Because UHT milk is heated so much more than regular pasteurized milk, it is less prone to spoiling. Pasteurized milk can be refrigerated for 2 or 3 weeks before spoiling. UHT milk go for as long as 2 or 3 months and still remain good.
Pasteurized milk is heated to 72-75 degrees Celsius and the temperature is maintained for 15-20 seconds before milk being cooled.
UHT milk is heated to 140 degrees Celsius, the temperature is maintained 4 seconds before cooling.
The pasteurization eliminates 99.99% of the original bacteria, but it doesn't eliminate the spores that can germinate after opening the milk box.
The Ultra high pasteurization can destroy both bacteria and spores. Thus, ultra-high temperature treatment removes any dangerous bacteria