Milk contains lactic acid, which develops in the milk over time. Lactic acid originates when bacteria break down lactose, which is a sugar found in the milk. This acid is not found in milk that has just been expressed.
Milk fat also contains about 400 different fatty acids. The pH of milk is neutral or slightly acidic if tested with litmus paper. A neutral pH is 7, and milk has a pH of about 6.7. Although the composition of milk varies depending on how cows are bred, fed and managed, it usually contains about 80 percent water. Milk from other mammals has varying composition but a similar pH.