Cashew Processing

Cashew nuts, which can be very aptly called a wonder nut due to its immense potential to offer a great variety of uses, is one of the most widely used in the nut family. Basically, placed between the peanuts and almonds in terms of affordability and usage, cashews are very profitable in terms of harvesting and processing.

Cashew nuts cannot be put into edible use as soon as the ripened fruit falls to the ground. The cashews are never plucked but rather allowed to fall by itself before being collected for processing. The cashew ripens as a fruit, which can also be called a false fruit as the nuts are outside these fruits. The edible nuts, which we normally know of, are outside the fruit and has a thick shell covering it.

In the whole fruit, all the different parts can be put to use, as the false fruit can be processed to make juices and wines, the cashew nuts themselves are very nutritious and high energy source snack food and even the shells that cover these nuts are processed to give various by product rising out of the oil extracted from these shells called CNSL (Cashew Nut Shell Liquid). More over these shells can also be used as low-cost fuel replacing the wood or oil as alternate fuel source.

Since the harvested cashews cannot be put into use immediately, it becomes necessary to prepare the same for use. This gives rise to cashew processing where the harvested cashew nuts are dried, the shells removed, testa removed, the cashew kernels cleaned, graded and probably value added by roasting / salting/spicing and finally marketed after packing it in convenient packs.

Raw Cashew Harvesting, Handling and Storage

Cashew nut is hand-picked and manually handled in various stages of operation – right from picking at farm level to processing at factory level and to storage and transportation.

To preserve the quality of harvested nuts, it is imperative that proper safeguards to be observed against natural and man-made contamination.

Quality of raw nuts in harvesting, handling and storage can be ensured by

  1. Proper Harvesting Techniques – only matured nuts from fully grown apple must be harvested.
  2. Proper Drying Methods – Nut should be dried immediately after harvesting / collection. Drying should be on hard ground, preferably concrete / slate drying yards.

    Quantity of nuts dried shall not exceed 20 Kg per sq. m. uniform drying can be achieved by constantly turning nuts over to prevent partial/ non-uniform heating / drying.
    While storing it is necessary to provide proper wooden dunnage to prevent damage to the cashew nut in bags by avoiding direct contact of bags to the godown floor. This can be economically achieved by keeping bamboo crisscrossed and tied and placed on the godown floor.

  3. Proper Storing Methods – Dried nuts should be stored in jute / sisal bags [80 Kg per bag] and not heaped or stored in silos to avoid warm humid condition in storage room.

    Storage rooms must be fly proof and well ventilated with good exhaust fans and properly insulated to prevent water seepage.

Process Flow Chart

Steam Cooking

The raw cashew nuts sun dried and stored in the godowns are steamed in boiler [without steam pressurizing the cooker vessel] for about 30 minutes. The cooking time is varied depending upon the conditions of cashew nut and atmospheric conditions.

The steaming expands the shell, softens the nuts due to penetration of steam into the shell. After steaming, the nuts are air-cured by spreading out on the floor in the shade. This ultimately hardens the shell and makes it fit enough for de-shelling in the manually operated cutting machine.

This steam processing preserves the original colour of the Cashew kernel inside the nut. The cut shells of this steam roasting process yield quality Cashew Nut Shell Liquid [C.N.S.L]

The proposed steam cooker is having the following specifications.

  • Capacity: 320 Kgs within 30 minutes
  • Operating steam pressure: 7 bar max.
  • Water feed operation: Electrical pump
  • Fuel: Wood/Cashew nut shell
  • Power: 2HP
  • Supply: 440 VAC

This comes as a whole system with a small boiler integrated with a cooker. The standard size of a cooker is one, which has a capacity of 320 Kg, and the boiler is matched with the cooker so as to complete the steaming of a whole batch in 30 minutes.

Boiler is shell/wood fired for economy and has three stages viz. preheater, water chamber and steam drum. The boiler is provided with a safety Valve to lift at 8 bar. The water level is maintained with an electrical pump and auto cut-off system.

The boiler can be filled with 320 kgs. of raw cashew nuts for a single batch. The steam from the boiler is fed into the cooler for 30 minutes at 7-bar pressure. The cooked cashews are then taken out by allowing it to fall by gravity. The whole system is mounted on a specially designed and made civil construction.

Steam bath process is having the following advantages over traditional roasting processes.

