The lactometer test for specific gravity in milk (Pearson).

As the specific gravity of milk is markedly changed when it is adulterated by the addition of water or the removal of cream, the lactometer is an important instrument to indicate such adulteration. It is of little use if both kinds of adulteration have been practiced on the same sample of milk, as the increase in weight due to removal of cream can be offset by the addition of water, which is lighter than skimmed milk. In connection with the Babcock test, the lactometer is most valuable, and several formulae are in use by which the solids not fat or the total solids of milk may be closely computed from the specific gravity and the fat test.

The lactometer is a form of hydrometer adapted especially for use in milk. Several styles are in use, the Quevenne being the most convenient because its readings indicate the specific gravity without the necessity of more than a simple mental calculation. The readings on the stem of the Quevenne lactometer are from 15 to 40, and they represent the second and third decimal figures of the specific gravity, the preceding figures always being 1.0 ; thus, a reading of 29 represents a specific gravity of 1.029. This instrument should be used in milk at a temperature of 60° F. If the temperature varies therefrom, a correction of the reading must be made, .1 of a lactometer degree being added to the reading for each degree of temperature of the milk above 60° F. or if the temperature is below 60° F, .1 of a lactometer degree is subtracted from the reading for each degree of temperature of the milk below 60° F. Thus, if the lactometer reads 31 at a temperature of 65° F., the corrected reading for 60° F. would be 31.5, and the specific gravity of this milk at 60° F. would be 1.0315. Special tables for making corrections for different temperatures are published in books treating on the subject. By the rule given, it is not advisable to attempt to correct for a variation of more than 10° from 60° F.

Another style of lactometer in common use is known as the New York Board of Health lactometer. Its graduations are from 10 to 120. The instrument stands at 100 in milk having a specific gravity of 1.029, and it would stand at 0, if graduated to that point, in a fluid having a specific gravity of 1. Thus, 100° in the B of H lactometer equals 29° on the Quevenne lactometer, and it is a simple matter to compute the equivalent reading of one lactometer for any given reading on the other by the formula : —

Q = .29 B of H, or B of H = Q


Test for boric acid or borax used as preservatives (Van Slyke).

Add lime-water to 25 cc. of milk until the mixture is alkaline to phe-nalphthalein ; evaporate to dryness and burn to an ash in a small porcelain or platinum dish. Add a few drops of dilute hydrochloric acid to the ash, care being taken not to use too much acid, then add a few drops of water, and place a strip of turmeric paper in this water solution. Dry the paper, and if either borax or boric acid is present, a cherry-red color will appear. This test is confirmed by moistening the reddened paper with a drop of an alkali solution, when the paper will turn to a dark olive color, if borax or boric acid is present.

Test for formaldehyde in milk.

This test can be performed in connection with the Babcock test. Measure into the Babcock test bottle 17.6 cc. of milk. Add five or six drops of ferric chloride solution and shake thoroughly. Add 17.5 cc. of sulfuric acid, but do not mix the acid and milk. If formaldehyde is present, a lavender-colored ring will appear at the point of contact of the acid and milk. If the contents of the bottle are mixed slowly, the entire mass of curd will turn a lavender color. This test will not work if the sample is too old.

Standardizing milk (Pearson).

Standardized milk is that which has been changed in its composition to cause it to contain a required amount of fat. This is usually accomplished by adding cream or skimmed milk. A convenient rule for determining the amount of ingredients to make a mixture testing a certain per cent of fat, is as follows, supposing cream and milk are to be used (in most States it is unlawful to add skimmed milk): —