Mustard is the product obtained by crushing and sifting the seeds of Sinapis nigra and Sinapis alba, of the genus BrassicaceÆ. In the manufacture of the condiment, both the black and white seeds are used. According to analyses made by Piesse and Stansell,[138] fine grades of the two varieties of mustard possess the following composition:—

Clifford Richardson regards the following proportions of the more prominent constituents of pure mustard flour as a basis for detecting adulterations:—

The following results were obtained by Messrs. Waller and Martin from the examination of 14 samples of very low grade dry mustard, as found on the New York market:[139]—

Eight samples were coloured with turmeric, 4 with Martius’ yellow, 12 contained starch, and 5 showed the presence of calcium sulphate.

The article usually sold as mustard is a mixture of mustard farina, prepared from different varieties of the seed, with wheaten flour or starch, and turmeric. It is claimed by the manufacturers that pure mustard possesses too acrid a taste to be suitable for use as a condiment; and its admixture with the foregoing substances is so generally resorted to and recognised, that the New York State Board of Health, in 1883, legally sanctioned the practice, provided the fact is distinctly stated upon the label of the packages. Other prevalent forms of sophistication consist in the partial extraction of the fixed oil from the mustard before its introduction on the market, and in the addition of cocoa-nut shells, terra alba, and “Martius’ yellow” (potassium dinitronaphthalate). The latter colouring matter is specially objectionable, being poisonous in its action. The presence of organic admixtures is usually recognised upon a microscopic examination of the sample. The anatomical structure of mustard seed is described by Fluckigen and Hamburg in ‘Pharmacographia.’ Wheaten flour or starch is readily identified by the iodine test. The following methods are employed for the detection of turmeric:—

1. A portion of the sample is agitated with castor oil and filtered. In case turmeric is present, the filtrate will exhibit a marked greenish fluorescence.

2. Upon treating the suspected sample with ammonium hydroxide, an orange-red colour is produced in presence of turmeric. Or, the mustard is boiled with methylic alcohol, the extract filtered, evaporated to dryness, and the residue treated with hydrochloric acid; if turmeric be present, an orange-red coloration takes place, which changes to a bluish-green upon adding an excess of sodium hydroxide. In addition to the above qualitative tests, valuable indications regarding the purity of mustard are to be obtained by the determination of the proportions of fixed oil, sulphur, and ash contained in the sample under examination.

Fixed Oil.—The amount of fixed oil is estimated by digesting a weighed portion of the mustard with ether in a closed vessel, filtering, and determining the weight of the residue left upon evaporating the ethereal solution to dryness over the water-bath. The oil possesses a specific gravity ranging from 0·915 to 0·920. The percentage of fixed oil in pure mustard is very considerable (usually over 34 per cent.), whereas the substances commonly added contain but a very small quantity. In case wheaten flour has been employed as an adulterant, the proportion of pure mustard (x) in a mixed sample, can be approximately calculated by the following formulÆ, in which y is the fixed amount of oil contained.[140]

33·9x 100 + 1·2(100 - x) 100 = y,
36·7x 100 + 2·(100 - x) 100 = y.

In the absence of flour, a low percentage of fixed oil indicates the presence of exhausted mustard cake.

Sulphur.—Blyth determines the total sulphur by oxidation with fuming nitric acid, diluting the liquid considerably with water, filtering and precipitating the sulphates formed by means of barium chloride. The proportion of sulphates (in terms of barium sulphate) found in the ash is to be deducted from the weight of the precipitate obtained; the remainder, multiplied by 0·1373, gives the amount of sulphur present in organic combination, and, as the quantity contained in this form in mustard is far greater than in any of the substances employed for its adulteration, the estimation is frequently very useful.

Ash.—The amount of ash is determined in the usual manner, i. e. by the incineration of a weighed portion in a platinum capsule. Genuine mustard contains about 5 per cent. of ash, of which nearly 1 per cent. is soluble in water. In presence of inorganic impurities, the quantity of ash is naturally increased, while a proportion under 4 per cent. is usually considered an indication of organic admixture.

The composition of the ash of mustard seed is given below:—

The adulteration of mustard is very extensively practised. Of 18 samples bought at random in the shops and tested for the New York State Board of Health, 12 were found to be impure; of 88 samples, examined in the year 1884 by the Massachusetts State Board, 20 were compounds (labelled as such, but in a manner designed to deceive the purchaser). 37 were adulterated with flour, turmeric, and, in some cases, with cayenne, and 31 were found to be pure; in 1885, 211 samples were tested, of which 124 were sophisticated; of 27 samples tested by the National Board of Health, 21 contained foreign admixtures, consisting chiefly of wheat or flour and turmeric, but also including corn-starch, rice, cayenne, and plaster of Paris.