I guess the first question is, does milk spoil when it is still inside the animal and if not, why does that not happen? Like cow milk. If I don’t milk that cow for a week, the milk wouldn’t spoil, right? But if I put a jug of pasteurized milk out in the environment, it would spoil, right?
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Milk spoiling in your fridge is caused by the fact that bacteria gave entered it and started to colonize it. Even when pasteurized little bacteria remains and will eventually start to spoil the milk. Inside the animal those bacteria are actively fought by the animals immune system and the immune cells in the milk. That’s why mother’s milk is good for infants. The immune cells within the milk itself keep any bacteria at bay for a developing immune system.
Now the milk within the animal will eventually break down if not consumed. As it does it is reabsorbed by the body. If that happens enough the body will send out a hormone signal telling the animal to stop producing milk. This is why cows need milked regularly or they’ll “dry up”…well that and the breeding done that makes them produce more milk than is comfortable to have stored in their udder when full.
Milk inside the cow is “sterile” (relative to the cow). Milk that has left the cow has now been exposed to airborne bacteria, which leads to spoilage if not properly treated and stored. While pasteurization kills bacteria, it doesn’t necessarily kill **all** bacteria, so it just extends the shelf life instead of making it not spoil.
That being said, cows typically use their milk to feed their calves (or have it stolen by us), so there wouldn’t normally be milk held there indefinitely.
Spoilage is caused by microbes such as mold and bacteria. The animal has an immune system to fight off these microbes. Once the milk leaves the body, there’s nothing to protect against the microbes anymore.
Your body is made up of about 60% water. If you took a glass of water and left it on the counter for a couple weeks, mildew would start growing in it. But the water in our body is much older than a couple weeks, and has no mildew. The mildew will only grow because microbes got in the water. Inside the body, those microbes would be killed by our immune systems. It’s the same thing with milk in mammals.
Milk isn’t stored in mammals. You might get 8 gallons of milk from a cow in a day, but the cow never has that milk inside. There isn’t even a glass of milk. The milk is produced while the cow is milked. Milk can’t exactly spoil when it doesn’t exist.
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