The cashew apple is a delicate accessory fruit. It's considered an accessory because it grows out of tissue other than the ovary of the flower. (In case you want a deep dive on flower ovaries, Wikipedia has you covered.)
Cashew apples go by the Spanish name marañón, and they can be made into juices, jams, and even a liqueur called fenny (reportedly, fenny is good with tonic water or cola). The ripe red and yellow apples are also used in Indian cooking, and can be used to sweeten up a curry. Unfortunately, the apples are delicate, and their juice can easily stain clothing -- in many countries, the apples are fed to livestock because they're so fragile.
Nutty Toxicity
The cashew nut seed is a kidney-shaped snack surrounded by several toxic layers intended to keep animals from gobbling it up. Outside the seed is a double shell filled with anacardic acid, which causes a skin rash on contact. On the bright side, the acid has medical uses -- it's mildly antibacterial and antifungal, and it's sometimes used to relieve toothaches. The acid can also be used to fight termites. In order to make the cashew seed edible, it's typically roasted to neutralize the acid -- but roasters urge caution, because the smoke that comes off the acid is itself toxic. Prior to becoming the rich beige we're familiar with, the seed itself is green as seen above.
Purdue University explains how the cashew made its way from Brazil to other tropical climates...and how the cashew apple was the main attraction for many years (emphasis added):
The cashew is native to northeast Brazil and, in the 16th Century, Portuguese traders introduced it to Mozambique and coastal India, but only as a soil retainer to stop erosion on the coasts. It flourished and ran wild and formed extensive forests in these locations and on nearby islands, and eventually it also became dispersed in East Africa and throughout the tropical lowlands of northern South America, Central America and the West Indies. It has been more or less casually planted in all warm regions and a few fruiting specimens are found in experimental stations and private gardens in southern Florida.
The production and processing of cashew nuts are complex and difficult problems. Because of the great handicap of the toxic shell oil, Latin Americans and West Indians over the years have been most enthusiastic about the succulent cashew apple and have generally thrown the nut away or processed it crudely on a limited scale, except in Brazil, where there is a highly developed cashew nut processing industry, especially in Ceara. In Mozambique, also, the apple reigned supreme for decades. Attention then focused on the nut, but, in 1972, the industrial potential of the juice and sirup from the estimated 2 million tons of surplus cashew apples was being investigated. In India, on the other hand, vast tonnages of cashew apples have largely gone to waste while that country pioneered in the utilization and promotion of the nut.
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