What is the next step for shell egg safety?

Implementation of the U.S. Food and Drug Adminsitration's (FDA) regulations for prevention of Salmonella enteritidis contamination of shell eggs by egg producers with more than 3,000 hens, the Egg Safety Final Rule, was a step forward for the U.S. egg industry as a whole. While many egg producers already were complying with many aspects of the Egg Safety Final Rule either voluntarily or through state programs, some had not been, and it is expected that the already low rate of eggs contaminated internally with Salmonella enteritidis was reduced further.

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Rapid cooling of shell eggs can restrict the growth of Salmonella inside the egg and improve the safety of eggs.
Rapid cooling of shell eggs can restrict the growth of Salmonella inside the egg and improve the safety of eggs.
Implementation of the FDA's regulations for prevention of Salmonella enteritidis contamination of shell eggs by egg producers with more than 3,000 hens, the Egg Safety Final Rule, was a step forward for the U.S. egg industry as a whole. While many egg producers already were complying with many aspects of the Egg Safety Final Rule either voluntarily or through state programs, some had not been, and it is expected that the already low rate of eggs contaminated internally with Salmonella enteritidis was reduced further.
The primary focus of the Egg Safety Final Rule is on biosecurity to prevent introduction of Salmonella enteritidis into the pullet or laying houses and environmental sampling to gauge the success of these efforts. Dr. Richard Gast, supervisory microbiologist specializing in egg safety and quality, USDA ARS, said this preharvest emphasis makes sense because laying hens infected with some Salmonella serotypes (particularly enteritidis and Heidelberg) can lay internally contaminated eggs as a consequence of the colonization of reproductive organs. He said the initial contamination of freshly laid eggs typically involves very small numbers of cells and rarely occurs inside the egg yolk.
Commenting on what the next step forward for shell egg safety might be, Gast said, "To my knowledge there are no new preharvest measures available that will do a significantly better job than what we are doing now. If we keep doing it over time, I think we will slowly, gradually bring our contaminated egg numbers downward, not to zero, but downward. If you are a producer and that is not enough, then it makes sense to do something else."
Refrigerate or pasteurize? 
"Even if we are doing a really good job, there is enough exposure out there that I think it makes some sense to take a look at what we can do to the egg after it is produced to try and prevent any problem that is generated," Gast said. "With what we know right now, we should still expect an almost unavoidable small rate of contamination of some eggs." Because of this, he said it makes sense to consider steps that help reduce the chance that the bacterial contamination causes human illness. 
The two potential post-harvest food safety measures at this time that can be taken with shell eggs that can have an impact on human health outcomes, according to Gast, are pasteurization and refrigeration. He said the potential magic bullet is inline pasteurization, but the technology for that is not available yet. The limiting factor with cooling is that it is neither a preventive nor a curative step; it is a limiting step. "Cooling doesn't prevent the problem from happening nor does it make the problem go away, but it does keep an existing problem from getting worse, which is arguably highly valuable," he said. Given what the technology is, the cost and the effect on quality, he said that refrigeration is probably the better choice now. 
To chill, or not to chill? 
Gast said that all of the evidence shows that eggs contaminated with Salmonella enteritidis are an infrequent event, and they tend to initially involve a relatively small number of bacteria per contaminated egg. Bringing the eggs down to 45F quickly with refrigeration can prevent a small number of cells from multiplying and becoming a more dangerous dose.
Gast stressed that what constitutes an infectious dose depends on the individual who consumes it, with the immune-compromised, the very young, and the very old being most at risk. For those at most at risk, even a just few cells could be dangerous whereas for a healthy adult a much larger dose would be needed.
"The Europeans are emphasizing that contamination occurs relatively infrequently, usually involves low numbers of cells, and most of the time is located outside of the yolk," Gast said. "It usually takes a few weeks for the yolk membrane to break down, even at room temperature, and this means that most of the contamination is going to stay in the albumin where, even at room temperature, it isn't going to grow much. So, with the emphasis on preharvest measures to prevent contamination of the egg in the first place, European regulators have taken the position that refrigeration of eggs is unnecessary."
