Peanut

• 1. Fruit Beneath the Soil

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  The peanut, Arachis hypogaea, buries its own fruit, and its Latin species name means "under the earth." In 2023 the world produced 54 million tonnes of peanuts, with China supplying 36 percent of the total and India 19 percent. The plant has a long list of other names. It is called groundnut, goober pea, and pindar, and in Britain it is also the monkey nut. Goober and pindar reached American English through Kikongo. Despite the name, the peanut is not a true nut. It is a legume, a relative of peas and beans. How does a plant manage to ripen its fruit in the dark? Where did it come from, and how did a South American crop end up in African stews and American lunchboxes? And why can the same small seed feed hungry children, nourish livestock, and threaten the lives of people who are allergic to it?

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• 2. The Underground Fruit

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  In 1753 Carl Linnaeus described the species in his Species Plantarum. It is an annual herb that grows 30 to 50 cm tall. Each leaf is opposite and pinnate, with four leaflets in two facing pairs and no leaflet at the tip. The leaflets measure 1 to 7 cm long and 1 to 3 cm across. At night they fold shut in what botanists call sleep movements. The flowers are yellowish orange with reddish veining, 1 to 1.5 cm across, and grow in clusters along the stems.

  After a flower is fertilized, a short stalk at the base of the ovary begins to lengthen. It becomes a thread-like structure called a peg. The peg bends down and pushes into the soil, and the pod develops there. This underground fruiting is known as geocarpy, and it is unusual among legumes.

  The finished pod is 3 to 7 cm long and normally holds one to four seeds. Its shell is mostly mesocarp, crossed lengthwise by several large veins. Peanut skins also contain resveratrol, and the seeds hold dietary fiber in amounts similar to several tree nuts.

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• 3. Andean Origins

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  The genus Arachis is native to South America, east of the Andes, around Peru, Bolivia, Argentina, and Brazil. The cultivated peanut came from a cross between two wild species, thought to be A. duranensis and A. ipaensis. That first hybrid should have been sterile. Spontaneous chromosome doubling restored its fertility and produced what scientists call an allotetraploid. Genetic analysis suggests the cross happened only once. It gave rise to A. monticola, a wild peanut found in a few places in northwestern Argentina. Southeastern Bolivia is another candidate for the origin, and the landraces with the most wild-like features grow there today.

  Artificial selection then reshaped the plant. Domesticated peanuts are bushier and more compact than their wild kin, with a different pod structure and larger seeds. Cultivation spread outward to form secondary and tertiary centers of diversity in Peru, Ecuador, Brazil, Paraguay, and Uruguay. Thousands of landraces evolved. They are grouped into six botanical varieties and two subspecies. The upright fastigiata types ripen on shorter crop cycles, while the hypogaea types sprawl along the ground and take longer.

  The oldest known pods, about 7,600 years old, were found in Peru, where dry conditions preserve plant remains. Cultivation almost certainly began earlier, in the moister center of origin. The Moche depicted peanuts in their art. Spanish conquistadors found the seeds for sale in the marketplace of Tenochtitlan, under a Nahuatl name that gave Spanish its word cacahuate.

  Peanut growing reached Europe in the 19th century through Spain, especially Valencia, where a small harvest continues. European traders carried the crop around the tropics and subtropics. In West Africa it largely replaced the Bambara groundnut, a relative whose pods also ripen underground.

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• 4. Rotation and Harvest

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  In Texas, peanuts grown in a three-year rotation with corn yield 50 percent more than peanuts grown year after year in the same field. Rotation helps because peanuts host nitrogen-fixing bacteria in their root nodules. When the nodules form properly, the crop needs little or no nitrogen fertilizer and leaves the soil richer. Rotation also cuts disease, pests, and weeds.

  The plant prefers light, sandy loam with a pH of 5.9 to 7, and it needs warmth all season. It can get by on 350 mm of water, but yields are best with at least 500 mm. Harvest comes 90 to 130 days after planting for the fastigiata types and 120 to 150 days for the hypogaea types.

  Timing is tricky because the plant keeps flowering while earlier pods fill. Some pods are still immature on harvest day. Dig too early and many pods are unripe. Dig too late and pods snap off at the stalk and stay in the ground. Farmers pull up the whole plant, roots included.

  A mechanized harvester slices the main root just below the pods, lifts the bush, shakes it, and flips it upside down. The pods rest off the soil and dry slowly over three to four days, to a little less than a third of their original moisture. Farmers once pulled and inverted the plants by hand. Threshing then separates the pods from the rest of the plant.

  Drought and heat are the main limits in semi-arid regions, especially before flowering, at flowering, and in early pod development. Pod borers, aphids, and mites attack the crop, along with leaf spots and rusts. The most worrying disease is the fungus Aspergillus.

  Severe drought during pod formation, or poor storage, can let the mold Aspergillus flavus produce carcinogenic aflatoxins. The USDA tests every truckload of raw peanuts and destroys any load above 15 parts per billion. Peanuts with high aflatoxin readings can still be pressed into oil, because the mold can be removed from the oil.

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• 5. The American Peanut

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  Peanuts arrived in the United States during the colonial period and began as a garden crop. Starting in 1870 they were mostly livestock feed, and human consumption did not grow until the 1930s. Through the 19th century, vendors sold them at fairs and from pushcarts.

  The spread paste came next. Marcellus Gilmore Edson of Montreal patented a manufacturing process in the U.S. in 1884, and peanut butter was first made in Canada. It became well known in America after the Beech-Nut company sold it at the St. Louis World's Fair in 1904. Around the same period the U.S. Department of Agriculture started a program to promote both growing and eating peanuts.

