Exploring Different Methods of Cashew Processing: Techniques and Equipment

The cashew kernel you snack on looks simple enough – pale ivory, lightly curved, ready to eat. But getting it to that point involves a precise, multi-step processing chain where each stage directly determines the quality and commercial value of the final product. From managing moisture in freshly harvested nuts to vacuum-sealing the finished kernels, cashew processing is both a science and a craft. Here is a close look at how it works, step by step.

Drying the raw cashew nuts 🔗

Processing starts before the nuts even enter a factory. At the time of harvest, raw cashew nuts carry 12 to 16% moisture – far too high for safe storage or quality processing. Drying brings this moisture level down to a maximum of 9%, which slows or completely prevents kernel deterioration during warehousing.

Sun drying is the most common and cost-effective approach. Nuts are spread on concrete drying floors or bamboo mats in layers no thicker than 10 cm, allowing roughly 60 kg of nuts per square metre, and are turned regularly to ensure even drying. In humid regions or large-scale facilities, mechanical dryers with controlled temperature and airflow are used to achieve uniform moisture reduction faster. After drying, nuts are stored in dry conditions with relative humidity below 65% and temperatures below 10°C until they are ready for processing – which can be up to 12 months after harvest, since the cashew season lasts only three to four months per year.

Calibration before heat treatment 🔗

Before roasting, the dried nuts go through calibration – a sorting step that groups them by diameter. Common size categories are 18 mm, 20 mm, 22 mm, 24 mm, and above 24 mm. Uniformly sized nuts are critical for consistent heat penetration during the next stage, and they also reduce mechanical shelling errors downstream.

Heat treatment: the three roasting methods 🔗

The central purpose of heat treatment is to soften or make brittle the hard outer shell so that it can be removed without damaging the kernel inside. The shell contains cashew nut shell liquid (CNSL), a caustic substance composed of anacardic acid and cardol that is a skin irritant and must be neutralised before the nut can be handled safely. There are three established methods for achieving this: drum roasting, oil bath roasting, and steam cooking.

Drum roasting 🔗

This is the oldest of the three methods. The nuts are placed in a tilted, rotating drum that is heated externally. The drum rotates for 3 to 5 minutes, causing the CNSL to ignite and burn off. The roasted nuts are then collected and covered with wood ash to absorb residual surface oil.

About 6 to 7 bags of cashew nuts (each weighing approximately 80 kg) can be roasted per hour using this method. Drum roasting is valued for its simplicity and low capital investment, and it typically produces a high rate of whole kernels. However, it generates significant heat and acrid fumes in the working area, causes the loss of most of the CNSL (which has considerable commercial value as an industrial raw material), and can result in uneven heating across batches. This method is largely being phased out in India in favour of steam cooking.

Oil bath roasting 🔗

In oil bath roasting, nuts held in wire trays are passed through a bath of cashew shell oil maintained at 200-202°C for approximately three minutes, extracting the CNSL from the shells in the process. The vessel is embedded in brickwork and heated by a furnace that uses spent shells as fuel.

This method can recover 85-90% of the available CNSL from the nuts, making it commercially attractive where CNSL extraction is a priority. However, CNSL that leaches into the processing oil contaminates it over time, requiring frequent oil changes or filtration systems, and an extra degreasing step is needed after roasting. Oil bath roasting is described as an outdated process in much of India but continues to be used in certain facilities where CNSL recovery is economically justified.

Steam cooking 🔗

Steam cooking has become the dominant and most widely recommended method in modern cashew processing. Raw nuts are cooked in a steel vessel for 20 to 30 minutes using steam generated by a boiler, then cooled to room temperature for 18 to 24 hours before shelling.

The boiler operates at 7.0-8.5 kg/cm² and the steam is injected into the cooker until it begins escaping from the outlet, indicating that the nuts are sufficiently cooked. This typically takes 10 to 20 minutes. There are two sub-variants: the static or fixed cooking vessel, which is more common in India, and the rotating drum steamer popular in Vietnam, where rotary steam cookers can handle batches of 640 kg or more, supporting higher throughput and greater mechanisation.

Steam cooking is preferred over the other two methods for several important reasons. It eliminates the open flame and the acrid CNSL fumes that characterise drum roasting, greatly improving worker safety. It preserves the natural colour and nutritional quality of the kernels better than direct heat methods. It also minimises the risk of kernel browning and reduces acrylamide formation compared with conventional dry heat roasting. Additionally, it allows for CNSL collection during the process, turning a hazardous by-product into a revenue stream used in resins, brake linings, and waterproof coatings.

Shelling: extracting the kernel 🔗

After heat treatment, the softened nuts are allowed to cool and harden slightly before shelling – the step where the outer shell is physically removed to expose the kernel. The objective is to obtain the highest percentage of whole, undamaged kernels, since this is the most critical factor determining processing output value.

