So, in my dorm fridge there is a half-full gallon of milk with a use by (or expiration?) date of Dec 14, 2012. It is pasteurized, homogenized, 2%, and grain-fed (I assume). The jug is plastic. It has not been opened since probably Dec 10 at the latest, but obviously it was opened, and thus exposed to air.
I recently discovered an opened cardboard carton of milk, similarly past its date, in a different fridge. That one had blue, green, and white mold floating on it. (I threw it away.) But that milk was continuously exposed to external air. The milk I'm asking about is not.
From what I've seen elsewhere online, pasteurized milk simply goes bad rather than souring as raw does. Most did not explain; a few suggested that it was the absence of any bacteria, good or bad, that would cause it to "rot." But bacteria, or at least fungi or protists or something, are required for rot to occur, and no one gave any evidence. Besides, the jug is half gone; there is some bacteria in there.
State of the milk in question:
It is exposed to a half-gallon of air, but that air has not been exchanged for 4 months--the same 4 months by which it is past. The air is pressurized, so bacterial activity has taken place. Floating on the milk is a ~1cm layer of whiter material. It could be curds, or possibly cream (4+ months vs homogenization). It is a bit frothy. It is viscous (i.e., thick). The plastic is translucent rather than transparent, so I can not be sure, but I see no colors or other evidence of mold.
Fermented and good to drink, or unsafe and toss? I've got another month to decide.