ELI5 How does breast milk not get sour in breasts?

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My wife breastfeeds our newborn, and after (inadvertently) leaving milk out on the counter for 2 hours, I got to wonder: how is it that she can not feed/pump for 6 hours while our child sleeps, and still have good milk? Body temp is 37°C and room temp is around 20°C.

In: Biology

Because it’s circulating like blood in the human body, with new and old cells within the ‘milk’ dying and being born anew much like sperm.

The ducts in the breast are an isolated environment where the bacteria that would cause it to “sour” are unable to survive and do so. Once outside the breast it is vulnerable to all the bacteria in the air and can sour fairly quickly as a result.

For the same reason that you don’t rot like unrefrigerated meat—you’re alive and have an immune system working all the time to kill bad bacteria

Breasts aren’t just big bags of milk, despite how they might look. Its more similar to sweat: produced on-demand, as needed. The act of nursing stimulates milk production, both in the short term, while the baby is actively feeding, and in the long term to keep the breasts producing milk until the baby is weaned.

Because breast milk isn’t stored in the breast like some kind of tank, they’re secreted on demand, and usually there’s a baby sucking every few hours anyway, otherwise the mother stops producing milk