  1. Cashew nuts do not get scorched.
  2. Kernels so produced is white and is of superior quality.
  3. Further processing operations become easier. Kernels shrink in size and becomes easy to detach from the shell thus decorticator is not necessary.

Shelling

Shelling is the process in which the cashew kernels are finally separated from its protective shell. This is a very critical operation as much success of the processing itself lies in this area of operation. The key idea of the shelling is to ensure that the kernels are dislodged from the shells without damaging the kernels themselves, since any blemish / damage to the kernels will considerably increase the further processing difficulties while reducing the prices the kernels fetch in the market impacting the bottom lines of the operation.

In earlier days, even as back as only a few years, the cashew shelling typically was done is Hand and leg operated machine, operated by two workers, working 8 hours a day to shell about 80 kg (one bag). The efficiency after a small learning curve if 3 to 5 months could reach as high as 96%, meaning just 4% of the kernels are damaged in the process of shelling. However high labour cost, unpredictability and associated issues has pushed the industry to look for automatic shellers. While the efficiency per se has not reached the machine assisted manual system, the overall return on investment is impressive, with the payback period for the capital cost being recovered in 12 to 15 months period.

The shelling machinery in itself is a series of machinery that includes a RCN calibrator, shelling machine and vibratory grade separator. Depending on the necessity, a bucket conveyor could be used to load the shelling machine, though not very necessary.

The calibrator grades the RCN as per size, normally into four major sizes and each grade is fed separately into different set of shelling machine. The shelling machine in its core unit is a set of two specifically shaped blades that cut into the RCN and split them for the kernels to be scooped out. The blades are moved using a system of roller s and cams and hence sizing the RCN becomes very important. The RCN is fed into the blade one by one through a system of channel. The vibratory grade separator separates the cut RCN into wholes, brokens and uncuts. Each machine is capable of handling about 200 Kgs of RCN per shift and hence will be configured into a cluster depending upon the plant size and area od procurement. There is a 10% to 15% uncuts that can be recycled through the machine and / or fed separately to manual shelling operation using the manual hand and leg operated machines, which is also used to shell the very small and very large RCN that is separated in the grade separator.

Drying – Heat Treatment Of Unpeeled Kernels [Borma Treatment]

The kernels after they are removed from the shells have to be dried in order to loosen the red skin adhering to the kernel for easy peeling. A few processors adopt conventional tray drier commonly known as “Borma”. The Borma chamber outer sides are heated by means of flue gas generated by burning oil extracted shell-cake at the bottom. The inner chamber heats up by radiation of heat.

The above system is becoming obsolete due to poor heat transfer efficiency. In the absence of air-circulation inside the chamber, heating is non-uniform. This results in over heating of kernels causing scorching of kernels inspire changing of trays and turning the kernels in the trays periodically.

The conventional “Borma” is presently replaced by more efficient electrical/diesel operated tray driers. This system ensures uniform heating of kernels for a pre-set temperature [80C] and time [4½ to 5 hours] by means of reliable electronic controller and timer. This system reduces the heating time considerably and does not requires periodic changing of trays during heating, as uniform heat is maintained throughout the drier by forced air-circulation. This system preserves the original colour of the Cashew Kernel and quality consistency is achieved within a batch and in every batch.

Nominal cap. 500kg. Control panel fitted with electronic controller, Electronic timer [with digital reading] indicator lamps, main ON/OFF switch, motor ON/OFF push buttons and mode selector switch HRC fuses motor and heater contactor. Shell fired Heat Exchanger, Blower and Ducting. Inner chamber of the drier made of mild steel 18swg thickness. The drier includes 4 trolleys and 100 trays. Trolley made of 16SWG thick mild steel angle Frames. Bottom four number Heat resistant fibre wheel, back Revolving type and front fixed Type. Tray size 32” x16”x 1.25” Made of mild steel 18SWG thick. Each trolley to hold 25 trays of size 32” x 16” x 1.25”. [Other specific caution as per technical data.] Power Req.11 Hp.

The shelled kernel is covered with the testa and to facilitate removal, i.e. to peel in order to produce the blanched kernel, the shelled kernel is dried. This 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 important that the drying capacity is higher than the shelling capacity in case of periods of heavy rainfall because the drying operation will be lengthened since the kernels will absorb the moisture very quickly. Sun drying, where the kernels are spread out in thin layers under strong sunshine is possible, however artificial drying becomes necessary for medium or larger-scale producers. Drying usually takes six hours, at a temperature of around 80-90 deg C. A uniform temperature throughout the drier is essential to avoid under drying or scorching. 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.

