest hot oil process is one that consists of a tank in which CNSL is heated and a wire basket that contains the nuts to be roasted. The nuts are placed in the basket and weighted down with a piece of mild steel plate (1 mm thick). A thermometer is inserted in the side well below the liquid level. Trays on either side of the tank act as draining areas, allowing excess oil to run back into the tank. The tank is heated from below by a built-in furnace. The nuts are held in the hot oil for 1.5 minutes at a temperature of 185°C. The entire process is manually operated. After roasting, the nuts are placed on a wire mesh screen over a tank for further draining and cooling prior to shelling.

A slight modification of this simple method allows larger quantities to be processed in one day. The equipment involves three circular baths situated in close proximity, each with a separate furnace. The baths are approximately 900 mm wide by 900 mm deep and are fitted with wire mesh baskets which hold the raw nuts. The baskets are successively dipped into the three oil baths.

The Pierce Lesley ‘hot oil’ method was the first of many to be patented. All subsequent methods are copies or modifications of this method and the equipment used by it. Equipment used varies in its degree of sophistication, throughput and price. The Oltremare hot oil plant is one such modified version that operates on the same principles. One advantage of this equipment is that it grades the nuts prior to roasting, thus allowing the nuts to be roasted for different lengths of time in accordance with their relative sizes.

Clean, conditioned cashew nuts are packed in a feed hopper, from which they are discharged at a controlled rate on to a conveyor that carries them through a hot oil bath maintained at 190°C. The equipment is designed such that the residence time within the oil bath is 1.5 minutes. The roasted nuts are subsequently discharged from the conveyor. The liquid level in the bath is maintained by an overflow pipe.

The bath is heated to a constant temperature by a furnace that runs along its length. Two thermometers are mounted within the bath to monitor the temperature. The oil bath is cleaned on a daily basis by a device that scrapes the bottom of the tank, gathering all the sludge and debris that has built up during the day.

The liquid overflow is channelled into drums in which it is filtered and allowed to settle in order to remove pieces of shell and other debris. Excess oil is filtered into drums for shipment.

Decarboxylation of the oil is inevitable throughout the process. The oil begins to froth as soon as new material is fed into the system and frothing continues until a steady state is attained. The oil bath design allows adequate space to accommodate frothing. Soon after roasting, the froth is broken and surplus liquid formed overflows into the settling drums.

Raw conditioned nuts, having a moisture content ranging between 15 and 17 percent are fed into the hot oil bath and heated until moisture is given off as steam. This produces some frothing. The production of steam within the nut shell assists with the extraction of CNSL, thus causing the shell to become brittle. There is a direct correlation between the amount of oil removed from the shell, its degree of brittleness and hence ease of cracking.

Large quantities of water vapour are produced in the oil bath due to the production of steam, and the decarboxylation and evaporation of the lower fractions of the CNSL. Water vapour production can however be minimized by covering the bath and drawing off the vapour.

Fuel from a range of sources can be used to heat the furnace. The use of spent cashew shells, which also contain some excess CNSL provides one of the most economical methods of heating the furnace. Problems caused by fumes must however be addressed.

The bottom plate of the tank must be made of steel (20 mm thick) that is reinforced with chromium. Mild steel will pit and distort under the constant heat and the action of the hot CNSL.

The nuts are discharged from the bath on to a cooling conveyor. Scorching of the kernels occurs if the heated nuts are allowed to remain in a pile. It is essential that the kernels are cooled quickly on emerging from the hot oil bath. The conveyor is designed to draw off the liquid from the nut and allow air to circulate around it in order to facilitate its cooling. Excess liquid is collected in a tank situated below.

The nuts are discharged from the end of the cooling conveyor into a centrifuge which removes much or the remainder of the adhering surface oil. They are then either manually or mechanically shelled.

The hot oil method is preferred by larger scale processors. A power driven plant, however, requires capital investment and is less flexible than a less sophisticated drum roaster.

It has been shown that hot oil plants are successful when used for processing large quantities of nuts. This is due to the fact that the constant heating and cooling of the CNSL causes polymerization, which changes the characteristics of the oil, and seriously affect the efficiency of the roasting process. When large quantities are roasted in a single batch, new liquid added to the bath keeps the liquid fresh while displacing some of the liquid which has been heated and cooled a number of times.

