CASHEW NUTS AND CASHEW APPLES

Introduction

The cashew is one of the two costliest nuts marketed in quantity worldwide. Formerly second to the macadamia nut in retail value, it is now more expensive because of short supply. Few people in the northern hemisphere question why the cashew nut is never sold in the shell as peanuts, pecans, walnuts, and almonds often are. There is a good reason for this. The cashew, by nature, is much more complex than other nuts and a technical explanation of its processing is required.

Description

The cashew tree, Anacardium occidentale, of the family Anacardiaceae, is short-trunked, up to 13m high and normally with a very broad crown, although it is often stunted and bushy on coasts. There is an enormous, very old, wild cashew tree in Natal, Brazil, that covers 0.75 ha and is a great tourist attraction. Its leathery, evergreen leaves are clustered at the branch tips. The small, yellow-and-red flowers are borne in open sprays. The true fruit of the tree is the kidney-shaped, hard-shelled nut that is at first green, later turning an ashy brown; it grows to 3 cm or more in length and develops at the tip of a fleshy stalk. As the ‘nut’ matures, the stalk inflates to form a showy, pearshaped, smooth-skinned, succulent, juicy pseudofruit which is bright-red, orange, yellow, or two-tone and is usually viewed, and utilized, as a fruit. The weight of the expanded pseudofruit causes the nut to fall to the ground at its peak of maturity. A caustic oil in the honeycomblike cells within the double-layered shell of the ‘nut’ protects it from being destroyed by foragers that feed on the ‘apple.’ However, the oil seriously complicates the processing of the ‘nut’ and extraction of its kernel for food use.

Origin and Distribution

The cashew tree is native from southern Mexico to Peru, Brazil, and the West Indies. Wild stands extend along the coast of Brazil from Pará to Rio de Janeiro, and the nut and apple have always been of importance to the indigenous people. The cashew was one of the first tropical American fruit trees to be introduced by early Portuguese and Spanish voyagers into the tropics of the Old World. In the sixteenth century the Portuguese planted it, especially on the west coast of India and the east coast of Mozambique, in order to halt soil erosion and, in time, it formed extensive forests. The tree soon became commonly cultivated and naturalized at low altitudes in East, Central, and West Africa, South-east Asia, Sri Lanka, Malaysia, the Philippines, Mauritius, and the Seychelles. In addition, it has been planted around villages in most of the Pacific Islands, including Hawaii, and is occasionally grown in dooryards in southern Florida. The name ‘cashew’ is derived from the Brazilian acaju, usually abbreviated to caju. In the Orinoco region, the tree is known as pauji. In all the Spanish speaking countries of Latin America and the Caribbean area, the common name is maran˜ on, except in Venezuela where it is called merey. In French it is cajou or acajou.

Climate and Soil

The tree requires a tropical or subtropical climate. It is highly sensitive to frost when young, but later is able to withstand brief cold spells. It is well adapted to sandy soil, intolerant of heavy clay, and is drought- and salt-tolerant, succeeding on land too poor for most other crops. It thrives from sea level to an altitude of 1000 m.

Commercial Development

India pioneered the domestication and commercialization of the cashew and, for a long time, augmented supply by importing raw nuts from Mozambique. The 1950s saw large-scale planting of cashew trees in Africa, India, and Brazil. In north-eastern Brazil there was first a private plantation of about 1500 ha. Then, in 1957, a programme was begun to plant a million trees in the state of Ceará , the government supplying the seeds and a bonus for each tree set out in plots of 100 or more. From 1957 to 1972, a company in Pacajus set out 2000 ha. Another surge of government-stimulated plantings totalled nearly 136 000 ha by 1972, mostly in Ceará . Later, 18 000 ha were developed in Rio Grande do Norte. By 1986, more than 40 000 ha of cashew had been added in reforestation programs in Ceará , Piaui, Rio Grande do Norte, and Bahia. There have been sporadic attempts to develop cashew industries in Venezuela, Colombia, Peru, Guatemala, Belize, and Jamaica. Some have been moderately successful. The primary problem is that cashew trees are still mainly grown from seed and there is great variation in yield, and great variability in the form and size of the nuts from seedling trees. The nuts may range from 3 to 32 g in weight and the apples (mostly orange or yellow) from 15 to 650 g. Trees bearing small nuts tend to produce heavily, while trees with large nuts and apples are less productive and may bear in alternate years.

