The peanut, also known as the groundnut, goober (US), or monkey nut (UK), and taxonomically classified as Arachis hypogaea, is a legume crop grown mainly for its edible seeds. It is widely grown in the tropics and subtropics, being important to both small and large commercial producers. It is classified as both a grain legume and, due to its high oil content, an oil crop. World annual production of shelled peanuts was 44 million tonnes in 2016, led by China with 38% of the world total. Atypically among legume crop plants, peanut pods develop underground (geocarpy) rather than above ground. With this characteristic in mind, the botanist Carl Linnaeus named the species hypogaea, which means “under the earth”.
As a legume, the peanut belongs to the botanical family Fabaceae; this is also known as Leguminosae, and commonly known as the bean, or pea, family. Like most other legumes, peanuts harbor symbiotic nitrogen-fixing bacteria in root nodules. This capacity to fix nitrogen means peanuts require less nitrogen-containing fertilizer and also improve soil fertility, making them valuable in crop rotations.
Peanuts are similar in taste and nutritional profile to tree nuts such as walnuts and almonds, and as a culinary nut are often served in similar ways in Western cuisines. The botanical definition of a “nut” is a fruit whose ovary wall becomes hard at maturity. Using this criterion, the peanut is not a typical nut.However, for culinary purposes and in common English language usage, peanuts are usually referred to as nuts.
Cultivated peanuts (A. hypogaea) arose from a hybrid between two wild species of peanut, thought to be A. duranensis and A. ipaensis.The initial hybrid would have been sterile, but spontaneous chromosome doubling restored its fertility, forming what is termed an amphidiploid or allotetraploid.Genetic analysis suggests the hybridization may have occurred only once and gave rise to A. monticola, a wild form of peanut that occurs in a few limited locations in northwestern Argentina, or in southeastern Bolivia, where the peanut landraces with the most wild-like features are grown today. and by artificial selection to A. hypogaea.
The process of domestication through artificial selection made A. hypogaea dramatically different from its wild relatives. The domesticated plants are bushier and more compact, and have a different pod structure and larger seeds. From this primary center of origin, cultivation spread and formed secondary and tertiary centers of diversity in Peru, Ecuador, Brazil, Paraguay, and Uruguay. Over time, thousands of peanut landraces evolved; these are classified into six botanical varieties and two subspecies (as listed in the peanut scientific classification table). Subspecies A. h. fastigiata types are more upright in their growth habit and have shorter crop cycles. Subspecies A. h. hypogaea types spread more on the ground and have longer crop cycles.
The oldest known archeological remains of pods have been dated at about 7,600 years old, possibly a wild species that was in cultivation, or A. hypogaea in the early phase of domestication. They were found in Peru, where dry climatic conditions are favorable for the preservation of organic material. Almost certainly, peanut cultivation antedated this at the center of origin where the climate is moister. Many pre-Columbian cultures, such as the Moche, depicted peanuts in their art. Cultivation was well-established in Mesoamerica before the Spanish arrived. There, the conquistadors found the tlālcacahuatl (the plant’s Nahuatl name) being offered for sale in the marketplace of Tenochtitlan. The peanut was later spread worldwide by European traders, and cultivation is now widespread in tropical and subtropical regions. In West Africa, it substantially replaced a crop plant from the same family, the Bambara groundnut, whose seed pods also develop underground. In Asia, it became an agricultural mainstay and this region is now the largest producer in the world.
In the English-speaking world, peanut growing is most important in the United States. Although it was mainly a garden crop for much of the colonial period, it was mostly used as animal feedstock until the 1930s.The United States Department of Agriculture initiated a program to encourage agricultural production and human consumption of peanuts in the late 19th and early 20th centuries.
Peanut is an annual herbaceous plant growing 30 to 50 cm (1.0 to 1.6 ft) tall. As a legume, it belongs to the botanical family Fabaceae (also known as Leguminosae, and commonly known as the bean or pea family). Like most other legumes, peanuts harbor symbiotic nitrogen-fixing bacteria in their root nodules.
The leaves are opposite and pinnate with four leaflets (two opposite pairs; no terminal leaflet); each leaflet is 1 to 7 cm (⅜ to 2¾ in) long and 1 to 3 cm (⅜ to 1 in) across. Like those of many other legumes, the leaves are nyctinastic; that is, they have “sleep” movements, closing at night.