It is advantageous to cool the oil bath as quickly as possible subsequent to roasting. This is done by removing the heat source and allowing air to circulate freely. Cold CNSL can be added to the bath to assist in the cooling process (FAO, 1969).



6.4 Shelling

The objective of shelling is to produce clean, whole kernels, which are free of cracks. Shelling has always been manually performed in India. Other countries have difficulty in competing with the great skill and the low wages of Indian workers. India has therefore enjoyed a virtual monopoly of cashew processing for a long time. Manual shelling (described in Chapter 5) is still relevant to the small-scale processor, although a close look at mechanical options is recommended in all cases.

6.4.1 Mechanical shelling

Several pieces of equipment are designed to remove shells from cashew nuts. The main challenge with mechanical shelling is to remove the kernel without damage or contamination from the CNSL. This challenge is exacerbated by the irregular shape of the nut and the wide variation in the size of the nut. The most successful mechanized decorticators work on nuts that have been conditioned by the hot oil process, which makes the shell brittle and easier to break.

A semi-mechanized process that has been predominantly utilized in Brazil incorporates the use of 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 while the second opens them and separates the kernel from the shell. About 15 kg of shelled nuts are produced on a daily basis by this team.

The first mechanized shelling system, Oltremare, is also based on two nut-shaped knives. The nuts are brought to the knives on a chain, each nut aligned 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 18 mm cannot be processed, and output is reduced since not all the spaces on the chain can be filled. This can count for as much as ten percent of the production volume.

The shelling machines of the Cashco system are also chain fed but the nuts are automatically aligned. 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. This system is advantageous in that it is a fully mechanized operation with an output of about 75 percent whole kernel quality. Nuts smaller than 15 mm cannot however be processed using this system.

Centrifugal shellers use a system which is simple and enables a continuous flow. A rotary paddle projects the shells against the solid casing of the machine and the impact cracks open the shell without breaking the kernel. All sizes of nuts can be processed by this method. It is however, necessary to grade the nuts into about four sizes, since a different rotary speed is used for each of the various size groups. The percentage of whole kernels produced is around 75 percent. By weakening the shells with grooves before the operation begins, the percentage can be increased. The speed of the rotor can be reduced and the risk of damaging the kernels is minimized.

Development of a cashew nut sheller in India

A mechanical sheller was designed in India by researchers at the Post Harvest Technology Centre, Indian Institute of Technology in Kharagpur. The sheller uses principles of compression and shear, taking into account the physical and mechanical properties of cashew nuts. The machine consists of four compartments - power supply, transmission, feeding, shelling and discharging (Figure 10).

The feeding section consists of a hopper and horizontal screw conveyor for positive feed of the roasted nuts to the shelling section. The design criteria for the hopper and screw conveyor use size, bulk density, coefficient of friction and angle of repose of roasted cashew nuts. A flat plate sliding gate is used to control the feed rate of cashews into the sheller.

Figure 10: Cashew shelling machine

 Figure 10: Cashew shelling machine

Shelling of roasted nuts takes place between two wooden discs, one of which is stationary (fixed to the machine casing) while the other is mounted on to a shaft. The rotating disc is spring loaded in order to compress and shear the roasted cashew nut against the stationary disc. Sufficient pressure is exerted by the spring in order to compress the nuts between the two discs. The compression and differential speed of the discs causes the shells to be broken and removed. Initial designs of the sheller resulted in either a high percentage of broken kernels or a large number of unshelled nuts. Further to several re-design attempts, a sheller has been developed with a shelling rate of about 70 percent and minimal breakage (Jain and Kumar, 1997).

The entire disc assembly is encased in an 18 gauge mild steel cage. At the bottom is a square opening (150 x 150 mm) for discharging the kernels and shells. A conduit is provided (120 mm diameter and 250 mm long) to prevent the kernels from being damaged by falling onto the collecting tray.

The machine is powered by a 1.5 kW DC motor (1500 rpm), equipped with a belt and pulley arrangement to transmit power from the motor to the shaft.

