Owing to the very important sanitary relations of milk as a model food, the subject of its sophistication has during the past ten years received particular notice at the hands of the food-chemist. The investigations of our public sanitary authorities have shown that milk adulteration is exceedingly common. It is stated upon good authority that until quite recently (1883) the 120 millions of quarts of milk annually brought into New York city were intentionally diluted with 40 millions of quarts of water, the resulting product rivalling in richness the famous compound once lauded by the philanthropic Squeers.

The results of the examination of milk instituted by the New York State Board of Health are given below, in which, however, the specimens of skimmed milk are not included:—

From October 1883 to March 1884, of 241 samples of milk examined by the Public Analyst of Eastern Massachusetts, 21·37 per cent. were watered; of 1190 samples tested during the year 1884, 790 were watered.[18] Over 73 per cent. of the milk supplied to the city of Buffalo in 1885 was found to be adulterated. A very marked improvement in the quality of the milk received in New York city has taken place since the appointment of a State Dairy Commissioner (1884). Under the direction of this official the metropolitan milk supply has been subjected to a most rigid inspection, and with very satisfactory results. During the years 1884 and 1885 nearly 45,000 samples of milk were examined.

A very common sophistication practised upon milk consists in the partial or complete removal of its cream. This process of skimming is conducted at establishments called “creameries,” of which sixty-three were formerly known to send their impoverished product to New York city. The State Dairy Commissioner has likewise accomplished much towards stopping this form of adulteration.

Milk is the secretion of the mammary glands of female mammalia. It is an opaque liquid, possessing a white, bluish-white, or yellowish-white colour, little or no odour, and a somewhat sweetish taste. At times it exhibits an amphigenic reaction, i. e. it turns red litmus blue and blue litmus red. From the examination of nearly one thousand cows in the States of New York, New Jersey, and Connecticut, the minimum specific gravity of milk was found to be 1·0290, the maximum being 1·0394. The opacity of milk is only apparent, and is due to the presence of fatty globules held in suspension; these under the microscope are seen to be surrounded by a transparent liquid. Upon allowing milk to remain at rest for some time it experiences two changes. At first, a yellowish-white stratum of cream rises to the surface, the lower portion becoming bluish-white in colour and increasing in density. If this latter is freed from the cream and again set aside, it undergoes a further separation into a solid body (curd), and a liquid (whey). This coagulation of the curd (caseine) is immediately produced by the addition of rennet, and of many acids and metallic salts.

The essential ingredients of milk are water, fat, caseine, sugar (lactose), and inorganic salts. The following table, collated by Mr. Edward W. Martin,[19] exhibits the results obtained by numerous authorities from the analysis of pure cow’s milk:—

Mr. Martin obtained the following results from the examination of cream separated by centrifugal force, and of skimmed milk:—

The proportion of mineral constituents in milk usually ranges between 0·7 and 0·8 per cent. The average composition of milk ash is as follows:[20]—

The tabulation below gives the composition of human milk and the milk of various animals:—

Several varieties of preserved and condensed milk have, for a number of years, been placed upon the market. The composition of the best-known brands of these preparations is as follows:—

Preserved Milk.

Condensed Milk.

Analysis.

The principal adulterations of milk (watering and skimming), are detected by taking its specific gravity, and making quantitative determinations of the total milk solids, the fat, and the milk solids not fat. Of these criteria, the last-mentioned is the most constant and reliable.

Physical Examination.

