The groundnut or peanut,(Arachis hypogaea), is a species in the legume Fabaceae native to South America, Mexico and Central America. It is an annual herbaceous plant growing to 30 to 50 cm tall. The leaves are opposite, pinnate with four leaflets. The flowers are a typical peaflower in shape, 2 to 4 cm across, yellow with reddish veining. After pollination, the fruit develops into a legume 3 to 7 cm long, containing 1 to 4 seeds, which forces its way underground to mature.
The flower of the Arachis hypogaea is borne aboveground. After it withers, the stalk elongates, bends down, and forces the ovary underground. When the seed is mature, the seed coat changes color from white to a reddish brown. The entire plant, including most of the roots, is removed from the soil during harvesting. The pods begin in the orange veined, yellow petaled, pea-like flowers, which are borne in auxiliary clusters above ground. Following self-pollination, the flowers fade. The stalk at the base of the ovary, called the pedicel, elongates rapidly, and turns downward to bury the fruits several inches in the ground, where they complete their development.
The pods act in nutrient absorption. The fruits have wrinkled shells that are constricted between pairs of the two to four seeds per pod. The mature seeds resemble other legume seeds such as beans, but they have paper-thin seed coats, rather than the usual, hard legume seed coats. Peanuts grow best in light, sandy loam soil. They require five months of warm weather, and an annual rainfall of 500 to 1000 mm or the equivalent in irrigation water.
The pods ripen 120 to 150 days after the seeds are planted. If the crop is harvested too early, the pods will be unripe. If they are harvested late, the pods will snap off at the stalk, and will remain in the soil. Peanuts are particularly susceptible to contamination during growth and storage. Poor storage of peanuts can lead to an infection by the mold fungus Aspergillus flavus, releasing the toxic substance aflatoxin. The aflatoxin-producing molds exist throughout the peanut growing areas and may produce aflatoxin in peanuts when conditions are favorable to fungal growth.
Harvesting occurs in two stages. First 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 dirt. This allows the peanuts to slowly dry to a bit less than a third of their original moisture level over a period of 3-4 days. After the peanuts have dried sufficiently, they are threshed, removing the peanut pods from the rest of the bush.
