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The Relations Between the Hydrogen-Ion Concentration and the Bacterial Content of Commercial Milk

From the Department of Bacteriology and Experimental Pathology, Stanford University, California

ABSTRACT

During the summer of 1916 the senior author, while working in the department of pediatrics of the Johns Hopkins University, began studies to determine the relationship between the hydrogen ion concentration and the bacterial count of cows' milk. The work then interrupted has recently been continued. The object of this paper is to present the methods employed and the result's thus far obtained.

The hydrogen-ion concentration of fresh cows' milk and also that of sour milk, in other words, the range of acidity, has already been determined by a number of workers. Its pH runs from approximately 6.8, for fresh milk, to 4.6, for completely soured milk. On the basis of these facts standard solutions covering this particular range of acidity were prepared.

Preparation of Standard

The standard solutions are prepared by mixing in certain proportions fifteenth molecular solutions each of acid potassium phosphate and alkaline sodium phosphate. These initial solutions are prepared as follows: (a) Fifteenth molecular solution of acid potassium phosphate is prapared by dissolving 9.078 grams of the pure, recrystallized salt (KH₂PO₄), in distilled water and making it up to 1 liter.