Cows on dairy farms are milked multiple times per day by machines. These machines suck everything they can out of the cow’s udder: milk, blood, pus, bacteria, and even chemicals, hormones, and antibiotics used to keep cows productive and profitable for as long as possible until their trip to the slaughterhouse. Because there is no way for the machine to know what to take out and what to leave in, it sucks everything out. This milk is pasteurized, homogenized, and bleached to hinder contamination as well as make the look, taste, and texture more appealing than raw milk (which tastes much different than the common milk found in grocery stores, and also has a yellow tinge to it).
Cows do not need to be milked by humans. Their udders will not explode if we don’t milk them. Mother cows only need to nurse their young. In the dairy industry they are deprived of this ability so that humans can have it instead.