a. Specific Gravity.—The instrument employed by the New York health inspectors for testing milk is a variety of the hydrometer, termed the lactometer, and its use, which is based upon the fact that under ordinary conditions watered milk possesses a decreased density, is certainly of great value as a preliminary test. The Board of Health lactometer indicates specific gravities between 1·000 (the density of water) and 1·0348. On its scale 100° represents the specific gravity of 1·029 (taken as the minimum density of genuine milk), and 0 represents the density of water; the graduations are extended to 120°, equivalent to a specific gravity of 1·0348. In taking an observation with the lactometer, the standard temperature of 15° should be obtained, and the colour and consistency of the milk noted. If these latter properties indicate a dilution of the sample, and the instrument sinks below the 100° mark, it is safe to assume that the milk has been watered. The scale is so constructed that the extent of the dilution is directly shown by the reading, e. g. if the lactometer sinks to 70° the sample contains 70 per cent. of pure milk and 30 per cent. of water. As the standard of specific gravity (1·029) selected for the 100° mark of the lactometer is the minimum density of unwatered milk, it is evident that the readings of the instrument will almost invariably indicate an addition of water less than has actually taken place. It would therefore appear that, under normal circumstances, the standard adopted by the New York Board of Health errs on the side of too much leniency toward the milk dealer. Cream being lighter than water, a sample of skimmed milk will possess a greater specific gravity than the pure article, and it is possible to add from 10 to 20 per cent. of water to it and still have the resulting admixture stand at 100° when tested by the lactometer. Vehement attempts have been made in court and elsewhere to impeach the accuracy of the indications afforded by the lactometer. These have been mainly founded upon the fact that a sample of milk unusually rich in cream will have a lower density than a poorer grade, so that it is quite possible that milk of very superior quality may show a gravity identical with that of a watered specimen. Great stress has been laid upon this by the opponents of the measures to control milk adulteration adopted by the public sanitary authorities. They have contended that a chemical analysis should be made. Recourse to this method would, however, involve a greater amount of time than it is usually practicable to devote to the examination of the numerous samples daily inspected; moreover, the process is resorted to whenever the indications of the lactometer leave the inspector in doubt. With the exercise of ordinary intelligence this contingency seldom arises, as the proportion of cream required to reduce the specific gravity to that of a watered sample would be more than sufficient to obviate any danger of mistaking the cause of the decreased density. In this connection it should be stated, that the average lactometric standing of about 20,000 samples of milk, examined by the New York State Dairy Commissioner in the year 1884, was 110°, equivalent to a specific gravity of 1·0319.

The following table shows the value of lactometer degrees in specific gravity:—

Value of Lactometer Degrees in Specific Gravity.

Chemical Examination.

b. Water, Total Solids, and Ash.—Five grammes of the fresh milk are weighed in a tared platinum dish, having a flat bottom, which is placed on a water-bath, where it is allowed to remain for about three hours. It is then transferred to a water-oven, and the dish is subsequently weighed, from time to time, until the weight becomes constant. The loss in weight is the water present; the difference between the weight of the platinum capsule and its weight with the remaining contents gives the amount of total solids, which, in milk of good quality, should not be under 12 per cent. The inorganic salts (ash) can now be determined by carefully incinerating the residual contents of the capsule. Too high a temperature is to be avoided in this process, in order to prevent the fusion of the ash, which should, however, be ignited until it shows a greyish-white colour. The amount of ash in genuine milk ranges from 0·70 to 0·80 per cent. The addition of water naturally decreases this proportion as well as that of the total milk-solids.

c. Fat, Milk Solids not Fat, Caseine, and Milk Sugar.—An approximate estimation of the fat in milk was formerly made by the use of the creamometer. This instrument consists simply of a long glass tube, provided at its upper end with a scale. The milk under examination is introduced into the tube and allowed to remain at rest for about 24 hours, or until the stratum of cream has completely collected upon its surface; the quantity is then read off by means of the attached scale. The results afforded by the creamometer are, however, far from reliable. Cream is really milk rich in fat, caseine, etc., and the quantitative relation it bears to the true amount of fat present is not always a direct one. A recent form of lactoscope, devised by Feser, is less objectionable, and is in very general use for the rapid estimation of fat in milk. It consists essentially of a glass cylinder, provided with two scales, one being graduated into c.c., the other, into percentages of fat. In the lower end of the instrument is a contraction, in which is placed a cylindrical piece of white glass, graduated with well-defined black lines. In using the lactoscope, 4 c.c. of the milk are introduced into the instrument by means of a pipette, and water is gradually added, with shaking, until the black marks on the small white cylinder become just visible. Upon now referring to the c.c. scale, the quantity of water used to effect the necessary dilution is ascertained, and the corresponding percentage of fat in the sample is indicated by the percentage scale.[22]

