sealPurdue News


October 1995

Sunny side up? Yup.

WEST LAFAYETTE, Ind. -- Chocolate chip cookie dough and sunny-side-up eggs may be back on the menu thanks to Purdue University researchers who have developed a new method for pasteurizing eggs in the shell to kill Salmonella enteritidis .

The new, low-temperature, long-time egg pasteurization method can heat an egg yolk to salmonella -killing temperatures without solidifying the white or yolk. It is being patented by Purdue food process engineer Rakesh Singh, food microbiologist Peter Muriana, poultry process specialist William Stadelman and graduate student Huiying Hou.

"Only about one out of every 20,000 eggs may be contaminated with salmonella -- and even the contaminated ones are safe if the eggs are handled properly," Muriana says, "but that put shell eggs back on the USDA hazardous food list in 1991."

Salmonella is a type of bacteria that may cause no more than an upset stomach in a healthy person, but it can be life-threatening for small children, the elderly, pregnant women and anyone with a weak immune system, according to the Food and Drug Administration (FDA). FDA estimates that from 2 million to 4 million cases of salmonellosis occur in the United States annually, and the Centers for Disease Control has recorded more than 120 outbreaks due specifically to Salmonella enteritidis .

Once pasteurized eggs hit the market, however, young and old will be able to eat thick, Texas-style French toast and home-made hollandaise sauce, mayonnaise, ice cream and eggnog from shell eggs without worrying about salmonella . And the process will add only a few cents per dozen to the cost of eggs, according to Stadelman.

If you wonder how you snitched all that cookie dough as a kid without getting sick, it's because the problem is new. Salmonella was found in lots of other foods, but not in raw shell eggs until recently.

"In the 1980s Salmonella enteritidis bacteria adapted in a way that made them able to cause ovarian infection in chickens," Muriana says. "Now it's the number one type of salmonella isolated from egg-related food poisoning outbreaks."

Because Salmonella enteritidis can infect a chicken's ovaries, eggs are infected before they are laid. The bacteria get packaged with the egg yolk when the white and shell form around it.

Producers and grocers take care to handle eggs properly so that salmonella bacteria don't grow in eggs and create a hazard. But they cannot test for the bacteria within an intact egg, so they cannot promise that eggs in grocery stores are absolutely salmonella -free. They also can't control how shoppers handle the eggs after they take them home. And it's at home or in restaurants that most problems occur.

The FDA notes that most incidents of salmonella food poisoning have been traced to situations where the contents of several eggs were mixed, allowed to sit unrefrigerated, then not fully cooked. Refrigerating eggs below 40 F limits Salmonella growth, and fully cooking eggs destroys the bacteria.

When the Purdue team tested its procedure with artificially inoculated eggs, the researchers were able to kill 10 million Salmonella enteritidis bacteria per egg. Bacteria levels very rarely get that high. Most naturally infected eggs contain no more than 100 salmonella bacteria per egg, so the process provides a wide margin of safety.

The pasteurized eggs are just as good for cooking and eating. They act exactly like their unpasteurized counterparts when researchers use them in cakes, cookies or eggnog.

"There is no coagulation, protein loss or loss in functionality of egg components," says Singh.

The process looks good to shell-egg producers.

Currently, egg producers with Salmonella enteritidis -infected flocks cannot sell their eggs in the shell, although they can crack the eggs, pasteurize the contents and sell them as lower-value, liquid eggs. Often producers destroy infected flocks and sanitize the buildings that had housed them, although that doesn't assure that a new flock in the building will be infection-free.

In the future, eggs from infected flocks may be pasteurized and sold whole.

Processors whose pasteurized liquid eggs have a shelf-life of just a couple of weeks are also interested in the Purdue process. They compete with a company in Minnesota that has full rights to a pasteurization process that gives its liquid egg product a 12-week shelf-life and allows it to ship the product cross-country. If other liquid egg processors started with already-pasteurized shell eggs, they could extend their product's shelf-life and also ship their pasteurized liquid eggs to farther markets.

Before any producers will buy into shell-egg pasteurization, however, the researchers must develop a fast, easy and effective commercial version. Several Indiana egg companies already are working with the Purdue researchers to scale up the process for commercial use.

Muriana, Singh and Stadelman doubt that all shell eggs will be pasteurized in the future. More likely, "pasteurized shell eggs" will become an additional, specialty item on grocery store shelves.

Sources: Peter Muriana, (765) 494-8284; Internet,
Rakesh Singh, (765) 494-8262; Internet,
William Stadelman, (765) 494-8286; Internet,

Writer: Rebecca J. Goetz, (765) 494-0461; Internet,

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