Note 3 — Chapter I · Production, Composition and Characteristics of Milk, Milk Ferments, Pasteurization

The Acid Test depends upon what in the laboratory is called “titration” and makes use of a “burette,” a long, graduated measuring tube provided with a pinch-cock. This burette is filled with an alkali solution of known strength, usually a “tenth normal” solution of caustic soda. A certain amount of the milk to be tested is measured off into a glass or a white porcelain cup. As a 17.6 c.c. pipette belonging to the Babcock test usually is at hand, that may be used for this purpose. A few drops of an Indicator is added to the milk and under constant stirring the soda solution is allowed to drip into it until suddenly it turns pink. The color will quickly disappear, however, and a few more drops of the alkali are added and stirred in several times until a faint but distinct pink color remains for some time. That indicates that the acid in the milk has been neutralized and the amount of the soda solution consumed is then read off on the scale on the burette. By dividing the number of c.c. of the soda solution used by two, the tenths per cent of lactic acid in the milk is found. For example, if it takes 4 c.c. of the soda solution to neutralize 17.6 c.c. milk, the acidity is .2%. This depends upon the fact that 1 c.c. of a tenth normal soda neutralizes .009 gram of lactic acid and that therefore the per cent of acid in the milk is equal to .009 multiplied by the number of c.c. of soda solution used, divided by the number of c.c. of milk and multiplied by 100.

If 50 c.c. of milk is taken instead of 17.6 the calculation is changed accordingly.