Cow's milk is a very weak acid, with a pH just below the neutral value of 7. The pH of fresh milk ranges from 6.8 to 6.4. As milk goes sour, the pH drops and the acidity increases due to the buildup of lactic acid.
The pH scale measures how acidic or basic something is using a scale from 0 to 14. Water has a pH of 7 and is in the middle of the scale. Water ionizes into hydrogen ions and hydroxide ions, and the presence of these two ions in a solution determine whether it is an acid or a base. If the hydrogen ions are in excess, the solution is an acid. If there are more hydroxide ions, the solution is a base.