Tired of Your Milk Going Bad Before You Get to Drink It?

It's because of the way it's been processed.

The process that gives the milk a longer shelf life is called ultrahigh temperature (UHT) processing or treatment, in which milk is heated to 280 degrees Fahrenheit (138 degrees Celsius) for two to four seconds, killing any bacteria in it.

Compare that to pasteurization, the standard preservation process. There are two types of pasteurization: "low temperature, long time," in which milk is heated to 145 degrees F (63 degrees C) for at least 30 minutes, or the more common "high temperature, short time," in which milk is heated to roughly 160 degrees F (71 degrees C) for at least 15 seconds.

... the difference between UHT and normal pasteurization comes down to ...

The different temperatures hint at why UHT-treated milk lasts longer: Pasteurization doesn’t kill all bacteria in the milk, just enough so that you don't get a disease with your milk mustache. UHT, on the other hand, kills everything.

So, with UHT milk, you can buy it and then store it for up to 6 months in some cases. So if you want to make a run on milk, UHT is probably the route to go. Now, be warned ... all bets are off once the milk is opened. As long as it is properly sealed it's sterile, but once you open it and expose it to the environment it's going to get contaminated. It may take longer than regular, pasteurized milk would to spoil, but it will go bad eventually ... so thinking that you can buy your UHT milk tomorrow, open it up, and then drink from it for the next 6 months isn't going to be feasible.