Re: What is the pH of spoiled milk?

> yet I cannot find at which pH level milk is 
> considered spoiled (when it starts to smell bad but before it 
> actually curdles).  

I think that part of the reason you are having trouble finding this 
information is that this isn't really the right question to ask.

There are several parameters that are used as indicators of milk spoilage, 
but pH isn't one of them.

Dr. Ted Labuza's classic book "Shelf Life Dating of Foods", 1982, lists 

(1) "Flavor score" (as judged by human taste panelist) < 35
(2) Greater than 1 million Psychrophiles (cold-loving microbes)
(3) Greater than 1 million total microbes

As a few possible ways of defining fluid milk shelf life.

Since the psychrophiles mentioned above are not especially prolific acid 
producers, it is actually possible that a particular carton of milk might 
"spoil", i.e. not smell or taste good, and yet not be curdled.  Another 
container of milk, inoculated with yogurt making bacteria might curdle, 
and yet taste just great.

The shorter answer to your question is that the pH of spoiled milk will be 
quite variable, depending upon the exact mixuture of microorganisms the 
milk started out with.