In the gravimetric determination of the fat (butter), 10 grammes of the milk are put into a tared platinum dish, containing a weighed amount of dry sand. The milk is evaporated as previously directed, the mixture being constantly stirred with a small platinum spatula. The residue is repeatedly treated with warm ether or petroleum naphtha of 70° B., and the solutions poured upon a small filter. The several filtrates are collected in a tared beaker, and cautiously evaporated, until constant weight is obtained. This will give the amount of fat. The undissolved residue remaining in the platinum capsule, or the difference between the quantity of fat and that of the total milk-solids, affords the proportion of milk solids not fat contained, which, in unadulterated milk, should amount to 9 per cent. It has been determined by experiment, that every percentage of milk-solids not fat, increases the specific gravity of milk 0·00375, whereas each percentage of fat decreases the gravity 0·0010, and the proportion of solids not fat can be calculated from the data afforded by the lactometer and Feser’s lactoscope by means of the formula:—

(S - A) 0·00375 ,

where S is the specific gravity of the milk, as shown by the lactometer, and A is the remainder obtained upon multiplying the percentage of fat indicated by the lactoscope by 0·001 and subtracting the residue from 1·0000.

The residue remaining after the extraction of the fat is treated with warm water containing a few drops of acetic acid, or with dilute (80 per cent.) alcohol, in order to remove the sugar. The residue is dried until it ceases to decrease in weight, and is then weighed. The difference between the original weight of the sand and the weight of the sand and residue combined represents approximately the amount of caseine (albuminoids) present. As this contains a certain proportion of ash it is to be subsequently ignited, and the ash obtained deducted from the first weight. The alcoholic sugar solution is evaporated to dryness and weighed. The residue is then incinerated and the weight of ash is subtracted. The difference is the amount of milk sugar contained. The sugar may likewise be determined by means of Fehling’s solution (see pp. 37, 111). About 50 c.c. of the milk is warmed with a small quantity of acetic acid to precipitate the caseine, which is removed by filtration, and the filtrate diluted to 500 c.c.; the test is then applied. 10 c.c. of the copper solution represents 0·067 gramme of milk sugar.

The sugar in milk can also be estimated by the polariscope (see under Sugar, p. 112). In case the Ventzke-Scheibler instrument is used, 65·36 grammes of the sample are weighed out and introduced into a 100 cc. flask; about 5 cc. of plumbic basic acetate solution is added, and the liquid is well shaken, and then allowed to stand at rest for a few minutes. It is next filtered, its volume made up to the 100 cc. mark, and the 20 cm. tube filled and the reading made; this divided by 2 gives the percentage of sugar in the milk.

Mr. A. Adams[23] has recently proposed a method of milk analysis which consists in first placing 5 cc. of the sample in a tared beaker, and then introducing a weighed paper coil made of blotting paper from which all fatty matter has previously been removed by washing with ether. As soon as the milk is completely absorbed, the paper coil is removed and dried at 100°. The increase of weight gives the amount of total solids. The fat is next extracted by petroleum naphtha or ether, and its weight determined. The proportion of solids not fat is ascertained by again drying and weighing the exhausted coil.

The standards adopted by the English Society of Public Analysts for pure milk, are:—

In the State of New York, the legal standards for milk are that it shall not contain more than 88 per cent. of water, nor less than 12 per cent. of milk solids, and 3 per cent. of fat.

In Massachusetts the law fixing a chemical standard of purity for milk reads: “In all cases of prosecution, if the milk shall be shown upon analysis to contain more than 87 per cent. of water, or to contain less than 13 per cent. of milk solids, it shall be deemed, for the purpose of this Act, to be adulterated.”

The Board of Health of New Jersey fixes the minimum amount of total solids at 12 per cent. and the maximum amount of water at 88 per cent. In Paris, the minimum limits for condemnation are the following:—

Fat, 2·70; milk-sugar, 4·50; caseine, albumen, and ash, 4·30; total solids, 11·50.

The following proportion can be employed in the calculation of the amount of pure milk (x) contained in a suspected sample:—

From the total solids:—

12·5: total solids found = 100 : x.

From the solids not fat:—

9·30: solids not fat = 100 : x.

From the sugar:—

4·40: sugar found = 100 : x.

From the specific gravity:—

1·030 : sp. gr. = 100 : x.

In most cases the determination of the total milk-solids and the fat (the difference being the solids not fat) furnishes all the data required for determining the amount of watering which a sample of milk has undergone. The Society of Public Analysts use 9 as the average percentage of solids not fat in pure milk (which is generally considered as too low) and adopt the formula:—

100 9 S = x,

in which x represents the percentage of genuine milk, and S the solids not fat.

PLATE IV.

ARTOTYPE. E. BIERSTADT, N. Y.