Why don't people refrigerate eggs in Europe?

British supermarkets don't refrigerate eggs which can be found between the canned vegetables and boxes of dry cake mix in the grocery store aisle with other traditionally nonperishable foods.

Eggs are usually not refrigerated in European grocery stores, nor are they washed.

It seems that different egg storage conditions come down to the different ways that eggs are farmed and processed in the U.S. compared to the U.K. and other European nations. In the U.S., the Department of Agriculture (USDA) requires that eggs destined to be sold on supermarket shelves - called graded eggs - are washed and sprayed with a chemical sanitizer before they are sold to the public to reduce the risk of Salmonella infection.

In the U.K., Grade A hen eggs may not be washed because the process is thought to "aid the transfer of harmful bacteria like Salmonella from the outside to the inside of the egg," according to the Food Safety Authority of Ireland . In fact, eggs from the U.S. could not be legally sold in the U.K. (and the other way around) due to these different preparation methods.

A typical large scale egg laying facility in the U.S.

That still leaves the question of why American eggs have to be washed in the first place and how this is related to refrigeration. There are two ways that Salmonella can infect eggs. The bacteria can be passed on from an infected hen to the inside of the egg as it's developing, or it can get onto the outside of the shell after the egg is laid by coming into contact with the hen's feces. In the U.S., large-scale laying houses are preferred over free-range system because farmers can produce more eggs on a smaller amount of land. Even with good sanitary practices, the factory farm environment makes eggs more susceptible to contamination.

Eggs are moved directly from the hen house to a conveyer belt that takes them through a washer. The eggs are sprayed afterward. It's critical that the eggs are washed properly, otherwise this method can actually increase the chances of bacteria seeping into the shell from feces on the outside of it. "Wetting a dirty shell provides moisture in which bacteria may breed and assists their growth and penetration through the shell," the USDA's Egg Grading manual explains . To get around the chance of that happening, the washing solution has to be hot enough - a minimum of 90 degrees F - to prevent the inside contents from contracting slightly as it cools and draws dirty water in through the shell, according to the USDA.

Europe takes a different approach to prevent Salmonella contamination. According to food safety officials in Ireland, "The priority in egg production is to produce clean eggs at the point of collection, rather than trying to clean them afterwards." There is also a suggestion that not allowing the cleaning of eggs in the EU might help maintain good farm husbandry and practices," said Mark Fielder, a professor at London's Kingston University and medical microbiology expert.

Additionally, scientists have found that the washing process may damage an outside layer of the egg shell

known as the cuticle. Without that chemical barrier, it becomes easier for bacteria to penetrate the inside of a clean egg.

The delicate outer cuticle covering the egg helps to seal the pores in the egg shell that could serve as a pathway for the entrance of Salmonella.

Cooler temperatures might prevent the eggs from deteriorating as quickly as well as inhibit the growth of bacteria.  Once eggs are washed, the USDA stipulates that clean eggs be immediately moved to cooler rooms that maintain a temperature of 45 degrees F or lower. Dirty eggs may be stored in temperatures of up to 60 degrees F.

After an egg is refrigerated, it must be kept at that temperature.  "A cold egg left out at room temperature can sweat, facilitating the growth of bacteria that could contaminate the egg," according to the United Egg Producers association , "Refrigerated eggs should not be left out more than two hours."  That's why the Centers for Disease Control and Prevention recommend that U.S. consumers keep eggs refrigerated at temperatures of 40 degrees F , in order to prevent illness from bacteria.  "In the EU it is generally suggested that eggs are stored at an ambient temperature of around 17 to 23 degrees C (62 to 73 degrees F)," said Fielder.

But there's another reason the U.K is not as concerned about washing eggs as the U.S.: Salmonella is not as big of a health concern in Britain. Egg farmers have begun vaccinating their hens since 1997 after thousands of people were sickened by the bacteria. Although vaccination has been linked to a rapid decline of Salmonella cases in the U.K ., U.S. regulators have still not mandated immunizations, although many eggs producers do vaccinate their hens today . In 2010, the FDA said they would not legally require the vaccination of hens because "there was not enough evidence to conclude that vaccinating hens against Salmonella would prevent people from getting sick." The New York Times reported that farmers also complained that it would be expensive.

Instead, the FDA controls the threat of Salmonella through regular testing, refrigeration standards, and strict sanitary codes in hen houses and processing areas, the Times said.