Legume

• 1. Introduction

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  Legume is the everyday name for the plant family Fabaceae, which holds some 19,000 known species in about 751 genera. That is roughly seven percent of all flowering plant species. Only two families on land are larger, Orchidaceae and Asteraceae. Yet most people meet this enormous family only as a bowl of lentils or a handful of peas. How does a plant pull its own fertilizer out of thin air? Why does the United Nations treat a dried bean differently from a green one? And how long have people been cooking these seeds?

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• 2. Pulse or Vegetable

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  The Food and Agriculture Organization of the United Nations, the FAO, reserves the word pulse for a legume crop harvested only for its dry seed. Green beans and green peas are therefore filed as vegetables. Soybeans and peanuts are left out because they are grown mainly for oil. Clover and alfalfa are left out because their seed is used only for sowing forage. Everyday speech blurs these lines, and many varieties used as dried pulses are also picked young, with the beans still in their pods.

  The FAO recognizes 11 primary pulses. Dry beans lead the list, and the common bean, Phaseolus vulgaris, covers the kidney, navy, pinto, black turtle and haricot types. Lima beans, adzuki beans, mung beans and black gram, also called urad, sit beside them. The list continues with dry broad beans, dry peas, chickpeas, cowpeas, pigeon peas, lentils, bambara groundnuts, vetch and lupins. A catch-all group called Pulses NES adds lablab, jack beans, winged beans, velvet beans and yam beans.

  Botanists define a legume by its fruit. It is a simple dry fruit that grows from a single carpel and usually splits open along a seam on two sides. That definition is wider than a farmer's. Scotch broom is a leguminous member of Fabaceae, but farmers tend to keep the word legume for food crops, so it rarely earns the label.

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• 3. Nitrogen Factories

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  Inside swellings on the roots called nodules, bacteria named Rhizobia do chemistry that most plants cannot. Many legumes host them, and plants of the genus Styphnolobium are a known exception. The bacteria take molecular nitrogen, written N2, from the air and turn it into ammonia, written NH3. Ammonia is then converted into ammonium, a form that some plants can use. Because the nodules supply nitrogen, legumes end up rich in nitrogen-bearing amino acids and in protein.

  When a legume dies in the field, for example after harvest, the nitrogen held in its remaining parts goes back to the soil. There the amino acids are converted to nitrate, which later plants can take up. In many traditional and organic farms, growers alternate legumes with non-legumes, or plant both together for part of a season. The field then receives enough nitrogen compounds that no nitrogen fertilizer needs to be added.

  Sri Lanka developed a practice called coconut-soybean intercropping. Grain legumes are grown among coconut palms, either as a companion planting or as a cash crop, for their protein, vegetable oil and effect on soil fertility. The system has a limit. After 3-4 years of continuous cropping, grain yields drop significantly.

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• 4. Ancient Beans

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  Between 70,000 and 40,000 years ago, Neanderthals and early modern humans were already cooking wild pulses. Farming came much later. Traces of pulse production along the Ravi River in Punjab, home of the Indus Valley civilisation, date to about 3300 BC. Lentils turn up in Egyptian pyramids and in cuneiform recipes. Dry pea seeds found in a Swiss village are believed to date to the Stone Age.

  Archaeological evidence suggests peas were grown in the eastern Mediterranean and Mesopotamia at least 5,000 years ago. They reached Britain as early as the 11th century. In China, the soybean was domesticated about 5,000 years ago from a wild vine called Glycine soja.

  In the Americas, the oldest known domesticated beans come from Guitarrero Cave in Peru and date to around the second millennium BCE. Genetic studies of the common bean show it originated in Mesoamerica and then spread southward alongside maize and squash, its traditional companion crops. The soybean reached the United States later. In 1770, Benjamin Franklin sent seeds from France to Philadelphia, and the domesticated soybean was introduced there.