Gast explained that the U.S. perspective has been to look more at the worst-case scenario where the yolk is contaminated early and the warm temperatures will let the bacteria grow rapidly. U.S. egg producers are now required to store and transport eggs under refrigeration at 45F (7.2C) within 36 hours of lay. In contrast, the British Lion Program only requires 20C (68F) storage of eggs.
Getting to 45F 
The current practice at U.S. inline egg farm and processing facilities is that eggs are conveyed to the packing facility where they are washed in warm water, dried, weighed, graded and packed, and then placed in a refrigerated storage area. North Carolina State University researchers demonstrated in the 1990's that eggs located in the center of boxed pallets can take seven to 10 days to cool to 45F under these conditions.
Gast said the current egg industry means of refrigeration may prevent 10 cells of Salmonella enteritidis from becoming 10 million cells, but they might become 10,000 cells because of the slow rate of cooling. More rapid cooling might be able to prevent 10 cells from becoming 10,000 cells or even 1,000 cells. Gast said, "If you have a day or two of warm temperatures and the organism is near the yolk, you have the opportunity for just a few cells to go up to levels that are inherently dangerous. What we want refrigeration to do is to stop the small number from becoming really dangerous numbers."
Rapid shell egg cooling 
Dr. Patricia Curtis, professor, director of the food systems initiative, Auburn University, was on the team of researchers at North Carolina State that looked into egg cooling. This group developed a system for quickly cooling eggs with carbon dioxide prior to packing the eggs in the carton. A demonstration unit was built by Praxair for use within the existing footprint of egg plants over a decade ago. 
"The faster you get the eggs chilled down, even if there is SE in there, you are going to slow its growth," Curtis said. "The last thing you want is for SE to grow in the egg. There really isn't a negative to [rapid egg cooling]. You are enhancing the quality of the egg. There isn't a downside, other than the cost."
Rapid egg shell cooling could also be done with a forced air system like a spiral freezer. Curtis cautioned that if you freeze too much of the albumin of the egg, the shell will crack because of expansion. Also, she said forced air may result in premature aging of the eggs as this accelerates the loss of carbon dioxide from the egg. Cooling with carbon dioxide increases the carbon dioxide concentration within the egg slightly and increases shelf life and enhances egg quality.
Advantages of carbon dioxide cooling 
Dr. Kevin Keener, professor of food science, Purdue University, has continued the work with rapid shell egg cooling with carbon dioxide. He said eggs starting at 105F can be treated in a carbon dioxide tunnel for 80 seconds and then be packed in cartons. A thin ice layer forms inside the egg shell when the carbon dioxide is applied. This ice layer then pulls heat outside of the center of the egg as the egg equilibrates to 45F within 20 minutes inside the carton. 
In research on rapid shell egg cooling, Keener said that they didn't see any measurable shell damage chilling eggs to 7C (44.6F). In addition to slowing Salmonella enteritidis growth in the egg by lowering the temperature, rapid cooling with carbon dioxide also strengthens the vitelline membrane, according to Keener. The vitelline membrane serves as a barrier to Salmonella enteritidis' movement into the yolk, where Salmonella enteritidis can grow rapidly.
Keener said Praxair demonstrated the rapid-cooling carbon dioxide unit in egg plants from 2002-04, but no egg producers decided to implement the technology. He thinks that now might be a good time for egg producers to reconsider rapid shell egg cooling. 
Keener said that rapid cooling can double or even quadruple the shelf life of eggs. He said that this extended shelf life could be valuable for exporting eggs, particularly to countries where the cold chain isn't completely developed. Specifically, he said the rapidly cooled eggs could be in transit for eight weeks and still have four to six weeks of shelf life when they arrive at their destination.
The cost of cooling eggs with carbon dioxide will vary greatly depending on the proximity to carbon dioxide sources. Keener said that carbon dioxide cooling could be done for as little as $0.03 per dozen eggs if there is a surplus of carbon dioxide in the area, as is sometimes the case in close proximity to ethanol plants. He estimated the cooling costs at $0.07 per dozen when carbon dioxide is brought in from other areas.
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