The flowers are 1.0 to 1.5 cm (0.4 to 0.6 in) across, and yellowish orange with reddish veining.They are borne in axillary clusters on the stems above ground, and last for just one day. The ovary is located at the base of what appears to be the flower stem, but is actually a highly elongated floral cup.
Peanut pods develop underground, an unusual feature known as geocarpy.After fertilization, a short stalk at the base of the ovary (termed a pedicel) elongates to form a thread-like structure known as a “peg”. This peg grows down into the soil, and the tip, which contains the ovary, develops into a mature peanut pod. Pods are 3 to 7 cm (1.2 to 2.8 in) long, normally containing one to four seeds.
Peanut seeds separated into their halves showing the embryos with cotyledons and primordial root
Parts of the peanut include:
Shell – outer covering, in contact with dirt
Cotyledons (two) – main edible part
Seed coat – brown paper-like covering of the edible part
Radicle – embryonic root at the bottom of the cotyledon, which can be snapped off
Plumule – embryonic shoot emerging from the top of the radicle.
Peanuts grow best in light, sandy loam soil with a pH of 5.9–7. Their capacity to fix nitrogen means that, providing they nodulate properly, peanuts benefit little or not at all from nitrogen-containing fertilizer, and they improve soil fertility. Therefore, they are valuable in crop rotations. Also, the yield of the peanut crop itself is increased in rotations, through reduced diseases, pests and weeds. For example, in Texas, peanuts in a three-year rotation with corn yield 50% more than nonrotated peanuts.Adequate levels of phosphorus, potassium, calcium, magnesium, and micronutrients are also necessary for good yields.To develop well, peanuts need warm weather throughout the growing season. They can be grown with as little as 350 mm (14 in) of water,but for best yields need at least 500 mm (20 in).Depending on growing conditions and the cultivar of peanut, harvest is usually 90 to 130 days after planting for subspecies A. h. fastigiata types, and 120 to 150 days after planting for subspecies A. h. hypogaea types. Subspecies A. h. hypogaea types yield more, and are usually preferred where the growing seasons are sufficiently long.
Peanut plants continue to produce flowers when pods are developing; therefore even when they are ready for harvest, some pods are immature. In order to maximize yield, the timing of harvest is important. If it is too early, too many pods will be unripe; if too late, the pods will snap off at the stalk, and will remain in the soil.For harvesting, the entire plant, including most of the roots, is removed from the soil.The pods are covered with a network of raised veins and are constricted between seeds.
Harvesting occurs in two stages.In mechanized systems, a machine is used to cut off the main root of the peanut plant by cutting through the soil just below the level of the peanut pods. The machine lifts the “bush” from the ground and shakes it, then inverts the bush, leaving the plant upside down on the ground to keep the peanuts out of the soil. This allows the peanuts to dry slowly to a little less than a third of their original moisture level over a period of three to four days. Traditionally, peanuts were pulled and inverted by hand.
After the peanuts have dried sufficiently, they are threshed, removing the peanut pods from the rest of the bush. It is particularly important that peanuts are dried properly and stored in dry conditions. If they are too high in moisture, or if storage conditions are poor, they may become infected by the mold fungus Aspergillus flavus. Many strains of this fungus release toxic and highly carcinogenic substances called aflatoxins.
There are many peanut cultivars grown around the world. The market classes grown in the United States are Spanish, Runner, Virginia, and Valencia. Peanut production in the United States is divided into three major areas: the southeastern United States region which includes Alabama, Georgia, and Florida; the southwestern United States region which includes New Mexico, Oklahoma, and Texas; and the third region in the general eastern United States which includes Virginia, North Carolina, and South Carolina.
Certain cultivar groups are preferred for particular characteristics, such as differences in flavor, oil content, size, shape, and disease resistance.Most peanuts marketed in the shell are of the Virginia type, along with some Valencias selected for large size and the attractive appearance of the shell. Spanish peanuts are used mostly for peanut candy, salted nuts, and peanut butter.
