Science Questions

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

Mon, 5th Sep 2016

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Mohammed Laher asked:

Cows eat grass and drink water and produce pure, wholesome milk which resides in the body close to the the intestines. Yet the smell does not penetrate the milk. How?




We put this question to biologist Andrew...Cow

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 mustnít get out all the bacteria which could be toxic. You donít want to get it into your bloodstream and get septicaemia. You donít 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 didnít have a good barrier with your intestines, you would be getting really sick over time. So it wouldnít just be going in milk. It would be going everywhere. Actually, if we think about the milk, for the one thing you probably donít 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 itís really unpleasant for the mother. Of course, the solution today is just antibiotics but you do have to make sure you donít get bacteria in there because they have these horrible side effects.

Kat - And also, there are some bacteria in milk and thatís presumably why we pasteurise it so that we donít 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, itís a complicated product, milk.




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Is what a "fluke of nature"? Is this a question about physiology? Biochemistry? Mammalian evolution? AndroidNeox, Sun, 4th Sep 2016

Milk is produced by the mammary glands.
The mammary glands take raw ingredients from the bloodstream and turn it into milk. Milk is close to the bloodstream, but it doesn't taste of blood.

The intestine breaks down food and turns it into small-molecule nutrients which make it across the intestinal wall and into the bloodstream. These small molecules don't smell much like the intestines.

The liver takes these small molecules  in the bloodstream and detoxifies the blood.

In the case of a cow (or ruminants in general), there are several stages in the processing of grass, through several stomachs, each with a soup of microbes breaking it down into nutrients.

There are several barriers between the intestines and milk, with radical changes in chemical composition at each barrier. It's not surprising that milk doesn't smell like intestines. 
evan_au, Sun, 4th Sep 2016

But woman can, as does every other female mammal, and IIRC human milk does not reek of fish and chips. That said, some small molecules such as allicin (garlic) and ethanol can be excreted through the mammary system, and some radionuclides and labelled pharmaceuticals can turn up in significant quantities in milk from any source.   

We can indeed manufacture artificial fats and proteins but the raw material, coal or oil, is generally more expensive than grass for which we have no other use, so cows, sheep, camels and goats turn out to be economic, if chemically inefficient, sources of dairy products. The fact that different mammalian milks have different flavors suggests that individual diet and physiology play a small part in milk production and I've no doubt that a baby seal or whale would find fresh cow's milk decidedly "grassy". alancalverd, Sun, 4th Sep 2016

Different glands in our bodies produce substances as diverse as sweat, insulin, prolactin, saliva, oxytocin, milk, bile, adrenalin, stomach acid, tears, earwax and pituitary hormone, all from the same basic mix of starting materials in the blood.

Beyond that is muscle cells, bone, brain, hair and toenails, all from the same blood supply.

A pretty amazing creation, huh?

We are just starting to understand the details of how it all works together, now we can read and manipulate DNA more easily. evan_au, Mon, 5th Sep 2016

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