The cashew-nut tree is a fast grower and an evergreen tropical tree. It grows to a height of 12 m. Blossoming takes place between November and January. Seedling trees flower in the third year after planting. The fruit ripens fully within 2 months.
The fruit (nut)
The nut is attached to the lower portion of the cashew apple which is conically shaped. The cashew nut (seed) hangs at the bottom of the apple, and is c-shaped.
The cashew seed has within the outside shell the edible kernel or nut. In its raw form the cashew kernel is soft, white and meaty. When roasted it changes colour and taste. Salted, it appeals to the palate as the most delicious nut.
Cashew apples and cashew nuts are excellent sources of nutrition. The cashew apple contains five times more vitamin C than an orange and contains more calcium, iron and vitamin B1 than other fruit such as citrus, avocados and bananas.
Cashew shell oil extracted from the shells is caustic and causes burns on the skin. The mucous membranes of the mouth and throat are severely affected when it comes into contact with shell oil or the irritating fumes emitted during roasting. The oily shell liquid has many uses.
Cashew trees are genuinely tropical and very frost sensitive.
The trees grow in a wide spectrum of climatic regions between the 25 °N and S latitudes.
Although the cashew can withstand high temperatures, a monthly mean of 25 °C is regarded as optimal.
Yearly rainfall of 1 000 mm is sufficient for production but 1 500 to 2 000 mm can be regarded as optimal.
The cashew tree has a well-developed root system and can tolerate drought conditions. Rain during the flowering season causes flower abortion due to anthracnose and mildew.
During harvesting, while nuts are on the ground, rain and overcast weather causes the nuts to rot or start germinating.
Nuts germinate within 4 days when lying on wet soil.
Trees grow well at Pongola, Hluhluwe, Mtubatuba and Makhathini where the climate can be described as warm-subtropical. The Natal coastal region north of Empangeni as well as the Pongola valley are suitable areas for cashew production. Regions in the interior, such as Malelane and Hoedspruit, with warm summers and winters are also suitable. In other subtropical regions of South Africa, where the absolute minimum temperature falls below 7 °C, commercial plantings will be at a high risk.
The cashew is a strong plant that is renowned for growing in soils, especially sandy soils, that are generally unsuitable for other fruit trees. For the best production deep, well-drained sandy or sandy-loam soil is recommended. Cashew trees will not grow in poorly-drained soils.
Self-pollination and cross-pollination play an important role in the formation of cashew seed. Seedlings therefore show great variation and no "true to type" trees can be grown from seed.
Selected trees should preferably be multiplied by grafting or air layering because vegetative propagation will ensure the best production and quality. Trees that are precocious bearers and grow vigorously are selected.
Nuts should weigh between 8 and 9 g with a density of not less than 1,0.
Trees with yellow to grey-brown apples have exhibited the most resistance against anthracnose and are associated with the best production.
Fresh seeds that sink in water are planted in an upright position in a planting bag containing a loose, sterilised soil mixture. Three to four seeds can be planted directly in the planting hole. The weakest ones are thinned out later and the strongest left to develop further. The seedlings are very susceptible to Phytophthora root rot. The plant bags should be 350 to 400 mm deep, as the tap-root grows very fast and bends around as soon as it touches the bottom.
Two grafting techniques, namely side grafting and wedge grafting are practised with success. Grafting should commence as soon as possible (seedlings of 3-4 months old) and planted out in the orchard to prevent the tap-root from bending.
Cashew seedlings are grown under shade (45 %) and hardened off before planting in the orchard. It is very important not to disturb the root system during planting. Young trees should be supported for the first 2 to 3 years so that wind will not blow the plants over.
Planting distances of 8 x 5 m is recommended. The trees grow vigorously in the first 3 years and as soon as the crowns touch each other alternate trees should be removed until the permanent planting distance of 10 to 12 m is reached. Branches hanging on the ground should be removed because they interfere with harvesting. In other parts of the world cashew trees bear well, in spite of the little attention devoted to the orchards.
Growth and production of cashew trees can be enhanced by establishing clonal orchards, and improving fertilising and irrigation practices
The application of nitrogen and phosphate are important. Approximately 75 g LAN and 200 g superphosphate per year age of the tree is applied annually with a maximum of 750 g LAN and 2 kg superphosphate. Cashew trees are subject to zinc deficiency that can be treated with 200 g zinc oxide/100 l water applied as a leaf spray.
Flowering to harvest time
Flowering is affected by weather conditions and also varies from tree to tree, but continues for a period of 3 months. High temperatures lead to earlier flowering. Both male and bisexual flowers are borne on one cluster. The flowers are very susceptible to mildew and control thereof on the leaves and flowers is a prerequisite for good production.
Pollination is mostly by insects. After pollination it takes 6 to 8 weeks for the fruit to develop. The nut develops first while the apple develops and enlarges only 2 weeks before fruit fall. Nuts should be harvested as soon as possible, especially under wet conditions and should be dried before storage.
