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November 5, 2001
Volume 79, Number 45
CENEAR 79 45 p. 40
ISSN 0009-2347
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Milk and shell eggs with extended shelf lives


The first time I saw the "sell-by" date on a carton of refrigerated ultrapasteurized milk, I thought it must be a misprint. The date was at least a month in the future--usually I'm lucky to find a carton of milk with a date 10 days out. Once I assured myself that it was no mistake, I started wondering what had been done to the milk to give it such a long shelf life.

The first ultrapasteurized milk I drank--from an eight-ounce refrigerated cardboard carton--tasted burned. Because I was swigging it in a fashion reminiscent of my elementary school days--straight from the carton with no straw--I wasn't sure if the problem was the milk or the carton. But I certainly don't remember milk tasting like that before.

Milk is pasteurized by heating, typically at 63 °C for 30 minutes or at 71 °C for 15 seconds, to kill bacteria and extend the milk's usable life. The process kills pathogens but leaves relatively benign microorganisms that can sour improperly stored milk. It's this second group of microorganisms that ultrapasteurization targets.

In ultrapasteurization, also known as ultrahigh-temperature (UHT) pasteurization, the milk is heated to temperatures on the order of 140 °C. The heating can be accomplished one of two ways--directly or indirectly, explains H. Douglas Goff, a professor who specializes in dairy products in the department of food science at the University of Guelph in Ontario.

In the direct method, the milk is brought into contact with steam at 140 °C for one or two seconds. The heating is instantaneous, and the milk must be cooled rapidly by evaporative cooling--exposure to a slight vacuum--to remove any water added to the milk by condensation of the steam.

"The key to high-quality products is in the heating and cooling process," Goff says. "You want to hit 140 °C, and no hold time is required to destroy microorganisms at that temperature. You have to get up to that temperature and get it cooled back down instantaneously. Otherwise, the milk is too hot for too long and it tastes burned."

The best direct method for ultrapasteurization employs "free-falling-film, steam infusion equipment," according to Goff. A thin film of milk falls through a chamber of high-pressure steam, heating the milk instantaneously. The bottom component of the equipment is cooled so that the milk never contacts any heated surfaces. The milk is flash cooled by application of a slight vacuum, which serves the dual purpose of removing excess water in the milk from condensing steam.

In the traditional, indirect method of ultrapasteurization, milk is heated in a plate heat exchanger. "It takes several seconds for the temperature of the milk to reach 140 °C, and it is during this time that the milk is scalded," Goff says. "Indirect UHT systems invariably lead to a burned taste in the milk.

"Fast heating and cooling times are necessary so the heat treatment does no damage to flavor or vitamin content," he adds. "What it does do, though, is completely eliminate all microorganisms and spores that might cause spoilage of the product. If this process is then coupled with aseptic packaging, so no downstream contamination can occur, the result is a long shelf life and a product that does not need refrigeration."

The aseptically packaged products are the ones that are found in bricklike boxes in the grocery aisle, rather than in the refrigerated case. Boxed milk can be stored unopened at room temperature for about six months. Once the box is open, the milk must be refrigerated and used within about 10 days. "In North America, it has limited use, but it is convenient for camping," Goff says. "UHT milk is very common in countries where temperatures are high and home or delivery refrigeration is not widespread."

After I learned about ultrapasteurized milk, I decided to give it another try. In my second experience of drinking the milk--a different brand, I must admit--it tasted just as good as the milk I was accustomed to.

ANOTHER FOOD in which bacteria present a problem is eggs. The bacterium Salmonella enteritidis, a major food-poisoning culprit, is found in approximately one in every 20,000 eggs. We are advised not to eat raw eggs, which turns such treats as homemade eggnog, hollandaise sauce, mayonnaise, and uncooked cookie dough into no-nos.

Now, however, pasteurized shell eggs, which defy the current conventional wisdom of the dangers of eating raw eggs, are available. They are sold in parts of the U.S. by Pasteurized Eggs Corp. under the brand name Davidson's Eggs.

The eggs are pasteurized by heating them in warm water. The temperature of the yolk must be controlled between 128 and 138.5 °F. At lower temperatures, the egg is not pasteurized, and at higher temperatures, the albumen (egg white) loses its functionality. Each egg is weighed and directed to a series of warm water baths. The combination of time and water temperature heats the eggs enough to kill Salmonella without cooking the egg. The entire process takes about an hour.

But doesn't heating the eggs denature the proteins? Well, yes--but not much.

With careful scrutiny, the yolk will seem more substantial and the white will appear better formed and appear to have a very mild opaqueness, according to the Davidson's Eggs website. It adds that the egg will taste and cook the same as a quality farm-fresh egg in all respects.

Pasteurized Eggs Corp. hopes to receive a market boost, courtesy of new labeling requirements from the Food & Drug Administration. A rule that took effect on Sept. 4 requires that eggs that have not been treated to kill Salmonella be labeled with a safe-handling statement.

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