The cashew nut is a ubiquitous snack, celebrated for its buttery texture and mild sweetness. Most consumers only encounter the smooth, pale, kidney-shaped kernel already shelled and ready to eat. This common presentation obscures the surprising reality of what the cashew looks like in its natural state. The raw product is hidden for a very good reason: it involves a distinct, double-shelled structure and a powerful, toxic compound that requires careful industrial handling.
The Cashew Apple and Nut Structure
The cashew comes from a small, evergreen tropical tree, Anacardium occidentale, native to Brazil. The nut does not grow in a husk or pod; instead, it is found dangling below a brightly colored, fleshy structure known as the cashew apple. This cashew apple is not the true fruit but is botanically classified as an accessory fruit, or pseudocarp, developing from the swollen stem of the flower.
When fully ripe, the cashew apple is a pear-shaped, yellow or red object, typically measuring 5 to 11 centimeters in length. The true fruit is the hard, grey, kidney-shaped drupe, which hangs conspicuously from the bottom end of the apple. This single, shell-encased seed contains the edible kernel that is eventually harvested. The overall appearance is unique, often described as a bell pepper with a boxing glove attached to its base.
The shell surrounding the kernel is thick and dense, composed of an outer shell (epicarp) and an inner shell (endocarp). These two layers sandwich a spongy, honeycomb-like matrix that is the source of the nut’s inherent danger. The kernel itself is curved and covered by a thin, reddish-brown seed coat called the testa. This complex, multi-layered structure heavily protects the kernel.
The Shell’s Toxic Composition
The cashew nut in its shell is never sold to the public because of the oil contained in the shell’s spongy layer. This layer is saturated with a dark reddish-brown, viscous substance known as Cashew Nut Shell Liquid (CNSL). CNSL is a mixture of potent phenolic compounds, primarily anacardic acid, which often makes up around 70% of the liquid.
The remaining compounds include cardol and cardanol, which contribute to the liquid’s corrosive properties. These chemicals are highly irritating and chemically analogous to urushiol, the allergenic oil found in poison ivy and poison oak. Direct contact with raw CNSL causes severe contact dermatitis, characterized by blistering, painful rashes, and chemical burns.
Exposure can lead to hyperpigmentation and serious internal issues if the fumes are inhaled or the substance is ingested. Historically, workers who manually processed raw cashews without proper protection suffered from persistent skin problems. This inherent toxicity dictates the entire process of safely turning the raw drupe into the finished product. The shell must be compromised and the liquid neutralized before the kernel can be safely extracted.
Safe Extraction and Preparation
The industrial process begins with heat treatment designed to neutralize the caustic CNSL compounds and make the shell brittle enough to crack. One common method involves placing the raw nuts in rotating drums and applying intense heat, known as drum roasting. Another technique is steam boiling, where the nuts are cooked in a pressurized steam chamber.
Steaming is preferred because it preserves the desirable white color of the kernel better than direct roasting methods. The intense heat from either method causes the CNSL within the shell layers to vaporize or degrade, rendering the remaining shell material non-toxic and easily breakable. The steam also helps separate the kernel from the inner shell wall, making the subsequent shelling step more efficient.
Once the heated nuts have cooled, they proceed to the shelling stage, where specialized machinery or manual cutters crack the outer shell to expose the kernel. Even after heat treatment, the exposed kernel is covered by the thin, inedible, reddish-brown testa, which must be removed. This final step, called peeling, is accomplished by drying the kernels further in hot chambers and then removing the skin mechanically or by hand to yield the pale, ready-to-eat nut.