Experiment No. 20

Testing for Watering or Skimming of Milk

a. Fat Content of Milk by Means of Babcock Test.—Measure with pipette into test bottle 17.6 cc. of milk. Sample should be carefully taken and well mixed. Measure with cylinder 17.5 cc. commercial H₂SO₄ and add to milk in test bottle. (See Fig. 25.) Mix acid and milk by rotating the bottle. Then place test bottles in centrifugal machine and whirl 5 minutes. Add sufficient hot water to test bottles to bring contents up to about the 8th mark on stem. Then whirl bottles 2 minutes longer and read fat. Read from extreme lowest to highest point. Each large division as 1 to 2 represents a whole per cent, each small division 0.2 of a per cent.

b. Determining Specific Gravity by Means of Lactometer.—Pour 150 cc. of milk into 200 cc. cylinder. Place lactometer in milk and note depth to which it sinks as indicated on stem. Note also temperature of milk. For each 10° above 60° F. add 1 to the lactometer number, in order to make the necessary correction for temperature. For example, if milk has sp. gr. of 1.032 at temperature of 70°, it will be equivalent to sp. gr. of 1.033 at 60°. Ordinarily milk has a sp. gr. of 1.029 to 1.034. If milk has sp. gr. less than 1.029, or contains less than 3 per cent fat, it may be considered watered milk. If the milk has a high sp. gr. (above 1.035) and a low content of fat, some of the fat has been removed.

(For extended direction for milk testing see Snyder's "Dairy Chemistry.")