Humidification of Unpeeled Cashew Kernels

The Borma treated cashew kernels are humidified [Moisturised by Water-mist] by means of electrical humidifier. The cashew kernels in trays are kept in trolleys are humidified in a closed room for a period depending on the atmospheric condition and moisture level of cashew kernel. The treatment time varies anywhere between 15 mts to 20mts. Depending on the condition of the cashew kernel under treatment and climatic conditions.

The brown skin [Testa] of the humidified kernels undergoes shrinkage due to sudden cooling. This helps to dislodge the hold [Adhesion] of skin to the kernel.

Peeling

Traditionally peeling was done manually with each worker being able to peel upto 20 Kgs of kernels per day. This is the process in which the testa or the skin is removed from the kernel and is highly critical as the kernel are moisture controlled to about 3 to 4% and hence are very brittle. Careless handling will break the kernels thus reducing the price instantaneously. An unskilled labour could pick the nuance of peeling within a period of 3 to 4 months and turn out impressive efficiency in terms of breakages. However, as in case of shelling, unpredictability and associated issues with the labour force has introduced the mechanized peelers, though with lower efficiency but providing better overall returns.

Featured above is a double head cashew peeling machine with pieces separator. Feeding into the hopper at the top automatically produces peeled kernels that is further separated into wholes, broken and pieces/splits thus helping the grading process immensely. The capacity of this machine is about 100 to 120 Kgs per hour and produces about 10 to 15% of unpeeleds and damages.

Grading

The peeled kernels are manually graded by skilled labour. The cashew kernels are graded on the basis of shape, size and colour. International specification and standard for grading are widely followed in the industry.

The colour grade chart officially issued by the Cashew Export Promotion Council of India serves as a guide for colour grading.

Normally a skilled labourer can grade anywhere between 20 to 60kg. of Cashew kernel per day depending upon grader’s skill and the size of the kernel being graded. For grading brokens into LWP, SWP and BB grades mechanised wire-mesh sieves are used.

Conditioning of Graded Cashew Kernel

The moisture percentage permitted in the graded cashew kernel is 5% max. by weight at the port of importation. While a moisture level above 4% at the time of final packing is not preferred as the same may cause fungi and aid infestation, low moisture level below 3% is undesirable as it causes breakage of kernel during shipment. Processors by and large control the final moisture level between 3 to 4%.
Cashew kernel are humidified to increase moisture percentage to the required level, heat treated to reduce the excess moisture.

Inspection, Filling and Packing

Packing is the most critical aspect of the cashew processing, in a sense that, the shelf life of the product and hence the quality of the product when it reaches the end customer depends on the efficiency of the packing.

It is generally believed that the product shelf life is increased if the product is insulated from oxygen and thus provided with less opportunity for oxidizing. This is achieved in two ways – either by replacing the atmosphere around the product with inert gas or completely removing the atmosphere by vacuumising. Once either of the above is achieved the product cover (in this case the pouch) is hermetically sealed to maintain the new atmosphere as long as possible.

Hence the efficiency of the system depends on the efficiency of reducing the oxygen rich atmosphere around the product, efficiency of sealing and maintaining the inert atmosphere around the product, efficiency of cleaning the product of other impurities and efficiency of correct weight.

In this case, the cashew kernel (end product) is first removed of pieces & debris using dust collector, packed into a plastic pouch, weighed correctly, vacuumed of oxygen rich atmosphere, flushed with inert gas (to replace the remaining atmosphere as well as maintaining equilibrium and reducing the breakages), sealed and released as a smart pack.

The inspection line that precedes the packing machine ensures that no foreign object get into the packed kernels and using visual, magnetic and pneumatic separators helps in removing all foreign matter. It is highly important that this line is established in very clean and sterile atmosphere and workers, though small in numbers, are trained to be hygienic.

Key Technical Issues in Cashew Processing

It has to be understood that the cashew processing, as in any other food processing, is very delicate and while machinery provides assistance in key areas, a large amount of human intervention at various levels are needed to ensure that the profitability and quality is not compromised. Thus, an important factor in the processing is processing knowhow refined over years of experience and use of modern techniques and computing to control what is processed, how it is processed and packed. Thus, along with the machinery and know how, the processes set up, hygiene & housekeeping and Processing Information System will play a key role in maximizing the efficiency of the plant.