_Ground-nut oil_, _Earth-nut oil_, _Pea-nut oil_, _Arachis oil_; F.
_Huile d’Arachide ou de Pistache de terre_; G. _Erdnussöl_.
=Botanical Origin=—_Arachis hypogæa_ L., a diffuse herbaceous annual
plant, having stems a foot or two long, and solitary axillary flowers
with an extremely long filiform calyx-tube. After the flower withers,
the torus supporting the ovary becomes elongated as a rigid stalk,
which bends down to the ground and forces into it the young pod, which
matures its seeds some inches below the surface. The ripe pod is
oblong, cylindrical, about an inch in length, indehiscent, reticulated,
and contains one or two, or exceptionally even four irregularly ovoid
seeds.
The plant is cultivated for the sake of its nutritious oily seeds in
all tropical and subtropical countries, but especially on the west
coast of Africa. It is unknown in the wild state. De Candolle[738]
regards it as a native of Brazil, to which region the other species
of the genus exclusively belong. But the opinion of one of us[739]
is strongly in favour of the plant being indigenous to Tropical
Africa, and so is that also of Schweinfurth. Arachis is one of the
most universally cultivated plants throughout Tropical Africa, from
Senegambia to lake Tanganyika. In Europe it has not proved remunerative.
[738] _Géographie Botanique_, ii. (1855) 963.
[739] Flückiger, _Ueber die Erdnuss—Archiv der Pharmacie_, 190. (1869)
70-84, with figure.
=History=—The first writer to notice Ground-Nut appears to be
Fernandez de Oviedo y Valdes, who lived in Hayti from A.D. 1513 to
1525; he mentions in his _Cronica de las Indias_[740] that the Indians
cultivated very much the fruit _Mani_, a name still used for Arachis
in Cuba and in South America. A little later, Monardes,[741] described
a nameless subterraneous fruit, found about the river Maranon and
held in great esteem by both Indians and Spaniards. But before, the
French colonists sent in 1555 by Admiral Coligny to the Brazilian coast
had become acquainted with the “Mandobi,” which Jean de Léry[742]
described quite unmistakably. Good accounts and figures of it were
given in the following century by Johannes de Laet (1625),[743] and
by Marcgraf,[744] who calls it by its Brazilian name of _Mundubi_. It
is enumerated by Stisser among the rare plants cultivated by him at
Helmstedt (Brunswick), about the year 1697.[745]
It is only in very recent times that the value of the Ground-Nut has
been recognized in Europe. Jaubert, a French colonist at Gorée near
Cape Verde, first suggested about 1840 its importation as an oil-seed
into Marseilles, where it now constitutes one of the most important
articles of trade.[746]
=Description=—The fat oil of _Arachis_, as obtained by pressure without
heat, is almost colourless, of an agreeable faint odour and a bland
taste resembling that of olive oil. An inferior oil is obtained by
warming the seeds before pressing them. The best oil has a sp. gr. of
about 0·918; it becomes turbid at 3° C., concretes at -3° to -4°, and
hardens at -7°. On exposure to air it is but slowly altered, being one
of the non-drying oils. At length it thickens considerably, and assumes
even in closed vessels a disagreeable rancid smell and taste.
=Chemical Composition=—The oil consists of the glycerides of four
different fatty acids. The common _Oleic Acid_, C₁₈H₃₄O₂, that is to
say its glycerin compound, is the chief constituent of Arachis oil.
_Hypogæic Acid_, C₁₆H₃₀O₂, has been pointed out by Gössmann and Scheven
(1854) as a new acid, whereas it is thought by other chemists to agree
with one of the fatty acids obtained from whale oil. The melting point
of this acid from Arachis oil is 34-35° C. The third acid afforded by
the oil is ordinary _Palmitic Acid_, C₁₆H₃₂O₂, with a fusing point of
62° C. _Arachic Acid_, C₂₀H₄₀O₂, the fourth constituent, has also been
met with among the fatty acids of butter and olive oil, and, according
to Oudemans (1866), in the tallow of _Nephelium lappaceum_ L., an
Indian plant of the order _Sapindaceæ_.
When ground-nut oil is treated with hyponitric acid, which may be most
conveniently evolved by heating nitric acid with a little starch, a
solid mass is obtained, which yields by crystallization from alcohol
_Elaïdic_ and _Gæidinic_ acids, the former isomeric with oleic, the
latter with hypogæic acid.
=Production and Commerce=—The pods are exported on an immense and ever
increasing scale from the West Coast of Africa. From this region,
not less than 66 millions of kilogrammes, value 26 millions of
francs (£1,040,000), were imported in 1867, almost exclusively into
Marseilles. From the French possessions on the Senegal, 24 millions of
kilogr. were exported in 1876.
[740] Lib. vii. cap. 5. Fol. 1074 f. (1547), as quoted by C. Ph. von
Martius in _Gelehrte Anzeigen der bayerischen Akademie_, 1839. 969.
[741] _Las Cosas que se traen de nuestras Indias Occidentales_,
Sevilla, 1569, part 2.
[742] _Histoire d’un voyage faict en la Terre du Bresil, autrement dite
Amérique_, 1586. 204 (first edition _La Rochelle_, 1578).
[743] _Histoire du Nouveau Monde_, Leyde, 1640. 503.
[744] _Hist. Rerum Nat. Brasil_. 1648. 37.
[745] _Botanica curiosa_, Helmst. 1697. 38.
[746] Duval, _Colonies et politique coloniale de la France_, 1864.
101.—Mavidal, _Le Sénégal, son état présent, son avenir_, Paris, 1863.
171.—Carrère et Holle, _La Sénégambie Française_, 1855. 84.—Poiteau, in
_Annales des Sciences nat., Botanique_, xix. (1853) 268.
The oil is exported from India where the ground-nut is also cultivated,
though not on so large a scale as in Western Africa. In Europe it
is manufactured chiefly at Marseilles, London, Hamburg and Berlin.
The yield of the seeds varies from 42 to nearly 50 per cent. The
softness of the seeds greatly facilitates their exhaustion, whether
by mechanical power or by the action of bisulphide of carbon or other
solvent.
=Uses=—Good arachis oil may be employed in pharmacy in the same way as
olive oil, for which it is a valuable substitute, though more prone to
rancidity. It has been introduced into the _Pharmacopœia of India_,
and is generally used instead of olive oil in the Indian Government
establishments. Its largest application is for industrial purposes,
especially in soap-making.