Some useful quotes on what makes milk go bad:
"Generally, if kept constantly cold, bacteria in milk will grow at a constant slow rate. The bacteria generally feeds on the lactose, turning it into lactic acid. As it accumulates, it makes the milk taste more and more sour. But our taste buds will generally ignore the small bit of sourness. The milk is considered bad when the sourness becomes noticeable. The proteins in the milk will start to stick to each other in an acid environment. However, for this to happen on a large scale requires the milk to be fairly acid (pH around to 5.5). Our taste buds register sour at a much higher pH (~6)."