Source, Etc

The black mustard plant, Brassica sinapioides, Roth (B. nigra, Koch, N.O. Cruciferoe), an erect annual plant attaining a height of 1 metre or more, is largely cultivated in Holland, England, Italy, Germany, and other countries. The fruits are smooth, erect, appressed siliquas, which dehisce, when ripe, by both sutures, disclosing about ten or twelve minute dark seeds. These are separated when ripe, and dried.

Description

Black, or, as they are sometimes termed, brown or red, mustard seeds are of a dark reddish brown or greyish brown colour, sometimes nearly black, and are frequently partially covered with very thin, whitish scales (dried mucilage from the epidermis, probably the result of damp). They are nearly spherical or flattened-ovoid in shape and about 1 mm. in diameter. Under the lens the surface appears minutely pitted, and the hilum can be distinguished as a paler point.

Although the seeds are hard, the seed-coats are thin and brittle. The kernel is greenish yellow and oily, and consists of the two folded cotyledons embracing the small radicle. The position of cotyledons and radicle can be well seen by cutting a seed in halves midway between the hilum and the apex. After the seed has been soaked in water, during which it surrounds itself with mucilage, the seed-coats can be easily removed, disclosing the cotyledons folded over and incumbent upon the radicle; there is no endosperm, the seeds being exalbuminous.

Black mustard seeds, even when powdered, have no marked odour. The taste is at first bitter, but rapidly becomes strongly pungent, and although the dry seeds are almost inodorous they develop, when moistened with water, a volatile substance of extreme pungency that rapidly attacks both the nostrils and the eyes. This volatile substance is not, however, developed if the seeds have been previously thrown into boiling water.

The student should observe: ,

(a) The minute size and spherical shape,

(b) The pitted surface,

(c) The incumbent and folded cotyledons,

(d) The pungent taste; and should compare these seeds with colchicum seeds, which are larger, rougher, and harder than black mustard, and have a bitter, not pungent, taste.

Fig. 85.   Black Mustard seed. A, entire seed, magnified 3 diam. B, transverse section, magnified 65 diam.: 4, 4, the two cotyledons; 5, the radicle. C, portion of the same, further enlarged: o, epidermis containing mucilage. (Berg.)

Fig. 85. - Black Mustard seed. A, entire seed, magnified 3 diam. B, transverse section, magnified 65 diam.: 4, 4, the two cotyledons; 5, the radicle. C, portion of the same, further enlarged: o, epidermis containing mucilage. (Berg).

Constituents

Black mustard seeds contain in the kernels about 27 per cent, of fixed oil (compare Expressed Oil of Mustard) which can be extracted by crushing and pressing the seed. In the seed-coal (in the cells of the epidermis) there is mucilage, which dissolves when the seeds are soaked in water. The seeds contain, further, a small quantity of acid sinapine sulphate (compare p. 149), and two substances, sinigrin (about 4 per cent.) and an enzyme, myrosin, which, by interaction in the presence of water, yield the volatile pungent body previously referred to; the latter is not a constituent of the seed, but is produced from two of its constituents under certain conditions. The assertion that black mustard seeds do not contain sufficient myrosin to decompose the whole of the sinigrin has been shown to be incorrect.

Sinigrin (also called potassium myronate), C10H18NS2O10K, may be crystallised from either alcohol or water. It may be extracted by first boiling the powdered seeds (not previously defatted) for a short time with 80 per cent, alcohol, by which the activity of the myrosin is destroyed; from the pressed and dried seeds cold water then extracts the sinigrin, which may be obtained by evaporating the aqueous solution in the presence of barium carbonate, extracting with alcohol, concentrating and crystallising (yield about 0.6 per cent.).

Sinigrin, when submitted to the action of myrosin in aqueous solution, decomposes into allyl isothiocyanate, acid potassium sulphate, and dextrose:

C1oH18NS2O10K

=

C3H5NCS

+

KHSO4

+

C6H12O6.

Sinigrin

Allyl isothiocyanate

Acid pot. sulph.

Dextrose

This reaction, however, does not take place in the seed, as the myrosin and sinigrin are stored up in separate cells.

Allyl isothiocyanate, or volatile oil of mustard, is a mobile volatile liquid with an extremely pungent taste and odour, and is the body to which the pungent taste and odour of black mustard seeds (after crushing and moistening with water) are due. The power of the myrosin to effect this decomposition is destroyed by boiling water, and therefore the seeds that have been subjected to such treatment do not develop allyl isothiocyanate when treated with cold water.

The cake that is left after the fixed oil has been expressed from the seed contains both the sinigrin and the myrosin. From this cake volatile oil of mustard may be prepared by crushing, macerating in water for several hours, and distilling; a mobile liquid collects at the bottom of the distillate, and this, after redistillation, constitutes volatile oil of mustard. It consists of allyl isothiocyanate, associated with small quantities of other bodies such as allyl cyanide, C3H5CN, etc.

Allyl isothiocyanate can also be produced artificially by decomposing allyl iodide with potassium thiocyanate; allyl thiocyanate (C3H5S.C:N) is formed, which on distillation is converted into the isomeric, pungent allyl isothiocyanate, C3H5N:C:S (allyl thiocarbimide)

Constituents 131

The seeds yield about 0.7 to 1.3 per cent, of volatile oil, Dutch seeds being the best. Official volatile oil of mustard (s.g., 1.014 to 1.025, distils at 148°-156°) contains at least 92 per cent, of allyl isothiocyanate.

Many Cruciferous plants, and some belonging to other natural orders, yield under similar conditions a pungent volatile oil, thus horse-radish root yields allyl isothiocyanate; scurvy grass (Cochle-aria officinalis, Linne) and nasturtium seed (Tropoeolum majus, Linne), isobutyl isothiocyanate (C4H9NCS); radish (Raphanus sativus, Linne) and watercress (Nasturtium officinale, Linne), principally phenylethyl isothiocyanate (C8H9NCS); and cress (Lepidium sativum, Linne), benzyl isothiocyanate, (C7H7NCS).

Black mustard seeds also contain about 29 per cent, of proteid matter and yield from 4.2 to 5.7 per cent, of ash. Starch, which is present in the unripe seed, is not found in any of the cells of the ripe seed.

Uses

Applied externally, black mustard acts as a rubefacient and counter-irritant; this effect is followed by loss of sensibility in the part, and consequently relief from previous pain. Prolonged action may result in vesication. Internally, mustard is used as a condiment, and in full doses as an emetic.

Varieties

Indian mustard (B. juncea, Hooker and Thorns) is widely cultivated in Southern Russia and India; the seeds resemble black mustard but are rather larger and browner in colour; the volatile oil is believed to contain about 40 per cent, of allyl isothiocyanate, and 50 per cent, of crotonyl isothiocyanate, C4H7NCS; they are sometimes sold as black mustard seeds.