Popular Science Monthly/Volume 32/February 1888/The Adulteration of Milk



THE absolute necessity of maintaining a certain standard of purity in food-products, has led, in most of the States, to a comprehensive and somewhat stringent legislation concerning adulteration. Particularly is this the case as regards the products of the dairy. The official inspection busies itself with both the qualitative and quantitative value of these articles. Of all the foods supplied directly by Nature, milk is the only one which contains all the elements of nutrition in the relative proportions required by man, and in a form easy of digestion; it is therefore the food best suited to young children and invalids—persons who can ill afford to have their food tampered with in any way. This consideration, together with the universal use of the article, has determined the adoption of a system of public inspection in nearly all of our larger cities. The States of Massachusetts, New York, and Michigan have given particular attention to the honesty of the milkman, and the standards of quality and methods of analysis established by their public analysts have been generally adopted by chemists all over the country. Yet, in spite of the vigilant eye which is thus constantly watching this department of the farm, pounds of butter weighed according to a system of units not recognized in the arithmetics, milk which contains an abnormal percentage of water, and cream whose composition will not bear investigation, are daily sold in the market-places of both city and country.

But there are, of course, two sides to the question. Not unfrequently the milkman is accused entirely without cause. There are few housekeepers who do not sincerely believe, in spite of an other-wise general faith in mankind, that he, at least, will bear watching. The analyst, however, is a perfectly unprejudiced person. He cares little for the protestations of the vender, or the suspicions of the customer. He simply says, "Your milk should have such and such a specific gravity; it should contain such and such percentages of fat, of other hydrocarbons, of mineral salts, and of water: if it contain these, it is an honest milk; if it do not, it is adulterated." And generally, the analyst is not open to argument. He has implicit faith in his hydrometer and analytical balance.

It is the purpose of the present article to point out the adulterants which are commonly found in milk, and to describe a simple method of analysis which will give conclusive results even when practiced by those having no great proficiency in the use of apparatus and reagents. The complete chemical analysis of a sample of milk is an operation which taxes the patience of even a trained chemist, and when accomplished, it has little value not possessed by the results obtained from the more rapid methods used by the State analysts. It is commonly believed that the list of adulterants at the disposal of the dairyman is practically unlimited, but if one will pause a moment, and reflect that the adulterant chosen must not affect the taste of the milk, or show any tell-tale sediment on standing, he will see that the possibilities of fraud are much smaller than at first suspected. Of all the adulterants, water is the most popular the land over, since it costs nothing and tells no tales unless questioned by an analyst. In addition to water, it is not uncommon to find a milk "doctored" by the use of preservative salts, such as borax or bicarbonate of soda, and colored to an ideal cream tint by means of a little burnt sugar or vegetable coloring-matter.

In the State laboratories, it is generally required that determinations shall be made of the specific gravity, the amount of cream, total solids, fat, mineral salts, and water. Mr. Sharpies, public analyst for Massachusetts, recommends also the determination of the amount of sugar in the milk in order to place its quality beyond peradventure. Most analysts, however, do not regard this determination as necessary. As it is our object to make the analytical process as simple as possible, and to bring it within reach of all readers having some delicacy of touch, whether they be chemists or not, we shall only describe those determinations generally considered essential, and then specify which of these will suffice in most cases to establish the quality of the sample under consideration.

The first requisite is to decide upon a standard of purity. Opinions differ as to what should be considered a normal milk. The chief discrepancy lies in the amount of total solids which it should contain. In England the standard in this respect is rather low, only 11·5 per cent of total solids being required. In Massachusetts, on the other hand, the legal standard exceeds even the generosity of Kature, as it calls for 13 per cent, when milk drawn directly from the cow, and therefore free from any possibility of dilution, frequently shows but a little over 12.

The following abbreviated table will give the composition of average milk, and will serve as a standard for comparison when the results of any particular analysis have been obtained:

Specific gravity 1·03
Total solids 12·50
Fat 3·20
Organic solids not fat 8·70
Mineral salts ·60
Water 87·50

The sample of milk selected for examination should be quite fresh, but never warm. The temperature should be about 60° F.

Of the several determinations required, that of specific gravity is at once the easiest and the most important. It is made either by means of the specific-gravity bottle or the hydrometer.

The specific-gravity bottle is a small glass flask which will contain a given weight of distilled water. The form used by the writer is shown in Fig. 1. It is adjusted to fifty grammes of water, and is accompanied PSM V32 D545 Fifty gramme specific gravity bottle.jpgFig. 1.—Fifty-Gramme Specific-
Gravity Bottle.
by a small brass weight to exactly counterpoise the weight of the flask. To determine the specific gravity of a sample of milk the bottle is completely filled with the milk and the stopper brought into contact with the fluid, care being taken that no air adheres to its under side. The stopper is then permitted to fall quickly into its bearing in the neck of the bottle. The excess of milk escapes through the fine perforation in the stopper. In this way an invariable volume of fluid is always weighed. The flask is then carefully wiped off, and the whole weighed, remembering, of course, to add the counterpoise weight to the scale-pan, in order to allow for the weight of the flask. The weight of the milk is obtained in grammes, and this multiplied by ·02 will give its specific gravity directly.