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• 5. On the Plate

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  A 100-gram serving of cooked chickpeas supplies 18 percent of the Daily Value for protein. The same serving gives 30 percent for dietary fiber, 43 percent for folate and 52 percent for manganese. Legumes are also an excellent source of resistant starch. Bacteria in the large intestine break it down into short-chain fatty acids such as butyrate, which intestinal cells use for energy.

  Grain legumes are grown for their seeds, which feed people and animals, or for oils with industrial uses. The group includes beans, lentils, lupins, peas and peanuts. Between 2013 and 2017, products containing legumes grew by 39% in Europe. They are a key ingredient in vegan meat and dairy substitutes, and their use as a plant-based protein is rising in the world marketplace.

  Many cooks believe that salting legumes before cooking keeps them from softening. The belief is a misconception. Beans stay hard because they are old, or because of hard water or acidic ingredients in the pot. Salting early actually seasons them better.

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• 6. Fodder, Dye, and Timber

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  Alfalfa, clover, vetch, stylo and Arachis are sown in pastures and grazed by livestock. Other forage legumes, such as Leucaena and Albizia, are woody shrubs or trees. Livestock browse them, or people cut them regularly to supply fodder. Legume-based feeds improve animal performance compared with a diet of perennial grasses, because animals eat more, digest faster and convert feed more efficiently. In cattle farming, Gliricidia sepium is planted along field edges to give shade, and the cattle eat its leaves and bark.

  Lupins are farmed commercially for their flowers and are popular in gardens worldwide. Indigofera species are grown for dye, and Acacia species for natural gum. Leucaena, Cyamopsis and Sesbania are grown as fallow or green manure crops. Timber comes from numerous Acacia species and from Castanospermum australe. The honey locust, Gleditsia, can be used in agroforestry. What a single crop is used for often depends on how mature it is when harvested.

  Not every legume tree is harmless. The black locust, Robinia pseudoacacia, and the Kentucky coffeetree, Gymnocladus dioicus, carry poisonous elements. So do Laburnum and the woody climbing vine Wisteria.

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• 7. Pests and Storage

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  Tiny flies of the family Agromyzidae, nicknamed bean flies, are considered the most destructive pest of grain legumes in tropical and subtropical Asia, Africa, Australia and Oceania. They attack a very wide range of cultivated legumes. Infestation can begin at germination and last until harvest, and the flies can destroy an entire crop at an early stage. Black bean aphids are a serious pest of broad beans, and their common hosts include fathen, thistle and dock. Pea weevils and bean weevils leave characteristic semi-circular notches along leaf margins.

  Anthracnose, caused by Colletotrichum trifolii, and downy mildew, caused by Peronospora trifoliorum, are among the common legume diseases. So are fusarium wilt, caused by Fusarium oxysporum, and Southern blight, caused by Sclerotium rolfsii. Root knot, caused by Meloidogyne hapla, completes a long list of biotic problems. Abiotic problems include deficiencies of nitrogen, phosphorus, potassium, copper, magnesium, manganese, boron and zinc. Pollutants and toxic concentrations of minerals also harm the plants.

  Seed viability falls the longer seed sits in storage. Studies of vetch, broad beans and peas show that they last about 5 years. Humidity and temperature are the main factors affecting germination. For moisture contents between 5 and 14 percent, seed life is longer when storage temperature drops by 5 degrees Celsius.

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• 8. A Year for Pulses

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  The Sixty-eighth session of the United Nations General Assembly declared 2016 the International Year of Pulses. The Food and Agriculture Organization was nominated to run the year with governments, non-governmental organizations and other stakeholders. Its aim was to raise public awareness of the nutritional benefits of pulses. It framed them as part of sustainable food production that supports food security and nutrition.

  The year was also meant as an opening to link every stage of the food chain, so that pulse-based proteins would be put to better use. Organizers wanted to further global production of pulses and encourage better crop rotations. They also pointed to unresolved challenges in the global trade of pulses.

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