Much research in Mozambique and India has been devoted to techniques of vegetative propagation (by cuttings, grafting, budding, or air layering) to achieve uniformity, but it is still not widely practiced, mainly because of the costs involved. India has accomplished much in rejuvenation of old groves by top-working beheaded, unproductive trees. Side-veneer grafting has been recommended in Venezuela. In Trinidad and India air-layered trees have poor form and tend to blow over in the wind. Once a method of propagation has been adopted, there must be a search for the best types to be grown. An experimental planting of grafted trees was initiated in Ceará but did not prosper because of a prolonged drought. In 1980, a selected clone of a ‘dwarf’ cashew was grafted on to Anacardium microcarpum and the 40 trees set out fruited early and were highly drought-resistant. This trial encouraged experiments with other rootstocks and selection of high-quality clones, but low yields have caused such work to languish. There are still no commercial orchards of selected, vegetatively propagated trees.

Varieties

Generally, named varieties have been distinguished merely by color and size of the apple, such as the Vermelho (yellow) and Amarello Gigante (large yellow) of Brazil, and the Marañon Amarillo (yellow) and Marañon Rosado (red) of Colombia. Natural hybrids in Colombia have been named La Gigante (huge) del Rio Magdelena, La Larga (long) de Nazareth, and La Pequeña (small) del Meta. Usually the nut and apple are larger in the yellow forms than in the red, and the apple is less astringent. One name pertaining not to the apple but to the nut is the Jumbo grown by the late Edward B Smith, Crescent Estate, Trinidad, who has supplied air layers to people from South America wishing to establish this large-seeded type in their countries. In evaluating cashew trees, preference should be given to a cashew tree of slender, compact form because the cashew needs light to flower and fruit and bears only on its outer circumference. The interior of a broad-spreading tree is devoid of foliage and fruit. Likewise, if cashew trees are planted too close together, the branches that overlap or shade each other will be nonbearing and may die back. From an experimental plantation of compact trees, the grower should select those that produce a high yield of large nuts and multiply them vegetatively, rather than increasing labor costs by simply planting more and more seedling trees with yields varying from 5 to 100 kg per tree. There is variation also in the color of the young leaves of cashew trees. On some, the new shoots at the branch tips are purplish-red, while on others they are more or less yellow. The yellow leaves are heavily attacked by leaf-eating insects while the red, being richer in phenolic compounds, are naturally insect-resistant.

Diseases and Pests

In humid atmospheres and in seasons of excessive rainfall, cashew trees are subject to attack by the fungus Colletotrichum gloeosporioides, which causes anthracnose, a disease affecting the foliage, especially young shoots, the twigs, flowers, the apple, and the developing nut. A parasitic fungus, Oidium anarcardii, appears on the leaves, shoots, and flowers when there are periods of alternating sun and rain. They become coated with a powdery white substance and this is followed by blackening, shrivelling, and shedding. The nut may be ruined by the fungus, Nematospora spp., which invades it through minute perforations caused by sucking insects. A large wasp sucks the juice from immature apples, causing them to shrivel and blacken while the nut is still green. Yellow, red-banded thrips (Selenothrips rubrocinctus Giard) often infest the foliage. Cashew apples are commonly eaten by birds, bats, monkeys, and squirrels.

Harvesting and Drying

Harvesting of cashew nuts is extremely labor-intensive. The entire crop does not ripen at one time, and harvesting may extend over a period of 40–75 days. Ideally the nuts should be allowed to fall (with the apple attached) when perfectly mature. Some people unwisely shake the branches to bring them down or knock them off with a pole. This results in slightly immature nuts of high moisture content being mixed with partly dried mature nuts and makes it impossible to assure uniform drying, thereby adversely affecting all subsequent steps in processing. In Brazil, cashews are plucked from the tree only if perfect apples are desired for dessert use. If allowed to fall, the apple will remain in good condition on the ground for 2–3 days. If it is not to be utilized, it is twisted off and left on the ground for cattle or pigs to consume. If it is wanted for preserving, it is salvaged and transported to a packing plant. The nuts are conveyed separately to be spread out in the sun and dried, with constant raking, and then stored if they are not to be immediately shelled. Thoroughly dried nuts can be stored for 1 or 2 years.