The performance of the sheller is optimal at 320 rpm. Performance statistics are as follows:

 

Capacity 18 kg/h
Shelling efficiency 70 %
Whole kernel yield 50 %
Half split yield 22 %
Broken yield 28 %


Case study: Small-scale cashew processing centre, Kurunegala, Sri Lanka

Processors at a small-scale centre in Kurunegala, Sri Lanka have invested in a mechanical cashew sheller that was designed by the Sri Lanka Cashew Corporation, based on an Indian model. The sheller is operated by two workers - one who places the raw nuts into the machine and the other who removes the kernels from the split shells. The workers carry out these two tasks in turn, alternating about every hour. The raw cashews are placed into a receiving receptacle situated on a chain driven belt. The chain belt is operated via a foot pedal. On depression of the food pedal, the chain belt rotates and a raw nut is placed in the free hole. As the chain rotates, the nut is taken to a pair of knives that split the shell. The split shell is discarded into a collecting bucket. Kernels are removed from the split shells by a second worker.

In order to reduce the quantity of CNSL, the nuts must be conditioned prior to being inserted into the sheller. Operators coat their hands with coconut oil in order to protect themselves against CNSL leaking from the shells. Use of this sheller at the processing unit has increased the efficiency of shelling and in turn the daily output of the unit.

 

Comparison of performance and labour requirements before and after the introduction of a mechanical sheller in Sri Lanka:

  With a shelling machine Before the shelling machine
Raw nuts per day (kg) 110 100
Kernel output (kg) 24.4 22.20
Wholes (kg) 20.74 (85%) 15.54 (70%)
Splits (kg) 3.66 (15%) 6.66 (30%)
     
Labour requirement    
Shelling 2 4
Peeling 4 4
Oven/grading/packing 1 1
Over all supervision 1 1
Total 8 10


6.5 Separation

After shelling, shell pieces and kernels are separated, and the unshelled nuts are returned to the shelling operation. Blowers and shakers are generally used to separate the lighter shell pieces from the kernels. Recovery of small pieces of kernel sticking to the shell, poses the greatest problem. This is usually done manually from a conveyor belt used to carry all the sorted semi-shelled nuts.

6.5.1 Pre-grading

Pre-grading can be done before or after drying the kernels and may greatly reduce the work involved in final grading. Pre-grading can be done mechanically for large-scale processes, separating mainly the whole from the broken kernels and sometimes separating the different size groups of whole kernels.


6.6 Drying

The shelled kernel is covered with the testa, the removal of which is facilitated by drying the shelled kernel, to produce the blanched kernel. Drying causes shrinkage of the kernel, thereby allowing the testa to be easily removed either mechanically, or by hand with a knife. Drying also protects the kernel from pest and fungal attack at this vulnerable stage. All processors dry the shelled kernels prior to peeling.

The moisture content of the kernel is reduced from approximately six percent to three percent by drying. It is important that the drying capacity exceeds the shelling capacity, should there be periods of heavy rainfall. Under such circumstances, the drying operation is increased, since the kernels absorb moisture very quickly.


Figure 11: Diagram of a tray dryer

Figure 11: Diagram of a tray dryer


Sun drying, where the kernels are spread out in the sun in thin layers is possible. It is however heavily reliant on a constant supply of sunshine. Although sun-drying does not pose any risk of scorching the kernels, it may be prolonged under conditions of bad weather, which can lead to mould development.

Artificial drying is more reliable and is required in medium or large-scale operations. 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. Various drier designs are available. Figure 11 shows a tray dryer, designed by ITDG, for drying cashew kernels. The dryer contains a series of mesh-bottom trays that are slotted into the drying cabinet. The trays should be of a size that can be lifted when full. A lever mechanism automatically moves the trays down when dried trays are removed and when new ones are entered into the cabinet. Hot air circulates over the trays and is exhausted through the chimney. The heat source can either be a gas or electric powered heater. Burning cashew shells or other sources of fuel (ITDG, 2000) can also be used to provide a heat source.

