This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
Brassica alba, Boissier (N.O. Cruciferoe), the white mustard, is cultivated like the black mustard, which it closely resembles, but not to so large an extent. It differs from the black mustard in producing more or less horizontal hairy fruits, those of the black mustard being erect, appressed, and smooth. Each fruit contains from four to six seeds.
White mustard seeds are yellow in colour, nearly spherical in shape and about 2 mm. in diameter. The seed-coat is very minutely pitted, the pits being so small that the seed appears smooth until examined with a lens. Internally the seed is yellow, and the oily kernel consists, as that of the black mustard seed does, of the two folded cotyledons embracing the small radicle; it becomes coated with mucilage when soaked in water, and can afterwards be easily deprived of its seed-coats.
White mustard seeds, either whole or powdered, are free from pungent odour, even when triturated with water. They have, nevertheless, a pungent taste.
The student should soak some white mustard seeds in water, remove the seed-coats, and observe the cotyledons and radicle, noting that the seed is exalbuminous; he should also crush the seeds, moisten them with water, and note that the taste is pungent, but the odour is not. He should also observe:
(a) The minutely pitted surface,
(b) The incumbent, folded cotyledons.
White mustard seeds contain a fixed oil (about 30 per cent.), mucilage (in the epidermis of the seed-coat), and proteids (about 25 per cent.). Starch is not present in the ripe seeds, which yield about 4 per cent, of ash.
They contain, in addition, a crystalline glucoside, sinalbin, and the same enzyme as is found in the black mustard seed - viz. myrosin. Sinalbin is readily soluble in water and in boiling alcohol, but only very sparingly in cold alcohol; it assumes an intense yellow colour when acted upon by alkalies. Under the influence of myrosin, and in the presence of water, it yields acid sinapine sulphate, dextrose, and acrinyl isothiocyanate. The decomposition may be represented by the following equation:
Acid sinapine sulphate
Of these three substances, acrinyl isothiocyanate is a yellow oily liquid with a pungent taste and powerful rubefacient action, but as it is not volatile it is destitute of pungent odour or pungent effect on the eyes. In this particular the pungent principle obtained from white mustard diff is essentially from that yielded by black, and since this principle is not volatile, it is evident that volatile (or essential) oil of mustard cannot be obtained from white mustard seeds. It is very remarkable, considering the close relationship of the two plants and similarity in other constituents, that white mustard should contain no sinigrin and black no sinalbin.
Sinapine is an alkaloid which is so unstable that it has not yet been isolated; the acid sulphate and other salts are crystalline and assume a bright yellow colour in contact with alkalies.
White mustard possesses rubefacient and vesicant properties similar to those of black mustard.