Cashew is a widely cultivated nut-bearing tree originating from South America, its natural range mostly limited to seasonally dry areas in the north of the continent, extending from the state of Pernambuco, in Brazil, north-west to the Guianas, Suriname, Venezuela and Colombia.
It is a slow to moderately fast-growing tree, depending on the growing conditions, reaching heights of up to 15 m (50 ft) on favourable sites and develops a densely leafy, rounded crown. On marginal sites, it is more typically 5 to 10 m (16 to 32 ft) tall with a wide-spreading crown, sometimes as wide as the tree is tall. Branching starts low on the trunk, which on young trees has smooth light brown bark, becoming rough as the tree ages.
Leaves oval, blunt-ended, large, up to 18 cm (7 in) long, emerging bronze-red, becoming dull green, prominently veined and with a thick, leathery texture. They are alternately arranged along the stems and remain on the tree in all seasons.
Flowers small and insignificant, creamy-white to pale yellow, changing to deep red with age. They are borne in pyramid-shaped clusters arising at the tips of the branches and come into bloom in the dry season, which extends from winter through to early spring in its native range.
The fertilised flowers are replaced by small, kidney-shaped fruit about 3 cm (1.2 in) long that are green when young, becoming grey-brown when mature. They have a thick, hard, oil-rich shell that surrounds a single seed. As the fruit matures, the stalk attaching it to the plant thickens and swells to form a large pear-shaped pseudo, or false fruit commonly known as the cashew apple. The cashew apple is about 6 to 12 cm (2.5 to 4.7 in) when fully grown, with thin yellow, orange or red skin, depending on the variety and has juicy pulp.
Cashew is cultivated mainly for its edible nuts, which are one of the most popular nuts in the world. The ripe fruit are harvested, separated from the false-fruit or cashew apple, then dried and roasted in the shell, which is afterwards peeled away leaving the creamy-white nut we know and eat. It is mostly eaten out of hand as a nut or is incorporated in dishes and recipes that call for nuts, including pestos, stir-fries, curries, baked goods, confectionery and desserts. It can also be processed into a nut-butter for spreading onto bread.
The nuts have a crude protein content of about 21% of their dry weight and those damaged and discarded during the shelling process are a good feed source for single stomach livestock, such as chickens.
The oil from the shell, extracted as a by-product of the roasting process, is a dark, viscous, tar-like substance. It dries on exposure to air and hence has found use in the manufacture of paints, varnishes and synthetic fibres, including natural plastics. It also has insect repelling properties, with good action against mosquito larvae.
The cashew apple pulp is edible and juicy, with an agreeable sweet to sub-acid flavour, but is astringent, due to a high tannin content, and is not usually eaten fresh. It is more commonly stewed with sugar, cloves and other spices and eaten as an accompaniment to ice-cream, or is used as a filling in pies and tarts. Other edibles made with the pulp include fruit syrup or cordial, jam, chutney, candied fruit, wine and brandy. In Goa, in southern India, cashew apples are fermented to make a liquor known as 'Feni'.
The bark yields a pale yellow, reddish or dark brown gum with adhesive properties and has been used in the past for bookbinding.
The wood is greyish to red-brown, moderately heavy, averaging out at about 500 kgs per cubic meter, with low natural resistance to decay and wood-boring insects. It is mostly used for firewood when available.
The flowers are a source of nectar for foraging honeybees and is reported as a major honey plant in India, Colombia and Guyana, but there does not appear to be much information on the honey itself, its colour, flavour or crystallisation properties.
Cashew nuts have a high edible oil content and are reported to be a good source of energy as well as protein for humans. They also contain high levels of Vitamins B1 (Thiamine), B2 (Riboflavin), B3 (Niacin) and K, as well as good supplies of Phosphorous and Potassium, while the fruit is rich in Vitamin C (Ascorbic acid).
The oil from the shell is corrosive and is used as a topical treatment against ringworm, warts, corns, as well as cracked skin on the sole of the feet.
Cashew trees need dry conditions during flowering and fruit-set to produce a good crop of quality nuts. High rainfall and humidity during this time encourage floral disease, particularly of the fungal type, leading to poor fruit-set and fruit development.
It is known to grow and fruit well in sub-humid to moderately humid subtropical and tropical climates, generally frost-free areas with annual lows of 18 to 25 °C, annual highs of 27 to 35 °C, annual rainfall of 1000 to 4000 mm and a dry season of 3 to 5 months. Cashew trees may fail to flower and fruit, or do so poorly, in areas where the average low of the coldest month is below 11 °C (52 °F).
Cashew is also cultivated with irrigation in areas receiving less than 1000 mm annual rainfall or with a long and pronounced dry season.
New plants are usually started from seed. Performs best on free-draining clay-loam, loam and sandy-loam or loamy-sand soils of a moderately acid to neutral nature, generally with a pH of 4.5 to 7.0 and on sites with full sun exposure.
Seedling trees start flowering and bearing at about four years old, yielding only a few nuts at first then gradually more over the years, producing up to 30 kgs (66 lbs) per year or more at their peak.
The oil in the shell is toxic and caustic, causing burning, blistering and swelling of the lips and mouth, and contact should be avoided between the shell and any sensitive part of the body. Fumes from roasting the shell are known to irritate the eyes and repository passage.
Cashew apple juice is known to permanently stain garments and other fabric-related items.
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