Drying programmes are generally organized so that the kernels from one day's shelling go directly into the oven for overnight drying. Kernels in the dried state are most vulnerable, since they are brittle and break very easily. It is essential that the kernels are carefully handled in order to minimize damage.


6.7 Peeling

At this stage, the testa is loosely attached to the kernel, although a few kernels may have already lost the testa during prior operations. Manual peeling is performed by gently rubbing with the fingers. Those parts still attached to the kernel are removed with the use of a bamboo knife. Approximately 10-12 kg of kernels can be peeled by one individual per day.

It is important that the kernels are neither cut nor damaged during the peeling process. The use of knives increases the likelihood of the kernels becoming damaged. It is also essential that the entire testa is removed. Gentle scraping of the testa with a blunt knife is the most effective way of removing it.

The peeled kernels can be separated into different grades by the peeler. At the most basic level, the kernels are separated into white wholes, scorched wholes, white pieces, scorched pieces, browns and refuse. More experienced graders are able to separate the kernels into more categories. It is preferable that grading is carried out at the time of peeling since this cuts down on handling of the brittle kernels. There is however an opportunity for further grading subsequent to peeling.

It is essential that the peelers work under well-lit conditions in order to enable them to remove the entire testa. At the end of the day, the removed testa is winnowed and all cashew pieces removed. The dust and very fine pieces that cannot be peeled, together with the diseased pieces, are classified as refuse and are thrown away. The browns, which are kernels that are badly diseased and which have not been separated out during the shelling operation, must also be removed and discarded (FAO, 1969).

Strict cleanliness in the peeling operation is essential, not only in the peeling room and its facilities, but must be observed by all personnel. All workers must follow basic codes of hygiene and wash their hands prior to handling the kernels.

The mechanized processes of peeling vary widely. They include air-blasting, suction, a freezing operation and a system of rubber rollers. These systems are of low efficiency due to the difficulty of removing the testa. The level of breakage can be as high as 30 percent. Currently research and development is taking place to improve the viability of the mechanization of this operation.

After peeling, the kernels are weighed in order to record daily production. The peeled kernels are vulnerable to insect infestation and mould growth. They are also prone to rodent attack and should be stored in rodent-proof containers or rooms.


6.8 Grading

The grading operation is important since it is the last opportunity for quality control of the kernels. After the kernels are extracted from the shells, dried and peeled, they are graded for export according to size and condition. The grading system is known as the American Standard, which is also incorporated in the Indian Government export criteria. Kernels are categorized on the basis of colour and condition.

Peeled cashew nuts can be classified into between 11 and 24 grades. These are roughly divided into three groups: white whole, white pieces and scorched grades. The three groups are further broken down as follows:

White wholes
W180 (super large) Between 120 and 180 kernels per lb (266-395 per kg)
W210 (large) Between 200 and 210 kernels per lb (395-465 per kg)
W240 Between 230 and 240 kernels per lb (485-530 per kg)
W280 Between 270 and 280 kernels per lb (575-620 per kg)
W320 Between 300 and 320 kernels per lb (660-706 per kg)
W450 Between 400 and 450 kernels per lb (880-990 per kg)
White pieces
Butts A kernel broken cleanly across the section of the nut.
Splits A kernel which has broken down the natural line of cleavage to form a cotyledon.
Pieces A kernel which has broken across the section but does not qualify for a butt and is above a specific size.
Small pieces As above but smaller.
Baby bits Very small pieces of kernel which are white in colour.
Scorched grades
Wholes Whole kernels that have been slightly scorched during the process but are otherwise sound. These are not graded according to size.
Butts Butts that have been scorched.
Splits Splits that have been scorched.
Pieces As for pieces, but which have been scorched during processing and contain all but the very small pieces.


White, whole kernels are graded according to their size on the basis of the number of kernels per pound (equivalent to 454 g). The most common count for Indian and African kernels is 300-320 per pound (W320) followed by 400-450 (W450), 220-240 (W240) and 200-210 (W210) per pound. In Brazil where the crop has a proportion of large kernels not found in other countries, another grade of 160-180 (W180) is available. Whole kernels of scorched and dessert types are not graded according to their count - but are sorted according to the colour of the kernel. White kernels which are not whole are graded according to the way in which they are broken. Splits are kernels that have divided naturally lengthways, while butts are kernels broken crosswise. Other pieces of kernels that have broken into more than two pieces are graded according to size - large white pieces, small white pieces and baby bits (Errington and Coulter, 1989).

