The Cause of Acidity of Fresh Milk of Cows and a Method for the Determination of Acidity. by Lucius

The usual method employed in determining the acidity of milk is to add a few drops of a solution of phenolphthalein as indicator to 100 cc. of milk and then titrate with 6 NaOH. By the use of this method it is found that 100 cc. of milk, when strictly fresh, will require the addition of 15 to 20 cc. of the alkali in order to produce a faint but permanent pink coloration. The acidity of fresh milk has been commonly attributed to the presence of acid phosphates and casein, and we will now consider the relation of these constituents to milk acidity. That the acidity of milk is due to the presence of acid phosphates (M HzP04) is indicated by the fact that milk is strongly alkaline to methyl orange. Further, it is well known that phosphates can not be titrated with any degree of accuracy in the presence of calcium salts, due to the fact that some of the insoluble dicalcium phosphate (CaHP04), which is formed during the titration, hydrolyzes, changing into calcium hydroxide and phosphoric acid, and then the calcium hydroxide unites with more dicalcium phosphate, forming tricalcium phosphate (CaaPzOs).’ These facts may be represented by the following equations: