In America, fresh eggs are kept in the refrigerator. Shutterstock/Freer British supermarkets don't refrigerate eggs. You'll find them sitting alongside canned beans, boxes of dry cake mix, and other nonperishable foods.

This is different from the US, where eggs are found in the refrigerated dairy aisle alongside butter, cheese, and milk.

The different storage conditions are a result of the way that eggs are farmed and processed in the US compared with in the UK and other European nations.

In the US, to reduce the risk of salmonella infection, eggs are washed and sprayed with a chemical sanitiser before they are sold to the public. Clean eggs are kept at cooler temperatures to prevent the eggs from deteriorating as quickly and to keep the growth of bacteria in check.

Europe takes a different approach to prevent salmonella contamination. Eggs are not washed and therefore are not required to be chilled, since the "priority in egg production is to produce clean eggs at the point of collection, rather than trying to clean them afterwards," according to food-safety officials in Ireland. This is done through "good management and hygiene of the poultry house," authorities say.

Because of these different food-safety regulations, Forbes contributor Nadia Arumugam noted in 2012 that US Department of Agriculture graded eggs could not be legally sold in the UK, or the other way around.

Poultry farming and egg washing in the US

In America, large-scale laying houses are preferred over the free-range systems commonly used in the UK. The factory-farm environment means more eggs can be produced in a smaller area, but it also makes eggs more susceptible to salmonella contamination, even with good sanitary practises. The bacteria can be passed on from an infected hen to the inside of the egg as it is developing, or it can get on the outside of the shell after the egg is laid by coming into contact with the hen's feces.

Caged hens seen at an egg farm in San Diego County in 2008. REUTERS/Mike Blake

To reduce the risk of infection, eggs are moved directly from the hen house to a conveyor belt that takes them through a washer before they are sprayed. It is critical that the eggs are washed properly, or the process can actually increase the chances of bacteria seeping into the shell from feces outside 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. The USDA stipulates that the cleaned eggs must be immediately moved to rooms that maintain a temperature of 7.2 degrees Celsius (45 degrees Fahrenheit) or lower.

After an egg is refrigerated, however, it must be kept at that temperature because 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. The organisation says refrigerated eggs should not be left out more than two hours. To prevent illness from bacteria, the US Centers For Disease Control and Prevention recommend that consumers keep eggs refrigerated at temperatures of 40 degrees Fahrenheit.

Husbandry and hygiene practices in Europe

In Europe, it is unlawful to wash eggs because this process is believed to damage an outside layer of the egg shell known as the cuticle, making it easier for bacteria to penetrate the inside of an egg. "The concern shown within the EU about allowing the practice of washing eggs arises first from the possibility of deterioration of the cuticle," according to a 2005 report in the EFSA Journal.

There's another reason egg washing — and subsequent refrigeration — has not become common practice in the UK. Salmonella is not as big of a health concern in Britain because egg farmers began vaccinating their hens in 1997, after thousands of people were sickened by the bacteria.

Eggs being prepared to be boxed on a James Potter free-range farm that produces eggs for Asda supermarkets, in Catton, northern England, in 2011. REUTERS/Nigel Roddis

Though vaccination has been linked to a rapid decline of salmonella cases in the UK, US regulators have still not mandated immunisations, though many of today's eggs producers do vaccinate their hens. In 2010, the FDA said it 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. 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.

In Europe, the goal is still is to prevent dirty eggs from being produced in the first place. There is "a suggestion that not allowing cleaning eggs in the EU might help maintain good farm husbandry and practices," Mark Fielder, a medical microbiology expert who is a professor at London's Kingston University, told Business Insider over email. As for refrigeration requirements, Fielder said: "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)."