why does mixing milk of the different temperature make it go bad faster?

Question

I’ve seen this rule for pumped milk: never add a new portion of warm milk to the portion pumped earlier and kept in the fridge, until the new milk cools down to the same temperature. Adding warm to warm is also fine. Why?

In: 2

Answer 1

Warmer milk is a much better place for bacteria to grow than is cold, however they do still grow in cold.

Adding warm milk to cold then makes a brew good for bacterial growth that due to the older age of the cold milk already had more bacteria in it to start with.

Warm milk tends to be low on bacteria. Its either fresh or the process of heating it may have killed some already.