Processing

In Latin America and the West Indies there have been crude precracking practices such as boiling the nuts in steel barrels, simply drying them in the sun for 2 or 3 days, or sweating them in cement bins for several months. There was much loss because many nuts were found to be infested with insects or fungi on opening. In some cases the nuts were cut open by machete wielders with their hands wrapped in layers of plastic. The half-kernels were then picked out and sent to local clubs or bars, but these nuts were too contaminated with the cashew nut shell liquid Sanitary processing of cashews in the past was best performed in India by roasting the nuts in shallow pans over open fires to expel the caustic CNSL, some of which was caught in jars below for various local uses. Then they rubbed the nuts with sand or ashes before handshelling, at which the Indian female workers were especially adept. But the fumes from the roasting were extremely irritating to eyes, nose, throat, and skin, and the shellers frequently had to dip their hands in lime, ash, or linseed or castor oil to minimize skin inflammation. A merciful improvement was a change to roasting in rotating cylinders over furnaces with chimneys to carry off the toxic fumes. The next advance, brought about by Mortimer Harvey, of New Jersey, who had investigated the chemistry and potential industrial uses of the shell oil, was a method of heating the nuts in baths of the oil kept at 188–194ºC. This method resulted in greater recovery of higher-quality oil. Semi-mechanized shelling was initiated in Brazil in  1946. Dried raw nuts were first steamed to moisten the shells and then fed into a device, operated by hand and foot-pedal simultaneously, which cut the shell into two halves, leaving the kernel whole or cut in half. Facing the cutter, a seated operator pries the kernel loose from the cut shells if it adheres. This system is still used in Ceara´ for shelling very large nuts. When East African countries achieved independence from colonial governments, they wanted to cease exporting their raw cashews to India in order to gain more revenue by processing the nuts themselves. A simple, raw-cashew opening-machine was invented by a mining engineer in Tanzania in 1960. The operator protected his hands with barrier cream. There followed the development of complete factory installations of cashew-shelling equipment in Italy, France, and Germany. Mozambique obtained its first full-scale factory in 1964 and 2 years later set a world record in cashew production; most of the crop went to India for shelling, while the rest (26 000 t) of the locally shelled kernels was exported to the USA, West Germany, and South Africa. The various systems of cracking by pressure, centrifugal cracking, and cutting or sawing were evaluated by the Tropical Products Institute of London (now the Natural Products Institute), and the Sturtevants Engineering Company began producing and exporting the centrifugal shelling equipment in 1970. This system is now used by Brazil Oiticica, and modified versions are used in north-eastern Brazil. An Italian factory installation of a series of machines, designed by the Instituto Agronomico per l’Oltremare, involves 15 steps. The dried nuts are cleaned, roughly sorted according to size, wetted for several days to the desired moisture content, roasted in CNSL, centrifuged to eliminate any oil residue, cooled, and mechanically calibrated to separate eight sizes. The nuts are then machine-cut, and the shell separated from the kernel and removed by pneumatic tubes for use as a fuel. The kernels are dried for 48 h in the sun or in ovens until the testa wrinkles and is removed mechanically. The kernels are rehumidified to resist breaking; they are then sorted into wholes and pieces. Lastly, kernels that still have clinging testa must be hand-peeled. Japanese processors freeze the nuts instead of roasting them before shelling. However, none of these or other factory systems have given ideal results, mainly because of the variability of the raw material. There is a great need for a portable, simple cashew sheller that can take dehydrated, raw nuts of any size, splitting them without contamination of the kernel. The shells would be sent to facilities for total extraction of the oil for industrial purposes, and the split kernels of Jumbo cashews should not be inferior to whole kernels from smaller nuts. Such shelling has been achieved experimentally with explosive decompression, but further refinement of the method is needed. The latest development in India is a cashew kernel drier, electronically controlled, with timer and alarm to avoid scorching, and adjustable dampers for air inflow; the drier turns out higher-quality kernels in 60–90 min.