There are other grades that do not fall into the above classification, but which are used for local consumption or are shipped to countries that have an outlet to the cheaper trade. These are the dessert grades and are classified as follows:

In practice, most processors do not produce all of the different grades. They produce such small quantities of certain grades that it is uneconomical to produce them all.

With the exception of a few grading aids, all grading is done by hand. Graders sit on high stools or stand at tables that are covered in blankets to provide a soft surface and reduce breakage. The blankets also hold any dust that is removed from the kernel. It is essential that the room is well lit as colour is an important grading criterion. With experience, graders become accustomed to picking out kernels of one particular size. A proven procedure is to have one or two workers picking out the 210 and 240 count grades, then have one or two more workers picking out the 450 count. The majority of the remaining kernels (and usually the largest quantity) will be the 320 count. In addition, all graders also pick out scorched kernels and broken pieces. It is important that care is taken to avoid breakage of the kernels during grading.

Throughout the grading of white whole kernels, the weight must be constantly checked. This is done with the use of a small counter scale with 250g (½lb) of graded kernels accurately weighed out and counted. From this, the count per kg (or per pound) can be calculated.

With the scorched grades, the degree of scorching must be evaluated by the grader, who must judge between scorching and discoloration due to disease. This applies to both wholes and pieces and to scorched butts and splits. Kernels that are discarded from the scorched category go into categories of either scorched wholes grade 2, dessert wholes or dessert pieces.

Several attempts have been made to mechanize the grading of kernels, with limited success. Power driven rotary sieves are one mechanical method, another being the use of two outwardly rotating rubber rollers aligned at a diverging angle. For large operations looking towards export markets, it is necessary to grade the kernels to an international level.

In all classes, there are certain minimum requirements to which operators must adhere:

1.   Kernels should be free from any deterioration likely to affect the natural keeping quality of the nuts and make them unfit for human consumption. They should be:

2.  Cashew kernels should have a moisture content no greater than five percent.

Quality has emerged, ahead of price, as the most vital criterion for any item if it is seeking entry into the global market. Quality aspects include safety, reliability, durability and acceptability of the product to the consumer (Nayar, 1995). Small-scale processors have to match the standards set by importers, consumers and standards agencies. Details of the minimum requirements for export will be available from the Ministry for Export or the Chambers of Commerce. Quality assurance procedures are an essential element of any processing operation to ensure product consistency between different batches. Simple HACCP (Hazard Analysis Critical Control Point) procedures suitable for small-scale operators to follow, can and should be applied to the process (Dillon and Griffith,1995).


6.9 Rehumidification

Prior to packing the kernels, it is necessary to ensure that their moisture content is increased from three percent up to around five percent. This makes the kernels less fragile, thus lessening 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.

The final moisture content is critical since, moisture contents in excess of six percent, favour mould growth. A moisture content of five percent is optimal. Many processors have facilities for adjusting the moisture content.

The moisture content of the cashews can be increased by transferring them to conditioning rooms which are completely closed rooms, in which trays of cashew kernels are placed overnight to absorb moisture from the surrounding air. Under low humidity conditions, the floor is sprayed with water prior to closing the door. Under high relative humidity conditions, it may not be necessary to add water to the floor. Steam is sometimes used for humidifying the kernels. Saturated steam is allowed into the conditioning room. The amount of steam to be injected is determined on basis of personal experience.

Determination of the moisture content is extremely important at this stage of the process. Two methods that are recommended for the determination of moisture content: use of the the Steinlite Moisture Meter and the Dean and Stark apparatus (Box 6.9).

Box 6.9. Measurement of the moisture content of cashew kernels

Source: FAO, 1969.

