Page:The story of milk.djvu/43

 milk and the percentage can be easily determined. Centrifugal force had already been used in the Fjord Cream Test, but it remained for Dr. Babcock to work out the splendid practical and reliable test which bears his name and in which he has given to the world an invention of incalculable value.

A two-bottle machine

Sampling.—Whether it is new milk or skim milk or buttermilk or cream that is to be tested the first thing to observe is to take a truly representative sample. The liquid must be thoroughly mixed by pouring it several times from one vessel to another, or stirring vigorously.

handled with care. Pour it slowly down along the wall of the bottle which is held at an angle and turned slowly during the operation.
 * [Footnote: the milk in the test bottle. The acid is a strong poison and must be

Now give the bottle a rotary motion to thoroughly mix the milk and the acid, shaking vigorously towards the end of the operation so as to be sure not to leave any of the acid which is heavier than the milk at the bottom of the bottle.

Whirling.—The bottles are then placed in the centrifugal machine and whirled for five minutes at the proper speed—from 600 to 1200 revolutions per minute—according to the diameter of the machine and as stated in the directions which come with the tester. The mixture of milk and acid is hot enough if the whirling is done at once, but if it is allowed to cool the bottles should be placed in hot water of 150 to 170° for about 15 minutes; whirling at full speed for 4 minutes brings all the fat to the top.

Hot water is now added until the bottle is filled almost to the scale on the neck and the bottles are again placed in the machine and whirled at full speed for one minute. Hot water is then again added until the lower end of the fat column is within the scale, preferably at the 1% or 2% mark on the neck of the bottle. Whirl once more for one or two minutes and then read off the percentage of fat on the scale. Each division represents 0.2% fat. The fat column is measured from the lower line between the fat and the water to the point where the top of the fat column touches the wall of the neck. A pair of dividers are handy for measuring the fat column and reading off the percentage of]*