The phrase "to contaminate the milk" means to make something pure or good become impure or bad by adding something harmful or dirty to it. Full definition
Well, a lot of things could lead to your dog having stomach issues, ranging from drinking of contaminated milk or water to eating foods which are very sensitive to their stomach. (itday.com)
This happened two years after the death of his son from contaminated milk. (healthimpactnews.com)
The worst thing in the world would be to give your baby contaminated milk due to not cleaning parts properly. (thelactatingmama.com)