The idea behind both pasteurization processes is to make milk safe to drink, Roberts said. Ultra-pasteurization also extends milk's shelf life.
Raw or nonpasteurized milk might or might not contain bacteria, but by pasteurizing the milk, consumers know it's safe, Roberts said. Outbreaks of tuberculosis and other food-borne illnesses in the early 20th century led to widespread pasteurization, he said.
UHT milk has been popular in Europe, South America and Asia for years because there were areas in those continents where refrigeration was uncommon or not available, Roberts said.
Sterilized UHT milk doesn't have to be refrigerated until the consumer opens it, said Lynn Little, family and consumer sciences educator with Maryland Cooperative Extension in Washington County. After opening, all milk is exposed to bacteria in the air and can spoil.
If ultrapasteurized milk is packaged in a sterile container using sterile equipment, the milk can be stored at room temperature. In stores, it might be dated to expire in six months or more, Roberts said. If ultrapasteurized milk is packaged to be sold in the refrigeration section, the package is almost sterile but not completely. Refrigeration is needed to slow or prevent the growth of surviving bacteria in the milk.
There is a decline in some vitamin levels with any pasteurization, Roberts said. The reduction depends on how long the product is heated. While raw milk contains more vitamins, it's not significantly more nutritious than pasteurized milk, he said.
For example, pasteurization reduces the amount of vitamin C in milk. However, milk doesn't have much vitamin C to begin with, said Isabel Maples, nutrition communications manager for the Mid-Atlantic Dairy Association. Both regular and ultrapasteurized milk are an excellent source of calcium, which is not affected by pasteurization.
One downside to ultrapasteurized milk is that it can taste different, Maples said. Some people say it has a bit of a cooked taste, to which they object.
The longer shelf life has helped organic dairies distribute their products, Maples said. The products can be shipped longer distances.
The longer shelf life also helps fast food restaurants and convenience stores that might not have a high turnover of milk products, she said.