The small Spanish types are grown in South Africa, and in the southwestern and southeastern United States. Prior to 1940, 90% of the peanuts grown in the US state of Georgia, were Spanish types, but the trend since then has been larger-seeded, higher-yielding, more disease-resistant cultivars. Spanish peanuts have a higher oil content than other types of peanuts. In the United States, the Spanish group is primarily grown in New Mexico, Oklahoma, and Texas.
Cultivars of the Spanish group include ‘Dixie Spanish’, ‘Improved Spanish 2B’, ‘GFA Spanish’, ‘Argentine’, ‘Spantex’, ‘Spanette’, ‘Shaffers Spanish’, ‘Natal Common (Spanish)’, “White Kernel Varieties’, ‘Starr’, ‘Comet’, ‘Florispan’, ‘Spanhoma’, ‘Spancross’, ‘OLin’, ‘Tamspan 90’, ‘AT 9899–14’, ‘Spanco’, ‘Wilco I’, ‘GG 2’, ‘GG 4’, ‘TMV 2’, and ‘Tamnut 06′.
Since 1940, the southeastern US region has seen a shift to production of Runner group peanuts. This shift is due to good flavor, better roasting characteristics and higher yields when compared to Spanish types, leading to food manufacturers’ preference for the use in peanut butter and salted nuts. Georgia’s production is now almost 100% Runner type.
Cultivars of Runners include ‘Southeastern Runner 56-15’, ‘Dixie Runner’, ‘Early Runner’, ‘Virginia Bunch 67’, ‘Bradford Runner’, ‘Egyptian Giant’ (also known as ‘Virginia Bunch’ and ‘Giant’), ‘Rhodesian Spanish Bunch’ (Valencia and Virginia Bunch), ‘North Carolina Runner 56-15’, ‘Florunner’, ‘Virugard’, ‘Georgia Green’, ‘Tamrun 96’, ‘Flavor Runner 458’, ‘Tamrun OL01’, ‘Tamrun OL02’ ‘AT-120’, ‘Andru-93’, ‘Southern Runner’, ‘AT1-1’, ‘Georgia Brown’, ‘GK-7’, and ‘AT-108’.
The large-seeded Virginia group peanuts are grown in the US states of Virginia, North Carolina, Tennessee, Texas, New Mexico, Oklahoma, and parts of Georgia. They are increasing in popularity due to demand for large peanuts for processing, particularly for salting, confections, and roasting in the shells.
Virginia group peanuts are either bunch or running in growth habit. The bunch type is upright to spreading. It attains a height of 45 to 55 cm (18 to 22 in), and a spread of 70 to 80 cm (28 to 31 in), with 80-to-90 cm (31-to-35 in) rows that seldom cover the ground. The pods are borne within 5 to 10 cm of the base of the plant.
Cultivars of Virginia type peanuts include ‘NC 7’, ‘NC 9’, ‘NC 10C’, ‘NC-V 11’, ‘VA 93B’, ‘NC 12C’, ‘VA-C 92R’, ‘Gregory’, ‘VA 98R’, ‘Perry’, ‘Wilson, ‘Hull’, ‘AT VC-2′ and’ Shulamit’.
Valencia group peanuts are coarse, and they have heavy reddish stems and large foliage. In the United States, large commercial production is primarily in the South Plains of West Texas, and in eastern New Mexico near and south of Portales, but they are grown on a small scale elsewhere in the South as the best-flavored and preferred type for boiled peanuts. They are comparatively tall, reaching a height of 125 cm (49 in) and a spread of 75 cm (30 in). Peanut pods are borne on pegs arising from the main stem and the side branches. Most of the pods are clustered around the base of the plant, and only a few are found several inches away. Valencia types are three- to five-seeded and smooth, with no constriction of the shell between the seeds. Seeds are oval and tightly crowded into the pods. Typical seed weight is 0.4 to 0.5 g. This type is used heavily for sale roasted and salted in-shell peanuts, and for peanut butter. Varieties include ‘Valencia A’ and ‘Valencia C’.
Tennessee Red and Tennessee White groups
These are alike, except for the color of the seed. Sometimes known also as Texas Red or White, the plants are similar to Valencia types, except the stems are green to greenish brown, and the pods are rough, irregular, and have a smaller proportion of kernels.