  George Washington Carver, who lived from 1864 to 1943, championed the crop in the American South. Repeated cotton plantings had depleted the soils there. Carver pushed peanuts through agricultural extension work and promoted hundreds of peanut-based products. They included cosmetics, paints, plastics, gasoline, and nitroglycerin.

  United States growers sort their crops into four market classes: Spanish, Runner, Virginia, and Valencia. Until 1940-90 percent of Georgia's peanuts were Spanish types. Since then, farmers have shifted toward larger-seeded cultivars that yield more and resist disease better. Naomi Chapman Woodroof is credited with building Georgia's peanut breeding program, which led to a harvest almost five times greater.

  Georgia now leads the country, with 49 percent of national peanut acreage and output. In 2014 farmers planted 591,000 acres and harvested 2.4 billion pounds. The most famous peanut farmer came from Sumter County, Georgia. Jimmy Carter became president of the United States in 1976.

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• 6. The Allergy Problem

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  Between 1.4 and 2 percent of people in Europe and the United States react badly to peanuts. Symptoms run from watery eyes to anaphylactic shock, and a reaction can be fatal.

  The hygiene hypothesis offers one explanation for the rise in food allergies. It holds that too little early childhood contact with germs and parasites may be a cause. Delaying a child's first exposure to peanuts can greatly raise the risk of allergy. Skin products containing peanut oil have been linked to allergy in children, though that evidence is inconclusive. Family history and soy intake are also associated with peanut allergy.

  Oil is a special case. Refined peanut oil does not trigger a reaction in most allergic people, and in the U.S. it is exempt from allergen labeling laws. Crude, unrefined oil still carries protein that may cause a reaction.

  Some U.S. school districts and others elsewhere have banned peanut products. A 2015 study in Canada then compared schools that prohibit peanuts with schools that allow them. It found no difference in the share of accidental exposures.

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• 7. Kitchens Around the World

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  In the Peruvian city of Arequipa, a dish called ocopa pours a smooth sauce of roasted peanuts and hot peppers over meat or potatoes. Both main ingredients are native to the region, and the sauce is finished with roasted onions, garlic, and oil. Another Peruvian dish, picante de cuy, serves roasted guinea pig in a ground peanut sauce. Onions and garlic came from Europe, so the dish marries two food worlds. Related sauces called ajíes, a word meaning hot peppers, top chicken or seafood.

  In Mexico, peanuts go into encacahuatado, a chicken dish in peanut sauce, and into red pipián, mole poblano, and oaxacan mole negro. Across Latin America they also become sweets, such as palanqueta, made from peanuts and honey. In Brazil, pé-de-moleque combines peanuts and molasses and resembles the Indian chikki.

  The Philippines has kare-kare, meat in a spicy peanut sauce. Indonesia has gado-gado, pecel, karedok, and ketoprak, all vegetable salads dressed with peanut sauce, plus satay.

  In the Indian subcontinent, chikki is a brittle made with refined sugar and jaggery. Cooks scatter roasted, crushed peanuts over salads and fold whole ones into leafy stews and rice for crunch. In South India, peanut chutney accompanies dosa and idli, and peanuts flavor pulihora tamarind rice.

  In Mali, peanuts enrich the meat stew maafe. In Ghana, peanut butter goes into the soup nkate nkwan, and crushed peanuts become nkate cake and kuli-kuli. Malawi and eastern Zambia serve peanut relishes beside nshima. Ugandan cooks boil ground peanut flour with cabbage, mushrooms, dried fish, meat, or vegetables to make the groundnut stew ebinyebwa.

  Boiled peanuts are a popular snack in India, China, West Africa, and the southern United States. In the American South, they are simmered for hours in briny water and sold at roadside stands. Canadians are enthusiastic eaters too. Ninety-five percent eat peanuts, averaging 3 kg per person each year. Americans eat about 6 lb of peanut products annually, at a retail cost of $2 billion.

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• 8. Butter, Oil, and Relief

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  Plumpy Nut, MANA Nutrition, and Medika Mamba are peanut-based pastes that are rich in protein, energy, and nutrients. Agencies including USAID use them as therapeutic food in famine relief in developing countries. Peanut milk, a lactose-free drink, was also developed to reduce malnutrition among children.

  In a 100 g serving, raw Valencia peanuts supply 2385 kJ of food energy and 9 g of dietary fiber. By weight they are 4 percent water, 48 percent fat, 25 percent protein, and 21 percent carbohydrate. They are a rich source of several B vitamins, vitamin E, manganese, magnesium, and phosphorus. Polyunsaturated and monounsaturated fats make up 83 percent of the fat. Studies link regular peanut eating to lower mortality from certain diseases, but their designs cannot show cause and effect. The FDA says the evidence suggests, without proving, that 1.5 ounces of most nuts a day may reduce heart disease risk. That applies to a diet low in saturated fat and cholesterol.

  Peanut butter is a spread of ground dry roasted peanuts, often with salt, sweeteners, or emulsifiers. Makers now sell chocolate, birthday cake, and cinnamon raisin versions. It fills sandwiches, most famously the peanut butter and jelly sandwich, and goes into granola bars and pastries. In the United States it accounts for half of peanut consumption and $850 million in annual retail sales.

  Other parts of the plant are useful too. The plant tops make silage. Hulls removed in processing feed ruminants, and the oilcake left after pressing becomes livestock feed and fertilizer. Groundnut cake is a valuable protein supplement for cattle and an important ingredient in poultry rations. Poorly stored cake can pick up aflatoxin.

  Peanut oil has a mild flavor, a high smoke point, and good resistance to rancidity. It is 46 percent monounsaturated fat, 32 percent polyunsaturated, and 17 percent saturated. Industry also uses it in paint, varnish, and furniture polish. A simple method using water and centrifugation can extract it from whole peanuts. NASA's Advanced Life Support program is considering that method for future long-duration human space missions.

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