The U.S. is one of the only countries on Earth that keeps chicken eggs in cold storage. But why?


One of the most common health risks, when it comes to eggs, is posed by Salmonella bacteria. There are really only two ways Salmonella can get at an egg: the first is to contaminate the egg externally, on the surface of its outer shell. The second is to spread from the inside. The former occurs after the egg has been laid, most commonly by coming into contact with feces containing Salmonella bacteria. The latter can occur if the egg develops in the reproductive tract of a Salmonella-infected hen.


Research has shown that Salmonella-infected eggs stored at room temperature for periods longer than three weeks tend to become overrun by bacteria in numbers far greater than those stored at colder temperatures. Given this insight, you might assume that Americans store their eggs in the fridge to extend their shelf life, or to lower the risk of bacterial contamination, and you'd be right on both counts.

But then, maybe the question should really be posed the other way around: Given the sanitary benefits of refrigeration, why don't other countries ship, package, and store their eggs at cold temperatures, like we do in the U.S.? Well, because, unlike America, they may not actually need to. Why? Because here in America, we wash our eggs – and while it may sound counterintuitive, the cleaning process may actually make eggs more susceptible to contamination.

We mentioned above that eggs run the risk of getting feces on them. Whether that feces contains traces of Salmonella or not, it stands to reason that if an egg gets poop on it, you should wash it off. And, in America, that's exactly what we do. In an elaborate automated process involving in-line conveyor belts, massive egg-scrubbing machinery, high-volume air-filtration systems and – last but not least – chlorine misters, American eggs are washed, rinsed, dried, and sanitized in an effort to remove as much dirt, poop and bacteria as possible, all while leaving the shells intact. (Read the details in the USDA's Egg-Grading Manual.)


Or rather, almost intact. When a hen lays an egg, she coats it in a layer of liquid called the cuticle. It dries in just a few minutes, and is incredibly effective at protecting the egg from contamination, providing what European egg marketing regulations describe as "an effective barrier to bacterial ingress with an array of antimicrobial properties." America's egg-washing systems strip eggs of this natural protection. "Such damage," the EU guidelines note, "may favour trans-shell contamination with bacteria and moisture loss and thereby increase the risk to consumers, particularly if subsequent drying and storage conditions are not optimal."

Washing eggs is therefore illegal throughout much of Europe. In an interview with Forbes, Chief Executive of Britain's Egg Industry Council Mark Williams gives another reason for the ban on industrial egg-cleaning facilities:

In Europe, the understanding is that [prohibiting the washing and cleaning of eggs] actually encourages good husbandry on farms. It's in the farmers' best interests then to produce the cleanest eggs possible, as no one is going to buy their eggs if they're dirty.


Okay, fine – but then why not just refrigerate the eggs, anyway? Wouldn't this just give unwashed eggs an extra line of defense? Perhaps, but the European Union laws again note that – like washing – refrigeration could actually wind up posing a risk to consumers. Again according to European egg marketing regulations, eggs that are stored cold and later left out at room temperatures could become covered in condensation, "facilitating the growth of bacteria on the shell and probably their ingression into the egg." EU guidelines therefore stipulate that eggs should be transported and stored at as constant a temperature as possible – a temperature between 66.2 °F and 69.8°F in the winter and between 69.8°F and 73.4°F in the summer.

The other reason Americans tend to refrigerate their eggs: our risk of Salmonella poisoning is often significantly higher than it is overseas, because our chickens are more likely to carry it. In the UK, for instance, it is required by law that all hens be immunized against Salmonella. This protection measure, enacted in the late 1990s, has seen Salmonella cases in Britain drop from 14,771 reported cases in 1997 to just 581 cases in 2009.


There is no such law in the United States, and while more farmers are electing to immunize their hens in the wake of a massive Salmonella-related recall in 2010, Salmonella infection remains a serious public health issue. Even in spite of our egg-washing and our refrigeration habits, FDA data indicates there are close to 150,000 illnesses reported every year due to eggs contaminated by Salmonella.

[USDA | EUR-Lex | Forbes | FDA | CHOW | New Scientist]