Q: Why does organic milk have a longer shelf life than regular milk?

A: The answer relates to the level of heat treatment used for the milk, wrote Carmen I. Moraru, an associate professor in the department of food science at Cornell University.

Most organic milk undergoes a pasteurization process called UHT, for ultrahigh temperature, which involves heating it to more than 280 degrees Fahrenheit for two seconds. Most regular or fresh milk, Moraru said, is heated only to 161 degrees for a minimum of 15 seconds, a process called HTST — for high temperature, short time.

The HTST process is designed to kill the disease-causing microorganisms that are of concern in milk, for example the Q fever agent, Coxiella burnetii, so the milk is safe for consumption. But some organisms that promote spoilage survive, and the milk will spoil after 14 to 21 days in the refrigerator.

The UHT treatment, on the other hand, “is a sterilization process, which means that it kills practically all microorganisms, both vegetative cells and spores,” Moraru said. As a result, the treated milk has a long shelf life, measured in months, even at room temperature. It is often labeled ultrapasteurized.

Milk with either treatment lasts about five days after the container is opened.

Most organic milk undergoes the hotter pasteurization process because the organic market is smaller, meaning the milk spends more time in the commercial distribution system and thus has to last longer, Moraru said.

-- C. Claiborne Ray, The New York Times