The essential oil of mustard was, apparently, noticed about the year 1660 by Nicolas Le Febvre (see in the article Rad. Inulae), more distinctly in 1732 by Boerhaave. Its acridity and high specific gravity were pointed out by Murray.[285] Thibierge in 1819 observed that sulphur was one of the constituents of the oil, and Guibourt[286] stated that it is not pre-existing in the seed.
Production—Mustard is grown in England only on the richest alluvial soils, and chiefly in the counties of Lincolnshire and Yorkshire. Very good seed is produced in Holland.
Description—The pod of Brassica nigra is smooth, erect, and closely pressed against the axis of the long slender raceme. It has a strong nerve on each of its two valves and contains in each cell from 4 to 6 spherical or slightly oval seeds. The seeds are about ¹/₂₅ of an inch in diameter and ¹/₅₀ of a grain in weight; they are of a dark reddish-brown. The surface is reticulated with minute pits, and often more or less covered with a whitish pellicle which gives to some seeds a grey colour.[287] The testa which is thin, brittle and translucent encloses an exalbuminous embryo having two short cotyledons folded together longitudinally and forming a sort of trough in which the radicle lies bent up. The embryo thus coiled into a ball completely fills the testa; the outer cotyledon is thicker than the inner, which viewed in transverse section seems to hold the radicle as a pair of forceps. The seeds when pulverized have a greenish yellow hue. Masticated they have for an instant a bitterish taste which however quickly becomes pungent. When triturated with water they afford a yellowish emulsion emitting a pungent acrid vapour which affects the eyes, and has a strong acid reaction. The seeds powdered dry have no such pungency. When the seeds are triturated with solution of potash, the pungent odour is not evolved; nor when they are boiled in water. Neither is the acridity developed on triturating them with alcohol, dilute mineral acids, or solution of tannin, or even with water when they have been kept in powder for a long time.
Microscopic Structure—The whitish pellicle already mentioned, which covers the seed, is made up of hexagonal tabular cells. The epidermis consists of one row of densely packed brown cells, radially elongated and having strong lateral and inner walls. Their outer walls on the other hand are thin and not coloured; they are not clearly obvious when seen under oil, but swell up very considerably in presence of water, emitting mucilage.[288] Seeds immersed in water become therefore covered with a glossy envelope, levelling down the superficial inequalities, so that the wet seed appears smooth. The tissue of the cotyledons exhibits large drops of fatty oil and granules of albumin.
Chemical Composition—By distilling brown mustard with water, the seed having been previously macerated, the pungent principle, Essential Oil of Mustard, is obtained.
The oil, which has the composition SCN(C₃H⁵), (allyl isosulphocyanate), boils at 148° C.; it has a sp. gr. of 1·017, no rotatory power, and is soluble without coloration or turbidity in three times its weight or more of cold strong sulphuric acid. To this oil is due the pungent smell and taste of mustard and its inflammatory action on the skin. As already pointed out, mustard oil is not present in the dry seeds, but is produced only after they have been comminuted and mixed with water, the temperature of which should not exceed 50° C.
The remarkable reaction which gives rise to the formation of mustard oil was explained by Will and Körner in 1863. They obtained from mustard a crystallizable substance, then termed Myronate of potassium, now called Sinigrin. It is to be regarded, according to the admirable investigations of these chemists, as a compound of
| Isosulphocyanate of allyl or mustard oil | C₄ | H₆ | NS | |
| Bisulphate of potassium | H | KS | O₄ | |
| Sugar (dextro-glucose) | C₆ | H₁₂ | O₆ | |
| so that the formula | C₁₀ | H₁₈ | KNS₂ | O₁₀ |
is that of sinigrin. It does in fact split into the above-mentioned three substances when dissolved in water and brought into contact with Myrosin.
This albuminous body discovered by Bussy in 1839, but the composition of which has not been made out, likewise undergoes a certain decomposition under these circumstances. Sinigrin may likewise be decomposed by alkalis and, according to Ludwig and Lange, by silver nitrate. These chemists obtained sinigrin from the seeds in the proportion of 0·5 per cent.; Will and Körner got 0·5 to 0·6 per cent. The extraction of the substance is therefore attended with great loss, as the minimum yield of volatile oil, 0·42 per cent. indicates 2·36 of potassium myronate.