The cashew is one plant that gives two utterly different foods, and a great deal of confusion follows from it. Anacardium occidentale belongs to the Anacardiaceae, the family of the mango, the pistachio, the sumac, and, less happily, the poison ivy, and it shares with its poisonous cousins a caustic oil that shapes its whole story. What the tree bears is, first, a swollen, fleshy, pear-shaped 'apple', brilliant red or yellow, juicy, sweet, and astringent: this is the cashew apple, not a fruit at all but a hugely enlarged flower-stalk, the peduncle. Hanging from its lower end is the true fruit, a hard, kidney-shaped, grey-green drupe, and inside that, protected by a double shell, lies the single seed the world knows as the cashew nut.

The nut is never sold in its shell, and for good reason. Between the shell's two walls runs cashew nut shell liquid, a dark, blistering oil rich in anacardic acid and the same urushiol that makes poison ivy so feared; it raises welts on bare skin and gives off acrid, irritating fumes when burned. The nut must therefore be roasted and laboriously hand-shelled before it can be eaten, which is why, even now, the cashew is one of the most labour-intensive of all the world's nuts and why its processing settled in lands of cheap and skilled hand labour. Unusually amongst the great food plants, the cashew is a single species with a single origin and no clutch of separate domestications: its variety lies not in different kinds of plant but in the two quite different harvests, the perishable apple and the travelling nut, that one tree provides.

The cashew is a Brazilian native that conquered the tropics, and it did so almost entirely in the holds of Portuguese ships. The Tupi of north-eastern Brazil had long valued the cashew, above all the apple, which they ate fresh and fermented into a drink, and the nut, which they roasted; but it was the perishability of the apple and the portability of the seed that decided the plant's future. When the Portuguese reached Brazil they took an interest in the hardy, drought-proof tree, and from about the 1560s they carried it east, planting it first on their Indian territory at Goa, where it was valued at the outset less for food than for binding the crumbling laterite slopes of the coast against erosion. From Goa the cashew spread down the Konkan and Malabar coasts and inland, and India in time became the heart of the world's cashew trade, both the great grower and the great sheller of the nut.

On the same Portuguese tide the cashew crossed to East Africa, taking hold above all in Mozambique, where it became at once a food and, for centuries, a mainstay of the colonial economy; Mozambique was for a long time the largest cashew producer on earth. Carried on along the maritime trade routes of monsoon Asia, the tree reached Siam, the south of China, and the lands of mainland South-East Asia, where the nut entered the wok; in the twentieth century Vietnam rose to become the world's foremost processor of cashews, and West Africa, from Côte d'Ivoire to Guinea-Bissau, the foremost grower of the raw nut. Meanwhile the shelled kernel, mild, rich, and crescent-shaped, travelled everywhere as a luxury snack and, at the turn of the present century, found a wholly new vocation in the Western plant-based kitchen, blended into the creams and cheeses of vegan cooking. From a single tree on the north-eastern Brazilian shore, the cashew had become a crop of every tropical coast and a nut on every continent's table.

The cashew leads two lives. As a nut it is global: the buttery, kidney-shaped kernel is eaten roasted and salted by the handful from Mumbai to Manhattan, and it runs deep through the cooking of the lands that adopted it. India, the greatest consumer of all, grinds it into the silky gravies of the Mughlai korma and the festive cashew curry, and boils it with sugar into kaju katli, the wafer-thin, silver-leafed diamond that is the most prized of all Diwali sweets. The wok kitchens of Thailand and southern China made the stir-fry of chicken and cashews, gai pad med mamuang and yaoguo jiding, into dishes famous the world over, and Vietnam, now the planet's leading sheller of the nut, turns out the salt-roasted cashews, hat dieu rang muoi, that are its own beloved snack. In Mozambique the nut is ground into matapa, the great stew of cassava leaves and coconut, and baked into Maputo's bolo polana.

The cashew apple, by contrast, has stayed close to home, for it bruises and ferments within a day of picking and cannot travel as the nut does. In its Brazilian birthplace it is pressed for the ubiquitous caju juice and clarified into cajuína, the amber, heritage-listed drink of the north-east; in Goa it is distilled into feni, the fierce country spirit that is the Goan liquor above all others. And in the twenty-first century the cashew has been reinvented yet again, this time in the West, where its mildness and high fat let it be soaked and blended into a smooth cream that has become the foundation of vegan cooking, standing in for dairy in everything from cheese to ice cream. Grown across the tropics, processed in Asia, and prized on every table, the cashew is one of the most thoroughly globalised of all the foods the New World gave the Old.