Supercrop: the yam bean, a tuber undaunted by drought, poor soil, or insects, produces astonishing yields. The crop is the focus of a worldwide effort to unlock its potential

Natural History, April, 2004 by Marten Sorensen

If you have ever sampled the menu at restaurants that offer such showstoppers as salmon encrusted with black sesame seeds, or breast of chicken in mango-ginger sauce, or mesclun salad with pear slices and Gorgonzola, there's a good chance you have come face to flesh with the ingloriously named yam bean. If you're a fan of Malaysian or Thai cuisine, there's an even better chance you've eaten it. How ironic, then, that although you've probably (albeit unknowingly) enjoyed at least one species of yam bean--the jicama--and although your local supermarket may well have these large, pale beige, flaky skinned, vaguely turnip-shaped tubers in stock, you've probably walked right past the small mound of them without giving them a second thought.

But if there is anything at all rational about the future consumption of food crops (a big "if"), the yam bean could become the hottest food item of the year. Few people realize what a treat they'd have in store if they merely took one home from the grocery, peeled it, cut it into slivers, and just crunched into it raw.

The yam bean (a name subsuming various members of the genus Pachyrhizus) is one of the most adaptable, low-maintenance, nutritious foods ever to grow in soil. Its agricultural properties, moreover, are well established: yam beans, indigenous to the tropical regions of the Western Hemisphere, have been grown there since well before the arrival of Columbus. The plant yields abundantly, produces well even in dry spells, does not need nitrogen fertilizers or good-quality soil, and resists pests and diseases. The harvest keeps well for months. Like the potato, the yam bean is a tuber--a fleshy subterranean stem. Like the bean and the pea, however, it is also a legume--generously endowed with proteins. And as a legume, its root nodules harbor symbiotic colonies of nitrogen-fixing bacteria: under good growing conditions, a season's crop of yam beans can release as much as fifty-three tons of usable nitrogen per acre into the soil.

Although the tuber is delicious eaten raw, there's also a truly dazzling variety of ways to prepare and consume the various species and cultivars of Pachyrhizus: as flour, juice, soup, tortillas, baby food, or moonshine; as a component of salad, a fruit, a stewed vegetable, or a preserved sweet; grated, sliced, squeezed, cooked, toasted, soaked, fried, dried, pickled, fermented, candied, mixed with milk in a porridge, or sprinkled with lime juice and chili powder as a street snack. With a bit of processing, every part of the plant becomes edible: the tubers, both young and mature; the seed pods, both young and mature, but always cooked; the oil from the pressed seeds; the leaves, as animal fodder. Pachyrhizus seeds also produce their own insecticide--which of course must be extracted before their oil can be used for cooking.

Yet despite the economic, environmental, and nutritional virtues of the yam bean, it is practically unknown in much of the West. In 1985, with support from the European Community's Science and Technology for Developing Countries program and the Danish Research Council for Development Research, I set up the Yam Bean Project. The aim of the project is to explore the plant's breeding and cultivation, as well as the potential for introducing it into new regions of the world. After all, out of some 6,000 cultivated species of plants, only a small fraction--perhaps nine or ten, such as the most commonly grown species of the cereals rice, wheat, maize, and millet; beans, especially soybeans; and several tuber and root crops, including potatoes, sweet potatoes, manioc, and taro--constitute the backbone of our planet's agriculture. Yet many less frequently cultivated species are well adapted to inhospitable growing conditions, and many crops that are nearly unknown globally may be--or may in earlier centuries have been--national or regional staples. The yam bean, tuber extraordinaire, is one such crop.

Of the three cultivated species of yam bean, the first to be scientifically recorded was the Mexican species P. erosus. That is the species commonly known as jicama (jicama in Spanish), and the one you've most likely tasted. It's also the species most widely grown today. The late-seventeenth-century English botanist Leonard Plukenet, gardener to the queen of England, depicted and described the jicama in his four-volume botanical catalog Phytographia; the eighteenth-century father of taxonomy, the Swedish botanist Carolus Linnaeus, relied on Phytographia for his pioneering Species Plantarum of 1753, a compilation of descriptions of roughly 6,000 plants from around the world, including P. erosus.

Archaeological evidence indicates that the jicama was grown by all the major early Mesoamerican civilizations, including the Olmec, Maya, Toltec, and Aztec. The Spanish introduced it to the Philippines in the sixteenth century; cultivation then spread to Southeast Asia, the Far East (including China), and the islands of the Pacific. Because yam beans stay fresh without being refrigerated, they may have been eaten on extended sea voyages in subsequent centuries, and those voyages may help account for the plant's almost pantropical cultivation. In 1821, believing the jicama to be an East Asian crop, the French botanist and explorer Gustave Samuel Perrottet played a central role in spreading the species westward from Indonesia. He introduced jicama seeds to the islands of Mauritius and Reunion in the Indian Ocean, proceeded on to what is now Senegal, in West Africa, and finally planted jicama in French Guiana, on the north coast of South America. In so doing, he came close to reintroducing the yam bean to its homeland.