Steinlite Moisture meter
The kernels are weighed to a specific weight and transferred to a special container designed for measuring electrical characteristics. The reading on the dial is converted into a moisture content by reference to a chart. Corrections have to be made for the temperature of the material.
Dean and Stark apparatus
A weight of cashew kernels is placed in a flask of toluene. The toluene is boiled and condensed. Water from the cashew kernels also boils, is liberated as steam and condenses. Since it is heavier than the toluene it drops to the bottom of the collection vessel. The water liberated is collected in a graduated vessel which allows measurement of the volume of water. This volume is then converted into the percentage moisture by a simple calculation.


6.10 Packing

The normal packaging used for the export of kernels is air-tight cans of 25lbs (11.34 kg) weight capacity. The packaging material needs to be impermeable, since cashew kernels are subject to rancidity and go stale very quickly. The can will be familiar to most tropical countries as it is a replica of the four gallon kerosene or paraffin oil can. Cans can be locally made in order to reduce costs. Parts purchased overseas can be locally fabricated. This may be done by arrangement with can manufacturers. The output of a can manufacturing line is usually too large for a single consumer, but some cashew nut processors have installed their own can manufacturing plants and supply other processors.

After filling and weighing, the cap should be soldered in preparation for the vita pack process. This consists of removing all the air from the can and substituting it with carbon dioxide (CO2). The advantages of packing cashew kernels in carbon dioxide are two-fold. Firstly, carbon dioxide will not support life so 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 can is sealed. Within a short period of time, a decrease in pressure takes place as the carbon dioxide goes into solution and the sides, top and bottom of the can are drawn inwards. The kernels are therefore tightly sealed in the can, thus preventing movement and breakage during transport. Carbon dioxide, being a heavy gas, causes the upward displacement of air and will remain in the cans after filling. Some large-scale machines will operate on six cans at a time, creating a vacuum in each and subsequently filling it with carbon dioxide.

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


6.11 Infestation

Far too little attention is paid to the infestation hazards to cashew kernels. These hazards are more prevalent at some times of the year than others. A good processor will be vigilant all the times. The main insect pests are:

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 white-washed on a regular basis. Some processors fill the corners and areas at which the wall meets the floor with a curved filling in order to eliminate all corners, so that the room can be properly swept.

The 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.

The kernels are most vulnerable when dry, 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.

6.12 Hygiene and safety

Successful business activity is dependent upon adequate food safety and hygienic practices. The main ways in which a producer can harm consumers are by selling food that:

Safe food can be produced by careful attention to hygiene and by the use of proper quality control.

Proper hygiene means careful attention to the cleanliness of the processing equipment and the personal hygiene of food handlers.

Proper quality control means careful attention to the selection of good quality raw materials; correct processing conditions, such as the temperature and time of heating; preventing contaminating materials, such as dirt, metal and stones, from becoming mixed with the food; and the use of suitable packaging materials to protect the food after processing. These factors will ensure that only wholesome food is produced without contaminants. Any bacteria in the raw materials will be destroyed, or controlled at a safe level, and prevented from growing and multiplying (Fellows, Hidellage and Judge, 1999).

6.12.1 Food hygiene and the law

In most countries, laws on food processing are designed to protect consumers against poisoning and injury. Nuts are prone to becoming contaminated by the mould aflatoxin. Readers are advised to contact their local Bureaus of Standards, Ministries of Health or other relevant government Departments in order to obtain full details of the specific laws of their country.

Although food laws can be enforced, in the end, the customer is the most effective food inspector. If customers become ill from eating a food, they will not buy food from that processor again. It is therefore in the processor's interest to prepare safe wholesome foods.

6.12.2 Food poisoning and its causes

The main cause of food poisoning is microbial activity. Microbes live almost everywhere: on animals and plants (hence on all fresh foods), in and on humans, in the soil, water, air and on all surfaces. There are many different types, but the most important for food hygiene are bacteria, yeasts, moulds and viruses.

Agents that cause disease (pathogens) can be transmitted to humans by a number of routes - soil, air, water, direct person-person contact and food. Some can be transmitted to food by animals or by an item of equipment. Cross contamination occurs when contaminants are transferred from one food to another via a non-food surface, for example, utensils, equipment or human hands.