Irrigation is important during establishment of young trees because it doubles the growth tempo of young trees in a dry season. Due to the deep root system the trees can survive several months without irrigation. Mature trees should receive 1 800 l of water per tree every 2 weeks.
Grass strips in the inter-rows between the tree lines are ideal to prevent erosion and should be cut regularly.
The objective of cashew processing is to extract the healthy, tasty kernel from the raw nut in the shell. Most modern factories are designed to obtain the maximum number of whole nuts and as much shell oil as possible. Processing can be subdivided into a series of steps.
Harvested nuts are dried in the sun for a few days. Properly dried nuts can be stored for 2 years before being shelled. Nuts are roasted to discharge the caustic shell oil and acrid fumes. Hand shelling is impossible if the shell oil has not been removed previously. Kernels must be protected from contamination by the shell oil because it would cause blisters in the mouth and throat when eaten. Before the nuts are roasted they must be soaked in water—the moisture in the shell facilitates the rupturing of the cells containing shell oil and retaining it in the shell. Moisture makes the kernel slightly rubbery and limits breakage of the kernels. The easiest method to wet the shells is to heap the nuts into big piles and to use sprinklers intermittently. Steam may also be used.
The simplest roasting method is to heat the nuts for about a minute in an open pan with holes. Acid fumes are released and if the nuts should catch fire the flames can be doused with water. A more efficient method is to use a slanting perforated cylinder that is rotated above a fire. The shell oil flows through the holes in the cylinder and is collected in a catch through. After the roasting process the nuts are dumped into ash or sawdust to remove the excess shell oil still clinging to the shells.
This is the most difficult operation in cashew processing. In India shelling is mostly done by cheap female labour. Shelling is carried out by using special wooden mallets and pieces of bent wire, at a rate of about 200 nuts per hour.
Mechanical shelling methods are difficult to design because of the irregular shape of the nut, hardness of the shell and brittleness of the kernel. In some mechanical processing plants compressed air is used to crack the nuts. The latest Windmer and Ernst method is to cut a groove around the shell and to place the shells in a modified centrifuge fitted with metal plates. The nuts are thrown against the plates and cracked by centrifugal forces when the machine spins. It is possible to obtain 85 % whole kernels with this method.
Removal of the testa
Before the thin, papery seed coat (testa) can be removed, the kernels must be dried. Nuts are dried on big racks in an oven at 70 °c. The testa becomes dry and brittle and is easily removed. The remaining traces of membrane are removed with bamboo knives. Modern factories use electronic machines to detect nuts with pieces of remaining testa which are then sorted and cleaned by hand.
Kernels, whole and broken, are sorted into 6 grading schedules. There is only a small demand for broken or dark and unevenly roasted kernels.
Kernels are dried to 3 % moisture content before they are packed.
Drying is necessary to extend shelf life and prevent fungal and other infections.
Dried kernels do not become rancid.
Nut kernels of export quality are vacuum packed in tins.
Shell oil represents about a quarter of the mass of an unshelled nut and approximately equal to that of the kernel. This fluid, that is not an oil as the term "shell oil" indicates, but a mixture of anacardic acid and cardol is the main by-product.
There are more than 200 registered patents of different uses of shell oil. One of the most important uses is in the manufacture of brake linings. Shell oil is used in the manufacture of numerous materials that have to be resistant to heat, friction, acids and caustic products, for example clutch plates, special isolators, varnish and plastic materials. The wood is insect repellent and used in making book cases and packing crates. The gum is a replacement for gum arabic and used as insect repellent glue in book bindings. In the nut and the apple, a compound has been found that combats tooth decay.
The apple is highly perishable but very healthy. It can be eaten fresh or juiced. Syrup, wine, brandy, gin, preserved fruit, pickles and glazed fruit are also made from the cashew apple. In Brazil, fresh cashew-apples are packed in trays and marketed in retail fresh produce outlets.
The indigenous people in cashew-producing regions use different parts of the plant such as the leaves, bark, gum, wood, juice and roots for the preparation of local medicines or insect-repellent mixtures. The bark is rich in tannins and is used in leather tanning. The papery seed coat around the kernel can serve as cattle feed.
India earns more than 200 million dollars a year by exporting 40 to 50 thousand tonnes of cashew kernels and the country's tradelinks are spread over 40 countries. Cashew is a craze in the United States which is by far the largest buyer. The other major purchasers are the eec countries, Japan, Australia, Canada, Hong Kong, Singapore and the countries in the Middle East.
Cashew is an excellent choice to grow around the house as a shade tree with healthy fruit (apple) and nuts.
Nuts determined the eating habits of prehistoric people. Along with berries and brook water, nuts followed humanity to civilisation. Cashew nuts are packed with proteins, fats and vitamins to a high degree.
Proteins, the tissue builders in our system, constitutes a large proportion of cashew contents. The cashew kernel contains 21 % of vegetable protein. Nutritionally they stand on a par with milk, eggs and meat. It also contains a high concentration of much-needed amino acids.