Take 1250 gr. of bread (from middle of loaf) or flour, and char thoroughly in a platinum dish or on foil over a gas lamp. Powder the char and mix it with sufficient pure strong hydrochloric acid to make a thin cream. Boil gently for a few minutes, then add 100 c. c. of water, and continue the ebullition a few minutes longer. Dilute to 150 c. c., stir well, and filter off 120 c. c., which will contain the alumina from 1000 gr. of the bread or flour. To this filtrate add a slight excess of solution of ammonia, boil for a few seconds. Then let the precipitate subside, and decant the supernatant fluid. Add boiling water to the sediment, and again set aside to settle, and decant the clear fluid. Pass the fluids through a small filter to collect any particles of the precipitate which may have been suspended therein, and throw the filtrate away. Now add to the partially washed precipitate about a gram of pure caustic potash (or soda), warm, and pass the solution through the same filter employed for the previously decanted fluids. Wash the filter with hot water, to which a little KHO may be added, and proceed to precipitate the alumina in the filtrate by adding a few drops of dilute phosphoric acid and excess of pure acetic acid. Heat the solution and precipitate to the boiling point, and then wash the latter by decantation and filtration. Finally dry, ignite, and weigh. The weight of the resulting Al2PO4 in grams, multiplied by 400, will give the amounts of ammonia alum in grains present in one pound of the bread or flour.
f. (Mr Crookes.) The bread of which at least 500 grains should be taken is first to be incinerated on a platinum or porcelain dish, until all volatile organic matter has been expelled, and a black carbonaceous ash remains. The temperature must not be raised much beyond the point necessary to effect this. Powder the coal thus obtained and add about thirty drops of oil of vitriol, and heat until vapours begin to rise; when sufficiently cool, add water, and boil for ten minutes. Filter and evaporate the filtrate until the fumes of sulphuric acid begin to be evolved, when 10 gr. of metallic tin and an excess of nitric acid must be added, together with water, drop by drop, until action between the acid and metal commences. When all the tin is oxidised, add water, and filter. Evaporate the filtrate until fumes of sulphuric acid are again visible, when more water must be added, and the liquid again filtered if necessary. To the clear solution now add tartaric acid, then ammonia in excess, and sulphide of ammonium. Evaporate the liquid containing the precipitate suspended to it, in a dish, until all the smell of sulphide of ammonium has disappeared. Filter, evaporate to dryness, and ignite to get rid of the organic matter. Powder the black ash, boil it in moderately strong hydrochloric acid, filter, add a crystal of chlorate of potash, and boil for a minute. Now add chloride of ammonium and ammonia, and boil for five minutes. If at the end of that time any precipitate is observed, it will be alumina. From the filtered solution, if oxalate of ammonia be added, the lime will be precipitated; and if to the filtrate from this, ammonia and phosphate of soda be added, the magnesia will come down.
Dr Dupré is of opinion that no baker should be fined in whose bread the amount of alumina found corresponds with less than 10 grains of potash alum in the 2-lb. loaf, unless there is direct evidence of adulteration by alum independent of the result of analysis.
Mr Crookes says, “By treatment with a trace of alum, flour with a doubtful soundness is endowed with soundness. For this purpose a proportion of alum is required which does not exceed 20 grains to a 4-lb. loaf.
2. Copper:—a. Moisten the suspected bread with a few drops of a solution of ferrocyanide of potassium. It will assume a pinkish-brown colour if copper be present.
b. A little of the bread may be steeped in hot water, or, better still, in water soured with a little nitric acid, and the clear liquor squeezed or poured off, and tested with ferrocyanide of potassium, as before.
3. Magnesia:—Bread adulterated with magnesia, on digestion in hot water acidulated with sulphuric acid, furnishes a liquid which gives a white precipitate when tested with a solution of either carbonate of potassa or of carbonate of soda, especially on boiling.
4. Soda; Potassa:—Hot water after digestion on the ashes or charcoal turns turmeric paper brown. The liquid may be evaporated to dryness, redissolved in distilled water, slightly acidulated with hydrochloric acid, and tested with bichloride of platinum. If a yellow crystalline precipitate forms, either at once or after some hours, it is potash; otherwise the alkali present is soda.
5. Chalk, WHITING, BURNT BONES, plaster of Paris, and similar substances are easily detected by calcining a little of the flour or bread in a clean open vessel, when the amount of ash left will indicate the quantity of adulteration. The quantity of the ash left by genuine bread or flour is very trifling indeed, about 2%.
Microscopic Characters of Bread. When bread is placed under the microscope, starch cells, broken up into angular masses, or greatly enlarged, and stringy masses of gluten are usually visible; besides these, when a microscope of high power is employed, bacteria of the rod-shaped variety may frequently be detected, the source of these being, probably, the yeast. Great caution and diligent observation are necessary to guard against the falling into the serious error of mistaking the many curious forms the broken-up wheat starch presents for adulterants. By practice and the constant examination of the characters of unadulterated bread, combined with a practical knowledge of the appearance different starch grains present, after being more or less changed in shape by cooking, the microscopist may identify rice-flour, bean-flour, and Indian millet. Barley flour and potatoes, however, are very difficult of detection. There is very little difference in the shape of the barley-starch granule and that of the wheat, and in the process of bread-making the potato granules are so changed as to confuse all their distinctive characters. Bone-dust and a few other mineral adulterations may be detected by the microscope.