If it’s the same Parmalat we have in Canada I think you’ll find that they get the extended shelf life mostly from ultra pasteurization, which is flash-heating, then instantly flash-cooling the milk. The temperatures are higher and for a shorter time (seconds) than for regular pasteurization, and it further degrades some of the qualities of the milk, which is why you would only use it when storage life is a priority.
They might irradiate some of the packaging itself (although sometimes simply with UV) to sterilize it and the volume of air (sometimes argon, sometimes nitrogen in packages, not sure specifically about milk) contained within the package. You will find a green circular logo on the package of food that has, in fact, been irradiated. I haven’t seen it here on Parmalat milk, but yours might have it? (Different regulations).
The package is also opaque, and sometimes metalized, which makes it less permeable to oxygen, and keeps light out; all things that inhibit the growth of microorganisms which would spoil the milk.
Also, the higher the fat content of milk the longer its storage life (table cream lasts months in the fridge).
*Trivia* It might seem strange to people from other countries to find out that Canadian milk is sold, and has been for decades, in plastic bags. In case you care.
Happy New Year!
- Brian.
(lip smacking)