Food Adulteration and Its Detection / With photomicrographic plates and a bibliographical appendix

by Jesse P. Battershall

Fat.—The inconsiderable proportion of fat in flour is best determined by exhausting the dried sample with ether and evaporating the solution.

Gluten (albuminoids).—As previously stated, gluten is separated by kneading the flour and repeated washing with water. After the removal of the amylaceous and soluble ingredients, the residue is carefully dried and weighed. A far more accurate method is to make a combustion of a small portion of the flour with cupric oxide, and determine the quantity of nitrogen obtained, the percentage of which, multiplied by 6·33, gives the percentage of gluten.[51] The proportion of gluten in flour ranges from about 8 to 18 per cent. From 10 to 12 per cent, is deemed necessary in order to make good bread, and, in England, any deficiency in this constituent is remedied by the addition of bean or other flour, but in the United States this practice is seldom required.

Substances soluble in cold water.—About five grammes of the flour are digested with 250 c.c. of cold water, and the solution filtered, and evaporated to dryness. Good flour is stated to yield 4·7 per cent. of extract when treated in this manner, the soluble matters consisting of sugar, gum, dextrine, vegetable albumen, and potassium phosphate. The latter salt, which constitutes about 0·4 per cent. of the extract, should form the only mineral matter present.

The Ash.—The ash of flour is determined in the usual manner, by ignition in a platinum dish. It varies in amount from 0·3 to 0·8 per cent., and should never exceed a proportion of 1·5 per cent.

When of good quality, wheaten flour is perfectly white, or has only a faint tinge of yellow. It should be free from bran, and must not show red, grey, or black specks, nor possess a disagreeable odour. It should also exhibit a neutral reaction and a decided cohesiveness, acquiring a peculiar soft and cushion-like condition when slightly compressed. Formerly, wheaten flour was mixed with various foreign meals, such as rye, corn, barley, peas, beans, rice, linseed, buckwheat, and potato starch; but at present this form of adulteration is probably but rarely resorted to, at least in the United States. The presence of mildew, darnel, ergot, and other parasites of the grain, constitutes an occasional contamination of flour. The most frequent admixture consists, however, in the addition of alum, which, although more extensively used in bread, is also employed in order to disguise the presence of damaged flour in mixtures, or to improve the appearance of an inferior grade; its addition to a damaged article serves to arrest the decomposition of the gluten, thereby preventing the flour from acquiring a dark colour, and disagreeable taste and odour.

It has recently been stated that in flour which has been kept for a long time in sacks, a transformation of the gluten sometimes occurs, resulting in the production of a poisonous alkaloid. This body may be separated by evaporating the ethereal extract of the flour to dryness, and treating the residue with water. The presence of the alkaloid in the filtered aqueous solution is recognised by means of potassium ferrocyanide. The presence of an excessive proportion of moisture is doubtless instrumental in the formation of toxic alkaloids or fungi in old flour and bread.

Pure wheaten flour is coloured yellow when treated with ammonium hydroxide, whereas corn meal assumes a pale brown colour, and the meals prepared from peas, beans, etc., become dark brown in colour when tested in this way. Nitric acid imparts an orange-yellow colour to wheaten flour, but fails to change the colour of potato-starch, with which it forms a stiff and tenacious paste.

Potato-starch is readily detected by examining a thin layer of the sample on a slide under the microscope, and adding a dilute solution of potassium hydroxide, which, while not affecting the wheaten starch, causes the potato-starch granules to swell up very considerably. Leguminous starches, such as peas, etc., contain approximately 2·5 per cent. of mineral matter; in pure flour, the average proportion of ash is only about 0·7 per cent., and this difference is sometimes useful in the detection of an admixture of the former.

The external envelope of the granules of potato-starch offers far less resistance when triturated in a mortar than that of wheat, and upon this fact a simple test for their detection is founded. It is executed by rubbing up a mixture consisting of equal parts of the sample and sand with water, diluting and filtering the paste formed, and then adding to it a solution of 1 part of iodine in 20 parts of water. In the absence of potato-starch, an evanescent pink colour is produced; in case it is present, the colour obtained is dark purple, which in time also disappears.