I’d never noticed that organic milk has a longer shelf life, but this is interesting:
Organic milk lasts longer because producers use a different process to preserve it. According to the Northeast Organic Dairy Producers Alliance, the milk needs to stay fresh longer because organic products often have to travel farther to reach store shelves since it is not produced throughout the country.
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.
UHT pasteurization has a greater impact on flavor than the standard process, so, oddly enough, organic milk is in one way less natural than conventional.
[Thanks to Julie for the link.]