Why does pasteurized milk left in a sealed container still spoil? I mean, what then is the purpose of the pasteurization? And is pasteurization different from canning?
Thank you in advance to all who reply 
From the Wiki page on Pasteurization:
Basically, not all the organisms are killed. Milk is something you keep in the fridge, and something you’re supposed to consume fairly quickly after it’s purchased, so there isn’t much chance for those few bacteria left behind to multiply and do much damage.
Leave them in there long enough, though, and they will grow and take over, spoiling the milk. Milk that stays in the fridge the whole time will take a lot longer to spoil because the bacteria are slowed down by the temperature, but it will still happen eventually.
Alas, pasteurization does kill the microorganisms that cause milk to go sour, which is a relatively benign process. It’s the pasteurization trade-off–drastically decreased likelihood of being sickened or killed by your milk, in exchange for increased difficulty in doing other neat things with it. If you want to make sour milk or cream, buttermilk, yogurt, cheese, etc. from pasteurized milk, you need to ‘seed’ it with a culture of the appropriate bacteria or mold. That’s the ‘cultured’ in the ‘cultured sour cream’ you find at the grocery store.
OTOH, UHT milk is widely available and lasts quite a while unrefrigerated.