In the UK, eggs are typically not refrigerated. Dina Spector/Business Insider British supermarkets don't refrigerate eggs. It's not unusual to find stacks of egg cartons sitting alongside canned beans, boxes of dry cake mix, or other traditionally nonperishable foods.

This is unlike the US, where eggs are found in the refrigerated dairy aisle with the butter, cheeses, and milk.

The difference is linked to the way that eggs are farmed and processed in the US compared with in the UK and other European nations.

In the US, the Department of Agriculture (USDA) requires that eggs destined for supermarket shelves — called graded eggs — are washed and sprayed with a chemical sanitiser before they are sold to the public to reduce the risk of salmonella infection.

In the UK, 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. Forbes contributor Nadia Arumugam has previously noted that USDA graded eggs could not be legally sold in the UK (and the other way around) because of these different preparation methods.

The Salmonella Risk

Salmonella can infect eggs in one of two ways. 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 US, 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 on a smaller amount of land, but it also makes eggs more susceptible to contamination, even with good sanitary practises. As a result, eggs are moved directly from the hen house to a conveyor belt that takes them through a washer. The eggs are then sprayed.

It's critical that the eggs are washed properly — otherwise the washing can actually the 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 32.2 degrees Celsius (90 degrees Fahrenheit) — to prevent the egg's contents from contracting slightly as the egg cools and drawing dirty water in through the shell, according to the USDA.

Europe takes a different approach to prevent salmonella contamination. "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. "There is also a suggestion that not allowing cleaning eggs in the EU might help maintain good farm husbandry and practises," Mark Fielder, a professor at London’s Kingston University and medical microbiology expert, told Business Insider.

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. Cooler temperatures might prevent the eggs from deteriorating as quickly as well as the growth of bacteria.

Fielder believes that refrigeration is related to "whether local advice recommends this practice or not." Once eggs are washed, the USDA stipulates that clean eggs be immediately moved to cooler rooms that maintain a temperature of 7.2 degrees C (45 degrees F) or lower. Dirty eggs may be stored in temperatures of up to -17.8 degrees C (60 degrees F).

The Right Refrigeration Methods

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 recommends that US consumers keep eggs refrigerated at temperatures of 40 degrees F — 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)," Fielder says.

But there's another reason the UK is not as concerned about washing eggs as the US: Salmonella is not as big of a health concern in Britain. Egg farmers began vaccinating their hens in 1997, after thousands of people were sickened by the bacteria.

Although vaccination has been linked to a rapid decline of salmonella cases in the UK, US regulators have still not mandated immunisations, although 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.

Salmonella is the most common cause of food poisoning in the US, according to the US Food and Drug Administration (FDA). The organisation estimates that more than 140,000 people get sick each year from eating eggs contaminated with the bacteria, which triggers non-life-threatening (though unpleasant) symptoms like diarrhoea, cramps, and vomiting.