Microscopic Structure—The epidermal cells of white mustard afford a good illustration of a mucilage-yielding layer such as is met with, under many variations, in the seeds of numerous plants. The cuticle consists of large vaulted cells, exhibiting very regular hexagonal outlines when cut across.[297] The inner layer of the epidermis is made up of thin-walled cells, which when moistened swell and give off the mucilage. In the dry state or seen under oil, the outlines of the single cells of this layer are not distinguishable. The tissue of the cotyledons is loaded with drops of fatty oil and with granular albuminoid matter; starch which is present in the seed while young, is altogether absent when the latter reaches maturity.
Chemical Composition—White mustard deprived of fatty oil yields to boiling alcohol colourless crystals of Sinalbin, an indifferent substance, readily soluble in cold water, but sparingly in cold alcohol. From the able investigations of Will (1870) it follows, that it is to be regarded as composed of three bodies, namely:
| Sulphocyanate of Acrinyl | C⁸ | H₇ | N | S | O |
| Sulphate of Sinapine | C₁₆ | H₂₅ | N | S | O₉ |
| Sugar | C₆ | H₁₂ | O₆ | ||
| so that the formula | C₃₀ | H₄₄ | N₂ | S₂ | O₁₆ |
represents according to Will the composition of sinalbin. It is actually resolved into these three substances when placed at ordinary temperatures, in contact with water and Myrosin, the latter of which is a constituent of white mustard as well as of brown ([p. 66]). The liquid becomes turbid, the first of the above-named substances separates (together with coagulated albumin) as an oily liquid, not soluble in water, but dissolving in alcohol or ether. This Sulphocyanate of Acrinyl is the rubefacient and vesicating principle of white mustard. It does not pre-exist, as shown by Will, in the seed, and cannot be obtained by distillation. By treating it with a salt of silver, Will obtained crystals of cyanide of acrinyl, C₈H₇NO: by warming it (or sinalbin itself, or an alcoholic extract of the seed) with caustic potash, sulphocyanide of potassium is produced. The presence of the latter may be indicated by adding a drop of perchloride of iron, when a blood-red coloration will be produced.[298]
Sulphate of Sinapine imparts to the emulsion of white mustard, in which it is formed, an acid reaction. Sinapine is itself an alkaloid, which has not yet been isolated, as it is very liable to change. Thus its solution on addition of a trace of alkali immediately assumes a bright yellow colour indicating decomposition, and a similar colour is produced in an aqueous extract of the seed.
The above statements show, that the chemical properties of sinalbin and its derivatives correspond closely with those of sinigrin ([p. 66]) and the substances which make their appearance in an emulsion of black mustard.
The other constituents of white mustard seed are nearly the same as those of black. The fat oil appears to yield in addition to the acids mentioned at [p. 67], Benic or Behenic Acid, C₂₂H₄₄O₂. White mustard is said to be richer than black in myrosin, so that, as explained in the previous article, the pungency of the latter may be often increased by an addition of white mustard. By burning white mustard dried at 100° C., with soda-lime, we obtained from 4·20 to 4·30 per cent. of nitrogen, answering to about 28 per cent. of protein substances.[299] The fixed oil of the seed amounts to 22 per cent. The mucilage as yielded by the epidermis is precipitable by alcohol, neutral lead acetate, or ferric chloride, and is soluble in water after drying.
Erucin and Sinapic Acid, mentioned by Simon (1838)[300] as peculiar constituents of white mustard, are altogether doubtful, yet may deserve further investigation. The sinapic acid of Von Babo and Hirschbrunn[301] (1852) is a product of the decomposition of sinapine.
Uses—White Mustard seed reduced to powder and made into a paste with cold water act as a powerful stimulant when applied to the skin, notwithstanding that such paste is entirely wanting in essential oil. But for sinapisms they are actually used only in the form of the Flour of Mustard which is prepared for the table and which contains also Brown Mustard seed.