Another important industry in the district is the cashew industry. It was started for the first time in India in Mangalore in the second decade of this century. Mangalore is thus the birthplace of this industry. Thereafter, the industry spread to other parts of the country. Now, nearly 80 per cent of this industry in India is centered in and around Quilon in the Kerala State, and South Kanara accounts for about 12 per cent (on the basis of the exports made by the factories). The area under cashew nut cultivation in the district in 1971-72 was 1,00,94 acres. But the produce of the district is not sufficient to meet the requirements of the cashew nut factories. Hence large quantities of raw cashew nut are being imported from Africa. Some quantities are also obtained from the North Kanara district and also from Kerala State. The raw cashew nuts grown in India and made available to the cashew factories constitute hardly one-third of the total requirements of the cashew factories.
There are three varieties of raw cashew nuts used in the factories at Mangalore. They are:
1. Local nuts of Puttur, Vittal and other places, which are uniformly large and good for out-turn and oil contents
2. African nuts, which are smaller than the local nuts, and noted for their out-turn which are more by about 5 to 6 lbs, but their oil content is less by 10 to 14 lbs. per bag of 168 Ibs;
3. North Kanara nuts, which resemble those grown in Kundapura but are not so good as the local nuts. The cashew nut consists of a Kernel covered with a thin pink skin enclosed in a double walled shell, the space between the outer and inner walls being a thin-walled honeycomb structure. The outer wall is smooth, grey-green in colour and leathery. The honeycomb structure contains what is known as cashew shell liquid. The cashew kernel forms about 5 per cent of the whole nut by weight and its extraction requires special, and careful processing. The processes of extracting kernels from raw cashew nuts are roasting, shelling, drying and peeling.
As the shell of the cashew nut is leathery, it is necessary for the raw nuts to be roasted before the kernels can be extracted from them. Until 1955, the roasting process resulted in the loss of casesw liquid, which is a versatile product. It was only in 1935, that a process of roasting, whereby the cashew shell liquid could, be extracted was evolved. The nuts are roasted at high temperatures either in steel drums out of contact with air, or in cashew nut shell liquid oil bath (this process is known as Hot Oil Bath Process) to roast the outer hard shell and prepare them for being shelled. During the process of roasting, cashew kernel is extracted from the shell, which, at the same time, is made brittle and easier to break. The roasted nuts coming out of the roasting plant have a small coating of cashew shell liquid, which is removed by centrifuging before taking to the sellers for shelling.
After roasting, the nuts are to be shelled. This denotes the process of breaking open the outer thick shell and removing the kernel with its husk or peel attached to its surface. This is done either by hand shelling in which the roasted nuts are cracked open by beating with a wooden mallet provided with a metallic cap at the breaking end or by machine shelling which denotes shelling by cutting open the outer shell by a machine operated manually: the latter process is more economical and easy. This work requires patience and precision lest the kernels would be broken to pieces.
The shelled kernels have a pink skin covering which has to be removed before the kernels are made edible. This skin is attached fast to the kernel and can only be loosened after the kernels have been dried and the moisture in the peel or husk is removed by contact with hot air. After the kernels are kept for several hours in a hot room or a drying chamber, they lose much of their moisture and shrink slightly. Thereafter the thin skin is removed by hand without difficult. The process of removing the thin skin is known as “peeling”. This is again a precision job done exclusively by female labour. The kernels, which are dried, are then peeled by hand by means of a blunt metallic knife. Care is to be taken to avoid breakage of the kernels as much value will be lost if the kernels are allowed to break. As in the case of shelling, peeling has to be done with one kernel at a time.
The peeled kernels are then oraded into different grades depending on the size, colour, appearance, either wholes or pieces, and such other considerations. Grading is a delicate operation, done manually by hand picking. Although the accuracy of the cashew shell liquid is an important by-product obtained while processing cashew nuts and extracting the kernels. During the process of roasting most of the oil in the shell oozes out and this is collected. The shells obtained after shelling are crushed in expellers to separate the oil, which is left in the shells. The commercial shell oil obtained by the roasting of cashew nuts is dark brown viscous oil easily soluble in most of the organic solvents. Although not a glyceride, it is a drying oil. With suitable driers, it gives a smooth, shining film of dark-brown colour.
In India, the oil is employed as a waterproofing agent, and also as a preservative in the painting of boats, fishing nets and light woodwork. It is only in recent years, mainly, as a result of the investigations of Messrs Harvey and Caplan that the oil has attained technical importance, and America, Japan and U. K. are now the chief buyers of the oil produced in India. The industrial applications of the shell oil are based upon its polymerization to a rubber-like material under the influence of acids, and on the formation of a wide range of condensation products with aldehydes. The latter are generally hard, infusible and extremely resistant to the action of chemicals such as acids and alkalies. The cashew shell liquid is now used in the manufacture of brake liners, paints, varnishes, lacquers and insulators.