What Is the Difference Between Pasteurized Milk and Ultra Pasteurized Milk?

Back in 1986, during a six-month residence in the South of France, I saw something I had never seen in the U.S.

The supermarkets kept their milk on the shelves without refrigeration. Instead of bottles or cartons, it was packaged in brick-shaped, cardboard-like boxes.

How can they do that, I wondered. Granted, milk is not the preferred beverage in France, but how do they get away with treating it in such a cavalier manner? Doesn’t it spoil? I promised myself to find out as soon as I returned to the States, but I seem to have procrastinated a bit.

The glass milk bottle, invented in 1884, began to be replaced after World War II by wax-coated paperboard cartons. The wax has since been replaced with a plastic coating, and today the coated paper carton competes with all-plastic, translucent jugs, especially in the larger sizes. Those brick-shaped, non-refrigerated containers are called aseptic packaging, which means, of course, germ-free packaging.

But isn’t all the milk that we buy in this country germ-free? Surprisingly, no, even though it has all been pasteurized in one way or another. There is a difference between killing all the germs dead and keeping the few that survive from multiplying.

The objective of pasteurization is to kill or deactivate all disease-causing microorganisms by “cooking” them. Just as you can roast a chicken at a relatively low temperature for a long time or at a higher temperature for a shorter time, effective pasteurization can be accomplished at a variety of time-and-temperature combinations.

Traditional pasteurization, originally intended primarily to kill tuberculosis bacilli, involved heating the milk to 145–150ºF and holding it there for 30 minutes. Traditional pasteurization isn’t used much anymore, because it doesn’t kill and deactivate heat resistant bacteria such as Lactobacillus and Streptococcus. That’s why ordinary pasteurized milk still has to be refrigerated.

Then came flash pasteurization, which keeps the milk at 162ºF for only 15 seconds. But today, modern dairy processing machinery can achieve sterilization by flash-heating it to 280ºF for a mere two seconds. It’s done by passing the milk through the thin spaces between hot, parallel plates, and then chilling it rapidly to 38ºF. That’s ultra pasteurization. Ultra pasteurized milk and cream still have to be refrigerated, but their shelf life is increased from 14 to 18 days to 50 to 60 days, depending on the refrigerator temperature. (It should never be higher than 40ºF.)

Did I say that ultra pasteurization heats the milk to 280ºF? Yes. But wouldn’t the milk boil first? Yes, it would, if it were in a container open to the atmosphere. But just as a pressure cooker raises the boiling point of water, the pasteurization equipment heats the milk under a high gas pressure that keeps it from boiling normally.

Europe has been ahead of us in adopting ultra pasteurization, and it is ahead of us in adopting aseptic packaging, those milk bricks I saw in France.

In aseptic packaging, the milk is sterilized at high temperature for a short time as in ultra pasteurization, and then sent to the containers and the packaging machinery, both of which had been sterilized separately with steam or hydrogen peroxide. The filling and sealing are done under sterile conditions. The resulting product has an unrefrigerated shelf life of several months or even up to a year. Moreover, because the package is hermetically sealed with no air inside, the butterfat won’t turn rancid from oxidation.

In our American markets, we rarely see aseptically packaged milk or cream. We see aseptic packaging mainly in soy milk products and tofu in the organic and “health food” sections, and in those little “drink boxes” of juice.

In Europe, aseptic packaging is more widely used, perhaps because it is more energy-efficient. The foods don’t have to be refrigerated during transportation and the packages are lighter than if steel cans or glass bottles had been used.

Another reason, industry sources tell me, is that American consumers just don’t trust milk that isn’t refrigerated. But many consumers have told me that high-temperature pasteurized milk has an unpleasant, cooked flavor.

No matter how your milk or cream has been pasteurized or packaged, it does have an expiration date, even as you and I. Always check the date printed on the package.