Illness is caused by eating food containing a significant amount of harmful bacteria. Poisoning bacteria can cause illness, either by producing poison in food before it is eaten, or by continuing to multiply inside the body after eating. The symptoms of an attack of food poisoning can include stomach pains, diarrhoea, vomiting, headache, fever and aching limbs. Sometimes the illness lasts for days, weeks or months and in some cases, it can cause death.

6.12.3 Personal hygiene

The main problems arise from contaminating food while preparing it, with microbes from the processors own hands or mouth, from dirty tools, work surfaces or from other food. All persons handling food should pay strict attention to good hygiene practices. This includes wearing clean, suitable clothing, including aprons, gloves and footwear, and covering the hair to prevent contamination of foods. Aprons, gloves or any other clothing that could touch the food should be thoroughly cleaned, everyday if necessary.

All cuts or wounds should be covered by a waterproof dressing and kept clean, even if they are not on the hands. Processors should not handle foods if they have a stomach upset or a skin disease, or if they are looking after someone else with these illnesses.

The processor should not smoke, eat or chew anything while preparing food. They should never spit near the food being prepared or cough or sneeze over foods as this spreads bacteria and can contaminate the food.

Everyone who touches food should wash his/her hands properly, using soap and clean water, especially after every visit to the toilet and between handling raw meat or poultry and any other food stuff, to avoid cross contamination.

After the basic preparation of ingredients has been completed, direct handling of the food can be avoided if appropriate care is taken. Food can be moved about with tongs or similar utensils. If the food is to be sold in bags, a good technique is to use the bag as a glove so that direct hand contact is prevented.

6.12.4 Cleanliness of equipment and the working area

Just as invisible harmful organisms live on the body, they can also be present on the utensils and surfaces with which food comes into contact. All surfaces and equipment must be clean before work, during production and after the process is finished. Cloths and sponges used to wipe down surfaces and towels used for hand drying should be washed and sterilized regularly by boiling in water.

There should be a supply of clean water for washing equipment as soon as it has been used and for use in food processing. If the water is not clean, it will contaminate the food. If the water comes from a stagnant pool or dirty source, it should be boiled for at least ten minutes to destroy bacteria, before it is used for washing food or utensils, or processing.

All the equipment must be in good condition and be properly repaired. Rusty, dirty or broken equipment must not be used to process foods as these can cause accidents as well as contaminate the food.

Equipment and utensils should be stored where they can be kept clean when not in use. Hang brushes and cloths up to dry after use. Store the cleaning equipment in a separate cupboard from the food and processing equipment. Keep all chemicals, pesticides, poisons and detergents away from food in a separate storage area.

Make sure that there is good lighting to help stop accidents and make working easier and safer.

6.12.5 Other sources of contamination

Food should always be kept covered and above ground level to protect it from contamination by insects, rodents and birds and the bacteria they carry. Flies, bluebottles, rats, mice and other vermin contaminate food with bacteria from their droppings or their bodies. All cupboard doors and lids of tins and jars should fit tightly. Table legs can be put in pots of water or kerosene to stop ants crawling up them.

All spills should be cleaned up as soon as they are formed. Wastes should not be left to accumulate on floors, in drains or on work surfaces. Lids should be kept firmly on bins and waste sacks should be securely fastened before putting them out for collection.

6.12.6 Packaging and preservation

Food poisoning bacteria multiply very quickly in moderate temperatures (between 20-40°C) so all foods should be stored in the shade and out of the sunlight. Storage areas and surfaces should be kept dry so microbes do not have the moist environment they need to breed.

FOOD HYGIENE SUMMARY

Transferring bacteria from outside sources

  • Using dirty implements
  • Cutting food up on a dirty board or surface
  • Infecting cooked food with bacteria from raw food
  • Preparing food with unwashed hands
  • Using food that has fallen onto the floor
  • Biting fingernails and licking fingers
  • Serving food on dirty plates
  • Washing up with dirty cloths

Creating ideal growth conditions for microbes

  • Not heating food to a high enough temperature Cooking food too soon
  • and letting it cool for too long and too slowly before serving Reheating
  • food slowly or insufficiently


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