By the use of the hydrometer (or lactometer, as it is usually called, when graduated for milk), the determination of the specific gravity may be made much more rapidly, A convenient form of this instrument is shown in Fig. 2. It will require no explanation beyond brief mention of the system of marking employed. The upper graduation on the stem of the tube is marked 1, and is the point to which the instrument will sink when placed in pure, distilled water, at 60° F. The lower graduation is marked 1·05, and is the point to which the instrument will sink when placed in a fluid of that specific gravity. As normal milk averages 1·03, the variations on either side will be fully covered by these limits. A low specific gravity indicates that the pump has been called into requisition, while a high specific gravity indicates a deficiency in cream. A popular form of lactometer, showing directly the proportion of water added to the sample, may be purchased of dealers in chemical apparatus for seventy-five cents, but some care PSM V32 D546 Lactometer with direct graduation.jpgFig. 2—
with Direct
must be exercised in its selection, since one instrument now offered the public has the point marked "skim milk" above that marked "pure milk." Evidently it should be below, as skimmed milk has a greater specific gravity than whole milk.

In some States the test of specific gravity by the lactometer is the only determination made. In spite of the importance of the test, however, it is not always conclusive. A very rich milk will show a suspiciously low specific gravity, while a skimmed milk, diluted with water to the proper density and colored to an agreeable cream-tint by "Richardson's Perfected Butter Color" or other dye, will escape so much as the breath of scandal.

The amount of cream is generally determined by permitting the sample to stand in a graduated jar until the cream separates, and then reading off the volume percentage directly. Centrifugal machines are also used, but a simpler test than either of these is usually sufficient. If a closed tube, blackened on the inside, is dipped into the sample, and slowly withdrawn, one can judge of the richness of the milk by the opacity of the film remaining on the tube.

To determine the total solids, five cubic centimetres of the sample of milk are placed in a small platinum dish of known weight, and the joint weight of milk and dish then obtained. The difference between the two will be the amount of milk taken for analysis. It is then evaporated to dryness on a water-bath without stirring. This will take about an hour, at the end of which time the dish will be found to contain a yellowish-white mass which shows a thin, transparent film on top and a honeycombed structure beneath. A number of cracks will extend throughout the mass, on account of the contraction on drying, but do not indicate that anything is amiss. The dish and contents are then put into an oven at 212° F., and at the end of half an hour are taken out and weighed. The known weight of the dish subtracted from the weight thus obtained gives the weight of the total solids. The ratio of this weight to the weight of milk taken for analysis will give the percentage value.

If it be desired to find the amount of fat, petroleum benzine is poured over the residue and is renewed at the end of an hour. After standing half an hour longer, the solvent is again decanted. The residue is thoroughly washed with benzine, and is dried in an oven at 212° F., in order to remove all traces of the fluid. The loss of weight experienced as the result of this treatment equals the amount of fat.

The amount of mineral salts is determined by igniting the residue until it is perfectly white. The heat must be very gradually increased in order to avoid loss from sputtering. The ash is then directly weighed, and its percentage value obtained as in the case of the total solids, that is, by the ratio of its weight to the amount of milk originally taken. The water is readily found by subtracting the per cent of total solids from 100.

The operations described above are all very easily made, and require comparatively but a short time. They presuppose, of course, the employment of a certain amount of apparatus, but if one can depend upon the courtesy of some chemical or pharmaceutical friend for the use of an analytical balance, all of the other requirements can be met at no very great expense. An analysis such as has been outlined, will show conclusively the quality of any suspected sample of milk, for the milk will deviate from the normal condition just in proportion as its constituents differ from those of the standard given. If, for instance, it is found that the specific gravity and percentage of total solids are both low, the addition of water is plainly indicated. By calculating the amount of whole milk corresponding to the total solids found, it is possible to state just how much water has been added. When it is found that the specific gravity is as it should be, and the total solids are between 12 and 12·5 per cent, it is seldom worth while to continue the analysis further than a determination of the mineral salts, for it would scarcely be possible for these three quantities to be what they should be unless the milk were quite normal. Thus, in a sample of milk recently analyzed by the writer, the specific gravity was found to be 1·033, the total solids 12·077 per cent, the mineral salts 0·598 per cent, and the water 87·923 per cent. Though the solid constituents were somewhat low, the discrepancy was too slight to fancy for a moment that the milk had been tampered with in any way.

Occasionally chalk or lime-water is added to give body to a diluted milk. Its presence will be readily detected by the increased amount of ash, which may be examined by the well known qualitative methods. During the warmer months, the difficulties of keeping milk fresh not infrequently lead to the addition of bicarbonate of soda or borax, but as a preservative the latter salt is an absolute failure. It simply keeps the curd from precipitating, but does not prevent the decomposition of the milk. After standing some time such a milk will be found to emit a very putrid, disagreeable odor.

Though the addition of these adulterants is far from commendable on the part of the milkman, the reflection is not without comfort that they exercise for the most part no injurious influence on the animal economy—a reflection, by the way, which can not be indulged in about the majority of food adulterants.