Regular milk is usually heated to between 145-161℉. This kills most bacteria inside, but not all and its that residual bacteria that can help turn your milk sour.
In contrast, organic milk is treated at 280℉. This is called "ultrahigh temperature" processing or UHT. It kills any bacteria inside and is why organic milk lasts longer than regular milk. But don't buy organic just for its longer shelf life.
There's also regular milk that has undergone UHT processing, but be warned. The high temperatures will caramelize some of the milk's glucose and lactose causing it to taste different from regular milk. Unopened, refrigerated regular milk has a shelf life of 15-17 days. While organic, or other UHT treated milk, should last 40 to 60 days.
To help keep any milk fresh for as long as possible, store it in the main part of your fridge, away from the door.
