Why isn't cow's milk contaminated by what's in the animal's intestines?

We put this question to biologist Andrew...

Andrew - Well, if we just move back from the question and think about what the intestines have to do. So intestines have to get all the stuff out that grass partially digested into the body but only the bits we want. So they mustn't get out all the bacteria which could be toxic. You don't want to get it into your bloodstream and get septicaemia. You don't want to get any toxins that are on the food. So, your gut really is a very, very clever system that only transport and get what it wants. So, some of that is done by osmosis. So the concentration of different molecules draws the water out and some of it is done actively. So energy tends to pour food out. Of course, you then have the blood system actually, not a tiny gap before you ended up with the other while you make the milk. Of course, the cells there are very carefully putting the right things in that milk, not just anything they come across. So, if you didn't have a good barrier with your intestines, you would be getting really sick over time. So it wouldn't just be going in milk. It would be going everywhere. Actually, if we think about the milk, for the one thing you probably don't think about is you do get infections in others or in people in breasts and you get mastitis. This is a real problem for people with breastfeeding because you got this milk which is very good for growing bacteria. You get this really sore infections and it's really unpleasant for the mother. Of course, the solution today is just antibiotics but you do have to make sure you don't get bacteria in there because they have these horrible side effects.

Kat - And also, there are some bacteria in milk and that's presumably why we pasteurise it so that we don't get sick from drinking it.

Andrew - Yeah. But they can come in different stage. They can come in through the pumps and stuff like that. What you give to a baby also come from many of the immunities from the mother as well. So, it's a complicated product, milk.