In detail the test is made as follows : The milk to be sampled is thoroughly mixed by pouring it several times from one vessel to another. By means of a milk pipette, or measure, graduated to hold 17.6 cc, this quantity of milk is transferred to a special form of bottle, which has a capacity of a little more than one ounce and a long neck with graduations or per cent marks from 0 to 10. The cubic capacity of the neck, from 0 to 10, is exactly 2 cc. This is the volume of 1.8 grams of melted fat, which is the substance to be measured on the scale. As the bottle is so graduated that 1.8 grams represents 10 per cent, it is necessary to use a sample weighing ten times as much, or 18 grams, and it is found that the 17.6 cc. pipette will deliver approximately this weight of milk. There is then added 17.5 cc. of concentrated commercial sulfuric acid, having a specific gravity of 1.82 to 1.83. The acid and milk are mixed by a rotary motion. The action of the acid on the water and solids of the milk generates considerable heat. The sample is promptly placed in a centrifugal machine and whirled for five minutes. Hot water is then added to bring the fat to the base of the neck. It is then whirled two minutes, and more hot water is carefully added until the fat rises in the neck so that it is opposite the graduations. The sample is then whirled one minute, to insure collecting as much fat as possible in the neck. While the fat is still warm, its percentage is ascertained by reading the marks at its upper and lower levels and taking the difference between them.

The cost of a small complete outfit for testing milk is $6 to $10.

Computing total solids of milk.

Babcock and Richmond have proposed formulae for computing the total solids of milk. One of the best is : —

L + 1.2 F + .14 = total solids. 4

L represents the second and third decimal figures of the specific gravity, or the Quevenne reading, and F represents the percentage of fat. This formula is used largely, and for practical purposes agrees closely enough with results of gravimetric analysis.

Test for acid in milk (Pearson).

It is not practicable to isolate lactic acid from milk and measure it as milk-fat is measured. But its quantity can be easily determined by slowly adding to a known weight of milk an alkali of known strength until all the acid is neutralized. The neutralization is indicated by phenolphthalein, which was previously added to the milk and which causes the milk to turn pink as soon as it begins to show an alkaline reaction. It is customary (Mann's test) to use deci-normal alkali solution, 1 cc. of which will neutralize .009 gram of lactic acid. The equipment includes, besides the neutralizer and phenolphthalein, a burette for measuring the neutralizer, cup and glass rod. If twenty grams of milk is used and it requires 6 cc. of alkali to neutralize the acid, it is known that the milk contains 6 X .009 or .054 gram of lactic acid, or .27 per cent. Alkali tablets (Farrington's), each capable of neutralizing .034 gram of acid, are on the market. They may be used in solution instead of the deci-normal solution.

Test for boiled milk.

It is sometimes desirable to determine whether milk has been subjected to 176° F. or higher heat. A successful test has been devised by Storch. To 5 cc. of the suspected milk add a few drops of potassium iodid and a similar quantity of starch solution, also a few drops of hydrogen peroxid. If the milk has not been cooked, an enzyme which is present will decompose the hydrogen peroxid, setting free oxygen. This combines with the potassium salt, and thus iodine is in turn set free and with the starch it forms a purple color. If the milk has been heated so that the enzyme is killed, no color will result.

Another test for cooked milk is given by Arnold, as follows : Tincture of guaiac is added, drop by drop, to a little milk in a test-tube. If the milk has not been heated to 176° F., a blue zone is formed between the two fluids. If it has been heated, there is no reaction. The guaiac-wood tincture is said to be more reliable than other tinctures, and it should not be used when fresh, but when at least a few days old and its potency has been determined.