Why eggs have to be refrigerated in the US but not Europe and the best places to store them

• Eggs in the US need to be refrigerated to slow the growth of any potential salmonella bacteria.
• Eggs in some other countries are less likely to have salmonella and don't need refrigeration.
• You can always refrigerate eggs to extend their shelf life, but make sure to store them correctly.

Eggs are a staple food in many homes, but how you store them largely depends on what country you're in.

For example, if you're in England you typically won't refrigerate your eggs whereas in the US you will.

Storing eggs properly is important since they are one of the leading causes of the food-borne illness called salmonellosis, which affects 1.35 million Americans and over 91,000 Europeans each year.

While it's perfectly safe to store any egg in the refrigerator, there's a very good reason why eggs in many countries don't need this extra step. 

How eggs can become contaminated with salmonella

There are two ways that an egg can become infected with salmonella (the bacteria that causes salmonellosis), says Ada Hagan, PhD, a microbiologist with Alliance SciComm & Consulting, LLC:

1. An egg can be laid with salmonella bacteria already within the shell. This happens when a hen is infected with salmonella and then lays the egg. 

2. Salmonella can permeate the shell after it is laid. This occurs if the egg comes into contact with feces and bacteria from the feces permeates the shell. 

For the latter case, bacteria can permeate a shell for a few reasons like if the shell is cracked or the cuticle, a protective outer layer, is damaged. 

Another way is if the eggshell develops condensation, which can promote salmonella growth, since bacteria thrive in warm, moist environments like those that produce condensation. The more salmonella on hand, the more likely it is that some will permeate the shell. 

"There are lots of ways to prevent both of these events from happening at many steps in egg farming and processing," says Hagan. 

Egg processing in the US versus Europe

There are four essential ways to reduce the risk of salmonella contamination in eggs, says Hagan. 

• Reduce the time between when the egg is laid and consumed so any bacteria that may be in the egg don't have time to multiply to dangerous amounts. 
• Wash and sanitize the egg in order to remove contaminants, like dirt or feces that may be on the outside of a shell.
• Keep eggs dry to avoid condensation and subsequent contamination.
• Avoid fluctuating temperatures that can cause condensation build-up.

Egg processing in the US

In the US, eggs are washed and dried. However, that process removes the cuticle, a protective coating on the egg. Because of that, American eggs are more prone to infection from salmonella. 

Therefore, it's legally required by US food processing regulations that commercial eggs be refrigerated at a constant temperature of 45˚F. 

Moreover, "eggs are refrigerated to avoid fluctuating temperatures and extend shelf life, essentially ignoring the first method," Hagan says. 

Egg processing in Europe

In Europe and most of the rest of the world, standards emphasize preserving the cuticle and getting the eggs to consumers quickly. 

In Europe, for example, eggs are marked "best before" 28 days after they were laid whereas in the US, commercially processed eggs have a "sell by", or expiration, date up to 30 days after they were packed in the carton.

"Countries that don't refrigerate their eggs, have a different method of avoiding contaminated eggs that lowers time to the table and never requires refrigeration," says Hagan. 

Eggs are packed near where they are laid, and don't go through the sanitization process. In addition, some countries like the UK vaccinate chickens against salmonella. 

While the processes are different, both are effective. 

"Neither method of food safety protocols are better at protecting against salmonella," says Brian Chau, a food scientist, trained through the FDA's foreign supplier verification program, and chef at Chau Time. 

How long can you keep eggs unrefrigerated

Exactly how long you can keep eggs will depend on how they are processed and stored. 

"Generally speaking, nest clean — unwashed, unrefrigerated — eggs have a shorter shelf life than washed, refrigerated eggs," says Hagan.

That's because refrigeration slows the growth of bacteria. Turns out, even if salmonella makes it into the shell, the egg has immune defenses that slow the growth of bacteria, Hagan says. 

If you purchase refrigerated eggs at the market, like most people in the US, you can safely store them in the fridge for up to 35 days (five weeks).

If, however, you purchase eggs fresh from the farm — or collect them from your backyard coop — those eggs can be kept unrefrigerated at room temperature for 14 to 21 days. But there's also no harm in putting them in the fridge to extend their shelf life a little longer.

In the US, whether you are purchasing commercially-produced eggs that need to be refrigerated or farm-fresh at room temperature, there are certain cases where you should discard your eggs, says Chau:

• If the shell is cracked or particularly dirty.
• If there is a strange smell.
• If there is any slime or other changes to the eggshell appearance.

The best place to store your eggs

When you store eggs, the key is keeping a consistent temperature. The USDA requires that eggs be stored at a temperature of 45 ˚F, or lower, says Corrigan Duffy, chef at Corrie Cooks. You should maintain that temperature at home, too. 

If you refrigerate your eggs, the best place to store them is obviously your fridge, but some spots are better than others.

"Despite how convenient the refrigerator door may appear, eggs should be kept on the middle shelf of your fridge, preferably toward the back," says Duffy. That's where temperatures are coolest and most consistent. 

If you're outside the US or have purchased farm-fresh eggs that don't require refrigeration, keep them on the counter or table, at a temperature between about 62-73 ˚F, say Chau. If it gets much warmer than that in your house, opt for the fridge instead. 

Insider's takeaway

Commercially sold eggs in America go through a washing and sanitizing process that clears contaminants, but also removes the natural protections that eggs have against bacteria. 

Because of this, eggs are refrigerated during processing and must remain chilled in order to reduce the risk of salmonella contamination. 

"Once the eggs get cold, they must stay cold," says Hagan. 

However, farm-fresh eggs and those in European countries and most of the rest of the world, can be safely kept at room temperature for up to two weeks, says Chau. 

There's no disadvantages to keeping eggs cold, however.  "If there are doubts, eggs in the fridge do no harm," Chau says.