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close this bookThe Winged Bean: High-Protein Crop for the Humid Tropics (BOSTID, 1981, 41 p.)
View the document(introduction...)
View the documentAcknowledgments
View the documentPreface
View the document1 Summary
View the document2 Agronomy
View the document3 Food Use and Nutritive Value
View the documentAppendix A. Pests and Diseases
View the documentAppendix B. Selected Readings
View the documentAdvisory Committee on Technology Innovation
View the documentBoard on Science and Technology for International Development

1 Summary

People in the hot, humid, tropical areas of the world need better plant sources of protein. But the soybean-the words's premier protein crop -grows well mainly in temperate climates. Although extensive research programs have been launched to adapt the soybean to tropical conditions, results have so far not achieved viable economic returns.

Some researchers are now hailing the winged bean (Psophocarpus tetragonolobus) as "a possible soybean for the tropics."

The winged bean is innately a tropical species. It is a poor man's crop that, until recently, was found chiefly in rural areas of Papua New Guinea and Southeast Asia. It grows in profusion in hot, humid equatorial countries such as Indonesia, Malaysia, Thailand, the Philippines, India, Bangladesh, Burma, and Sri Lanka.

The winged bean looks like a pole bean: a mass of twining, leafy stems climbing to heights of 4 m and more if the support is tall enough, set off by white, blue, deep-purple, or pink flowers that quickly develop into pods.

The pods are green, pink, purple, or red-20 cm long on average, and in some varieties as long as a man's forearm. They can occur in such abundance that they often enshroud the whole vine. Each pod has four sides and a square or oblong cross section. A distinctive flange or "wing" projects from each corner. When picked young, these pods lack noticeable fiber and make a succulent green vegetable that is eaten raw, steamed, boiled, stir-fried, or pickled to make a crisp, chewy delicacy.

But pods are only one of six different foods supplied by this plant, which has been described as "a supermarket on a stalk." With the winged bean, almost everything goes into the pot. Its leaves are cooked and eaten like spinach (they are rich in vitamin A, a deficiency of which blinds many tens of thousands of children each year in tropical countries); its succulent shoots resemble lacy thin asparagus; and its flowers, when steamed or fried, make a sweet garnish with the appearance and texture of mushrooms.

The winged bean seeds, however, have created the greatest interest internationally. They virtually duplicate soybeans in composition and nutritional value; both contain similar proportions of protein, oil, minerals, vitamins, essential amino acids, and other constituents. Once processed, both have similar high digestibility.

Perhaps the most unlikely feature of the winged bean is what, in some varieties, is produced underground. The plant's roots enlarge to form tubers the size of small carrots, if it is cultivated correctly. Their firm, ivory-white flesh has a pleasant nutty flavor when cooked. Highland tribesmen in Papua New Guinea esteem it so highly that they hold winged bean sing-sings (feasts) at harvest time. Winged bean tubers can be boiled, steamed, baked, fried or roasted. Thai food scientists have deep fried and salted them to make tasty snacks. The true value of these tubers became apparent only recently when it was found that they are exceptionally rich in protein, containing two to four times as much as potatoes, more than eight times as much as cassava.

What is known today about the winged bean is roughly equivalent to what was known about the soybean 60 years ago, shortly before its large-scale commercial production in the United States. Many of the advances in the genetic improvement and the processing of soybeans for food and feed could have similar application to the winged bean.

The winged bean may one day become as significant as the soybean in world agriculture. However, its potential for the immediate future is as a subsistence crop or as a cash crop for small markets.

The winged bean is unlikely to rival the soybean in the near future, largely because all varieties now available require staking to produce economic amounts of seeds. They are climbing plants, which if left unslaked, tangle into heaps of intertwined stems that produce few pods or seeds. Staking is laborious and relatively costly to carry out on a mass scale. The hope is that stiff-stem dwarf varieties (having short internodes) will be found somewhere in the winged bean's native region. So far, none has been located.

The winged bean, as noted, is a humid-tropic plant. Varieties currently available grow prolifically in temperate areas, but because of the long summer day length, will not produce seeds. The winged bean's flowering is initiated when days are short.

Like many legumes, the winged bean is able to convert nitrogen gas from the air into forms usable by plants. This is actually accomplished by soil bacteria belonging to the genus Rhizobium. The bacteria inhabit swellings (nodules) on the root surface. Within these nodules the rhizobia proliferate and thrive. They absorb air from the soil and "fix" the nitrogen. The plant, in turn, absorbs much of the nitrogenous product, which it transfixes to protein, some vitamins, and other nitrogen-containing compounds.