The aqueous solution of myrosin coagulates at 60° C. and then becomes inactive: hence mustard seed which has been heated to 100° C. or has been roasted yields no volatile oil, nor does it yield any if powdered and introduced at once into boiling water. The proportion of myrosin in mustard has not been exactly determined. The total amount of nitrogen in the seed is 2·9 per cent. (Hoffmann) which would correspond to 18 per cent. of myrosin, supposing the proportion of nitrogen in that substance to be the same as in albumin, and the total quantity of nitrogen to belong to it. Sometimes black mustard contains so little of it, that an emulsion of white mustard requires to be added in order to develop all the volatile oil it is capable of yielding.
An emulsion of mustard or a solution of pure sinigrin brought into contact with myrosin, frequently deposits sulphur by decomposition of the allyl sulphocyanide, hence crude oil of mustard sometimes contains a considerable proportion (even half) of Allyl cyanide, C₄H₅N, distinguished by its lower sp. gr. (0·839) and lower boiling point (118° C.).
The seeds, roots, or herbaceous part of many other plants of the order Cruciferæ yield a volatile oil composed in part of mustard oil and in part of allyl sulphide
| C₆H₁₀S = | C₂H₅ |  | S, |
| C₃H₅ |
which latter is likewise obtainable from the bulbs of garlic. Many Cruciferæ afford from their roots or seeds chiefly or solely oil of mustard, and from their leaves oil of garlic. As to other plants, the roots of Reseda lutea L. and R. luteola L. have been shown by Volhard (1871) to afford oil of mustard.[289] The strong smell given off by the crushed seeds or roots of several Mimoseæ, as for instance, Albizzia lophantha Benth. (Acacia Willd.) is perhaps due to some allied compound.
The artificial preparation of mustard oil was discovered in 1855 by Zinin, and at the same time also by Berthelot and De Luca. It may be obtained in decomposing bromide of allyl by means of sulphocyanate of ammonium:—
C₃H₅Br · SCN(NH₄) = NH₄Br · C₃H₅SCN.
The liquid C₃H₅SCN, boiling at 161°, is sulphocyanate of allyl; if it is gently warmed with a little alcoholic potash, and then acidulated, the red coloration of ferric sulphocyanate is produced on addition of perchloride of iron, but by submitting the sulphocyanate of allyl to distillation it is at once transformed in the isosulphocyanate, i.e. in mustard oil; the latter is not coloured by ferric salts, but it would appear that in the cold emulsion of mustard, even at 0°, a little sulphocyanate makes also its appearance.
Mustard submitted to pressure affords about 23 per cent.[290] of a mild-tasting, inodorous, non-drying oil, solidifying when cooled to -17·5° C., and consisting of the glycerin compounds of stearic, oleic and Erucic or Brassic Acid. The last named acid, C₂₂H₄₂O₂, occurs also in the fixed oil of white mustard and of rape, and is homologous with oleic acid. Darby (1849) has pointed out the existence of another body, Sinapoleic Acid, C₂₀H₃⁸O₂, which occurs in the fixed oil of both black and white mustard. Goldschmiedt, in 1874, ascertained the presence also of Behenic Acid, C₂₂H₄₄O₂ in black mustard. Sinigrin being not altered by the extraction of the fatty oil, either by pressure or by means of bisulphide of carbon, the powdered seed, deprived of fatty oil, still yields the whole amount of the irritating “essential” oil. This important fact has been ingeniously used by Rigollot[291] for the preparation of his mustard paper.
Mustard seed when ripe is devoid of starch; the mucilage which its epidermis affords amounts to 19 per cent. of the seed (Hoffmann). The ash constituents amounting to 4 per cent. consist chiefly of the phosphates of calcium, magnesium, and potassium.