The ash of leguminous meals is deliquescent and soluble in water, forming a strongly alkaline solution, which contains both chlorides and neutral phosphates. The latter give a clear yellow precipitate with nitrate of silver. Upon adding a solution of this salt to the aqueous solution of the ash, a pale yellow precipitate, which turns violet if exposed to the light, is therefore produced.
Detection of mineral substances.—The principal mineral substances, that are fraudulently added to flour, are ground calcined bones, sand, lime, plaster, alum, and sulphate of copper. The two last named salts are almost invariably added in small quantities; alum renders the flour white, even when used in the proportion of one per cent.; sulphate of copper is added to impart a good appearance to bread made from a damaged flour.
a. Ground bones (carbonate and phosphate of lime).—The washings of the gluten are placed in a conical vessel, and, after some time has elapsed, the clear supernatant fluid is removed by means of a syphon, a conical shaped deposit remaining on the bottom of the vessel: two hours later, the fresh layer of fluid that has formed is removed with a pipette. As soon as the residue becomes nearly solid, it is detached from the vessel, placed upon a fragment of plaster, and allowed to dry. The bones, being heavier than the amylaceous substances, are to be found in the apex of the cone formed by the residue. This is detached, and incinerated: in case the ash obtained contains phosphate and carbonate of lime, the addition of hydrochloric acid will cause effervescence, and, upon adding ammonia to the acid solution, a white precipitate will be formed. If the solution is then filtered and oxalate of ammonia added to the filtrate, a precipitate will be produced which, when heated to redness, leaves a residue of caustic lime possessing an alkaline reaction.
b. Sand.—As this substance possesses a much greater specific gravity than the usual constituents of flour, it is only necessary, in order to accomplish its separation, to repeatedly stir the flour with water, and remove the deposit at first formed, which, if consisting of sand, will be insoluble in acids, and will grate, when placed between the teeth.
c. Carbonates of lime and magnesia; vegetable ashes.—Carbonic acid is always evolved, upon treating flour with hydrochloric acid. If the base present be calcium, upon adding oxalate of ammonia to the filtered solution—which has previously been neutralized with ammonia—a white precipitate, possessing the properties mentioned above, will be formed; in case the base is magnesia, the addition of oxalate of ammonia will fail to cause a precipitate, but upon adding solution of phosphate of ammonia to the fluid a granular precipitate of phosphate of ammonia and magnesia is produced; if, finally, the flour contains vegetable ashes—i. e. carbonates of the alkalies—bichloride of platinum will produce in the acid solution a yellow precipitate: the addition of vegetable ashes, moreover, would render the ash of the flour deliquescent and very strongly alkaline.
d. Lime.—In presence of lime, carbonic acid produces a white precipitate, when conducted into the filtered aqueous extract of the flour.
e. Plaster.—The flour is boiled with water acidulated with hydrochloric acid, the fluid filtered, and lime detected in the filtrate by means of ammonia and oxalate of ammonia. The presence of sulphuric acid is indicated by the formation of a precipitate insoluble in acids, upon addition of solution of chloride of barium. Upon calcining the flour without access of air, sulphate of lime is converted into the corresponding sulphide: the residue of the calcination, when treated with hydrochloric acid, evolves sulphuretted hydrogen, and the lime present in the filtered acid solution is likewise precipitated by the addition of ammonia and oxalate of ammonia.
f. Alum.—A portion of the flour to be examined is treated with water, the fluid filtered, and the filtrate divided in two portions: in one, sulphuric acid is detected by means of chloride of barium; in the other, alumina by adding a solution of potassa, which gives with its salts a white gelatinous precipitate, soluble in an excess of the reagent.[R]
g. Sulphate of copper.—About 200 grammes of the bread under examination are incinerated; the ash treated with nitric acid; the mixture evaporated until it acquires a sticky consistence, and the mass then taken up with water. The aqueous solution is next filtered; an excess of ammonia and several drops of solution of carbonate of ammonia added; the fluid again filtered, the filtrate slightly acidulated with nitric acid, and divided into two parts. It is then ascertained if sulphuretted hydrogen produces in one portion of the solution a brown precipitate of sulphide of copper, and if solution of ferrocyanide of potassium produces in the other a reddish-brown precipitate of ferrocyanide of copper.[S]