The main commercial product of the cashew tree is the nut. In the main producing areas of East Africa and India, 95% or more of the apple crop is not eaten, as the taste is not popular. However, in some parts of South America and West Africa, local inhabitants regard the apple, rather than the nut kernel, as a delicacy. In Brazil, the apple is used to manufacture jams, and soft and alcoholic drinks. In Goa, in India, it is used to distill a cashew liquor called “feni”.
The cashew fruit is unusual in comparison with other tree nuts since the nut is outside the fruit. The cashew apple is an edible false fruit, attached to the externally born nut by a stem. In its raw state, the shell of the nut is leathery, not brittle. It contains the thick vesicant oil, CNSL, within a sponge-like interior. A thin testa skin surrounds the kernel and keeps it separated from the inside of the shell.
The primary products of cashew nuts are the kernels which have value as confectionery nuts. Cashew nut shell liquid (CNSL) is an important industrial raw material for resin manufacture and the shells can be burned to provide heat for the decorticating operation.
Processing Overview [click image to enlarge]
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. The CNSL is released when the nuts are roasted.
Its value makes collection in sufficient quantities economically advantageous. 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 be cracked either manually, using a hammer, or mechanically. Manually operated blade openers (as listed in the supplier’s section) are relatively inexpensive, 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 this or subsequent stages as whole kernels are more valuable than broken ones. Once the kernel is removed from the shell, it is dried, the testa is peeled off and the kernel is graded. Figure 3 gives an overview of cashew nut processing and the various choices in methods.
All raw nuts carry foreign matter, consisting of sand, stones, dried apple etc. The presence of foreign matter in the roasting operation can be avoided by cleaning the nuts. The raw nuts can be sieved by hand using a ¾ inch mesh sieve.
The next stage is to soak the nuts in water to avoid scorching them during the roasting operation. This can be done by placing the nuts in a 40-45 gallon drum or vat and filling it with water until all the nuts are covered. After being left to stand for about ten minutes, the water should be drained off via a plug near the base of the drum. The nuts should then be left for periods of not less than four hours in order to allow the water left on the surface of the nuts to be absorbed. The process of covering the nuts with water, draining and standing should be repeated with the same nuts about three times until a moisture content of 9% is reached.
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.
The application of heat to the nut releases the nut shell liquid and makes the shell brittle which facilitates the extraction of the kernel when breaking the shell open. Three methods of roasting exist: open pan, drum roasting and the “hot oil”method. The latter is more suitable to medium-scale operations with associated higher equipment costs and viability of CNSL collection.