Why does milk curdle?

Question

Why does milk curdle?

Answer 1

bacteria. its basically digesting the milk...

Answer 2

I know it happens when the milk becomes more acidic.

Answer 3

In milk there is a sugar called lactose (you may have heard of this from those who are "lactose intolerant" which means they do not produce the enzyme needed to break down lactose)

Bacteria feed off this sugar and as a byproduct of consumption they produce what is known as lactic acid. The introduction of this acid to the milk neutralizes the proteins present in the milk.

Normally the proteins repel each other due to their natural charges and because of that it stays in a liquidy state, but as these are neutralized they start clumping together. Eventually you are able to see the clumps and these are what we know as "curdles".

Answer 4

Below is a very good source for your question.