Manual shelling remains widespread, especially in countries like India where skilled labour is available. Workers use leg-operated cutters: the nut is placed between contoured blades; pressing the pedal pierces the shell, and the handle splits it open so the kernel can be separated by hand. Mechanical shelling machines use calibrated cutting blades that make two precise cuts in the shell. Automated machinery achieves whole kernel recovery rates of 85-90%, significantly higher than the 65-75% typical of manual methods. The shells removed at this stage are used as fuel within the processing plant or for CNSL extraction.

Drying the shelled kernels 🔗

Freshly shelled kernels carry high moisture – typically 8 to 12% – which causes the testa (the inner skin) to adhere tightly to the kernel surface, making it very difficult to peel. The kernels must therefore be dried before peeling can proceed. Borma dryers are most commonly used for this purpose: air is heated indirectly from a furnace – fuelled by cashew shells – and forced through a mesh at temperatures of 70-100°C.

Kernel moisture is reduced from around 7% down to approximately 3% during this drying stage. Kernels meeting the 3-4% moisture level satisfy export standards and are easier to shell cleanly. After Borma drying, the kernels are cooled for 24 to 48 hours before peeling begins.

Testa removal (peeling) 🔗

The testa is the thin, brown skin that surrounds the dried kernel. It must be completely removed before the cashew can be graded or sold, as any residual testa causes kernel grade downgrades and can lead to rejection by export buyers.

To make peeling easier, the dried kernels undergo a brief humidification step. Kernels are exposed to steam or a fine water spray for 30 to 90 seconds, raising surface moisture to 5-6% while the kernel core remains at 3-4%. This moisture gradient softens the testa without making the kernel itself wet or sticky. The window for peeling after humidification is 2 to 4 hours – too short and the moisture hasn’t penetrated enough; too long and it redistributes through the kernel, reducing peelability.

Peeling can be done manually or mechanically. In manual peeling, workers gently rub the kernel between their fingers or use a small knife to lift off the testa. In mechanical peeling, the humidified kernel passes through a rotating cylinder fitted with brushes, which strip the testa away. Well-configured peeling machines can achieve 80-85% whole kernel recovery with complete testa removal. For premium export orders, machine peeling is followed by a manual finishing pass to bring residual testa below 3%.

Grading: classifying by size, colour, and quality 🔗

Once peeled, kernels are sorted into commercial grades. The internationally recognised classification scheme developed by the Association of Food Industries (AFI) grades kernels by colour into white (W), scorched (S), and desert (D) categories, and by form into whole kernels and broken kernels. Broken kernels are further subdivided into splits, butts, pieces, and baby bits.

Within whole kernels, size is expressed as the number of kernels per pound. W-180 denotes approximately 180 kernels per pound – the largest and most valuable grade – while higher numbers like W-450 indicate smaller kernels commanding lower prices. Grading is performed using calibrated vibratory sieves for size separation, optical or colour-sorting machines for colour uniformity, and trained inspectors for defect identification. Automated colour sorters achieve sorting accuracy above 99%, compared with 85-90% for manual grading alone.

Final moisture content of graded kernels should be 3-5% for optimal storage life and prevention of rancidity. Kernels failing this threshold or showing scorching, blemishes, shrivelling, or insect damage are downgraded or rejected according to AFI defect specifications.

Packaging: preserving quality to market 🔗

Cashew kernels are hygroscopic – they absorb atmospheric moisture readily – and are susceptible to oxidative rancidity and mould if packaging is inadequate. Kernels are vacuum-packed in foil pouches with air extracted, followed by nitrogen gas injection and automatic sealing to prevent oxidation and extend shelf life.

The AFI standards require cashews to be packed in new, clean, dry, leakproof, lead-free containers with an airtight hermetic seal. Only food-grade CO₂ is permitted in modified atmosphere packaging, at a minimum mix of 60% CO₂ with the balance as nitrogen. Storage under temperature-controlled conditions of 10-15°C and relative humidity below 65% can maintain cashew quality for 12 to 18 months. For bulk export shipments, kernels are packed in moisture-proof containers with labelling indicating grade, weight, and processing date.

Why each step matters 🔗

Cashew processing is a tightly linked chain. A poorly dried raw nut leads to mould in storage. Insufficient heat treatment makes shelling dangerous and difficult. An over-roasted nut discolours the kernel and reduces its grade. Inadequate humidification before peeling drives up breakage rates and lowers the proportion of high-value whole kernels. And poor packaging erases all the quality gains made in earlier stages. The Global Cashew Council emphasises that every stage must be conducted under clean, hygienic conditions with regular equipment and surface sanitation to prevent biological, chemical, and physical contamination. The choice between processing methods – particularly the shift from drum and oil bath roasting to steam cooking – also has direct implications for worker safety, environmental compliance, and CNSL by-product recovery.

What do you think? Given that steam cooking is now the dominant method for its efficiency and quality advantages, do you think small-scale processors in developing countries can feasibly transition away from drum roasting without significant financial support? And with over 26 internationally recognised cashew kernel grades, how do you think grading technology – manual versus automated sorting – shapes market access for processors in different countries?

References

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