Economic Status

In the past, the cashew ranked as India’s second dollar earner. After mechanization, India found that the total crop of its cashew plantations could not keep its big factories busy as the productivity of the trees declined after they reached 35 years of age. In Kerala, the leading growing area, cashew orchards have been cut down and replaced by more profitable rubber plantations. From 1980 to 1987, the World Bank assisted India in a cashew development project to improve old plantations and establish new ones. In 1990, adverse weather conditions in India and other major cashew areas seriously affected the supply of cashews for export. Indian production was reduced by 20–25%, causing a sharp increase in prices. Nevertheless, the demand for cashew kernels has increased, especially with the opening of markets in eastern Europe. In 1989, world export of cashew kernels from India, Brazil, Mozambique, Kenya, Tanzania, and other areas amounted to 90 000 t. If horticultural improvement could be brought up to the level of cashew engineering, this figure could be doubled or tripled, and cashews would be less expensive and more widely enjoyed.

Inspection, Grading, and Packing

Before export, cashew kernels go through rigid in spection systems, are classified according to grade – wholes, butts, splits, large and small pieces and baby bits, also white, scorched, and dessert – and are carefully vacuum-packed in 11.3-kg capacity metal containers, two to a crate. They are then held in cold storage at temperatures below 9ºC. For retail sale, cashew kernels are usually reroasted in olive, peanut, or other oil, an adhesive and salt are applied and the kernels are packed in glass jars or cans. In recent  years, many have been dry-roasted without salt. High grades have a minimum of split or broken kernels. Lower grades are mainly utilized by the bakery or confectionery industries. They are familiar ingredients in cookies, chocolate bars, and other products, and are made into cashew butter.

Cashew Apple

Because of the difficulty of cracking cashew nuts, many Latin Americans have habitually discarded the nut and consumed the cashew apple or, in Spanish, manzana del maran˜ on, large heaps of which are seen on native markets at the height of the season. As it is fibrous, the apple is massaged and the juice is squeezed into the mouth as a thirst-quencher, especially out in the countryside. Inferior types that are still astringent when ripe are eaten with salt so that they will not irritate the throat. In the kitchen, cashew apples may be sliced and stewed or used to make juice, fruit soup, jam, jelly, paste, or chutney. The best cashew apples are preserved whole in syrup in glass jars. The less perfect are candied – cooked thoroughly in heavy syrup until well wrinkled, and then dried and sometimes rolled in granulated sugar. This is an excellent product resembling candied figs but it is, of course, seedless. In India, the Central Food Technological Research Institute at Mysore has found that the astringency and acridity of cashew apple juice, due to 35% tannin content and 3% of an oily substance, can be eliminated by pressure steaming for 5–15 min and thorough washing. Then the juice is mechanically extracted. In the Philippines, the Bureau of Plant Industry, Manila, designed a cashew-apple-crushing machine for this purpose. To free the juice of some other undesirable elements, casein, gelatin, pectin, or lime juice may be added before straining or centrifuging. After the addition of sugar and citric acid to arrive at 15º Brix and 4% acidity, the juice is boiled for 1 min and then bottled or canned, alone or with other fruit juices. In Brazil, it is estimated that less than 10% of the total cashew apple crop is processed, though the juice fermented into wine. Cashew apple brandy is subject to government control in East Africa and Goa. In the late nineteenth century, in Mozambique, the cashew was denounced as a source of vice and ruin because of the highly intoxicating liquor distilled from the apple. Cultivation of the tree was banned for a while; later a tax was imposed on plantations as a possible deterrent.

Cashew Nut Shell Liquid

This toxic oil is highly heat- and friction-resistant and is valuable for many industrial uses. When it was first used in aircraft paint-stripping products, mechanics suffered skin reactions. Mortimer Harvey promptly patented processes of detoxification and its uses have multiplied. It has been standard material for automobile clutch facings and brake linings and in insulation for electrical tools. It is incorporated into marine paints, water-resistant plywood, resins utilized in laminating, varnishes, floor tiles, cold-setting cements, molding powders for plastics, and many other products. In addition, the testa, which is removed from cashew kernels, has a high level of condensed tannin (as much as 25%) and the extract has been used for tanning leather.

Written by J F Morton, University of Miami, Coral Gables, FL, USA in "Enciclopedia of Food Sciences and Nutrition", Academic Press, USA, Editor-in-chief Benjamin Caballero, excerpts pp.958-964. Digitized, adapted and illustrated to be posted by Leopoldo Costa.