system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: 5 years ago in Australia I would buy a bottle of milk and it would last 3-4 days before going off. Now I buy a bottle and it doesn't go bad for over a month. What changed? And how does it work?
gpt
It's likely to be ultra-pasteurization. This heats the milk to a higher temperature (135C or 275F) killing off more bacteria. This milk can last for up to 2 months. Improved packaging is also an important aspect.  
Edit: People seem to be saying that this kind of milk is not UHT processed (same as ultra-pasteurization), but life of the product can also be drastically improved by changes in packaging (from clear to opaque, better sealing, and better sterilization of the packaging), a more sustained cold chain (the milk is always kept below a certain temperature) or that the milk is "newer" (fresher) when arriving to the store.