Fresh eggs last from two hours to a year, depending on where you buy them and how you store them.

Eggs from the grocery store or the farmers market can last for weeks in the refrigerator. That's often much longer than the expiration dates marked on the cartons. If you break them down and freeze the whites and yolks, they can last so much longer. The key is to store them correctly.

These guidelines from the U.S. Department of Agriculture (USDA) show how long you can store eggs before they go bad or lose flavor.

Do Fresh Eggs Have to Be Refrigerated?

A freshly laid egg has a natural moist coating called bloom that helps seal it and protect it from bacteria. If an egg is washed, that membrane disappears and the egg becomes porous and vulnerable.

Fresh eggs can carry the bacteria salmonella on their shells. Salmonella can cause a foodborne illness characterized by nausea, vomiting, stomach pain, diarrhea and fever. People often get sick by eating raw or undercooked eggs or egg products that are contaminated with the bacteria.

In the 1970s, concerns about contamination and spoilage led the USDA to require large-scale egg producers and processors to immediately wash, sanitize and refrigerate their eggs. Canada, Japan and Scandinavian countries soon began washing their eggs too.

In most of the European Union, however, eggs aren't washed or refrigerated, even in stores. Many Europeans believe the eggs are protected from bacteria because the shell's coating remains intact so they keep them at room temperature for weeks at a time. (In addition, many countries require poultry farmers to vaccinate their hens for salmonella.)

Because of this protecting coating, many people who have backyard chickens or who sell fresh eggs at farmers markets often say its safe to keep their unwashed eggs on the counter or in the pantry. They believe the protective bloom or cuticle is keeping the eggs safe from bacteria as long as you don't scrub it away.

But Deana Jones, an Agricultural Research Service (ARS) food technologist in Athens, Georgia, says that research has shown that this bloom degrades once the egg is laid.

"We know that the cuticle dries and comes off, and we also know that from an evolutionary standpoint, it isn't there to prevent salmonella in the egg, but to control respiration during incubation," she said in a statement.

Refrigeration doesn't just protect an egg from bacteria, it also protects its quality.

In a study published in Poultry Science, Jones and her team compared how eggs are stored in the U.S. and Europe, as well as other techniques. They found that the U.S. method is the most effective — even after as long as 15 weeks of storage.1

Researchers looked at 5,400 eggs and found that the ones washed and stored in the fridge were still Grade A (very high quality) eggs after 15 weeks on average. The ones stored at room temperature degraded from Grade AA (highest quality) to Grade B (lowest quality) in just a week. The eggs also lost 15% of their weight over the 15 weeks.

"Basically, the key is that egg quality stays high with refrigeration and degrades rapidly without it," Jones said.

How to Store Fresh Eggs

When buying eggs at the store, first check to make sure none of them are cracked. Broken shells can let in bacteria. If any eggs break on the way home, the USDA says break them into a clean container and cover it tightly. Refrigerate and use within two days.

It might be tempting to use the handy built-in egg tray in your refrigerator, but eggs should always be kept in their cartons. The carton is designed to protect eggs from cracking and from absorbing odors from other foods in your refrigerator.

Store the eggs where it's coldest — in the body of the refrigerator, not in the door. Your refrigerator should be kept at 40 degrees F (4 degrees C) or below.

Don't wash store-bought eggs before storing or using them. If you have unwashed fresh eggs, you should wash them before using them. The University of Lincoln-Nebraska Extension suggests:

• Washing gently in water that is 90-120 F (32-49 C) while using rubber gloves for about 30 seconds with an unscented detergent.
• Dip in a solution of 1 tablespoon of bleach to a gallon of hot water.
• Rinse thoroughly, then refrigerate.

How to Tell If an Egg Is Good or Bad

You want to make an omelet or some brownies, but you're not sure if the eggs in your refrigerator have been sitting there too long. Eggs might lose a little quality over time, but they are still safe to use for many weeks. Here are several ways to test if an egg is good or bad.

Check the Expiration Date

Always check the expiration date or "sell by" date on the carton before bringing eggs home from the store. Expiration dates can be no more than 30 days from the day the eggs were packed into the carton, according to the USDA. But eggs can last much longer than that.

If you store and refrigerate them correctly, fresh eggs in the shell can last three to five weeks. The quality of an egg may start to decline as an egg gets older, but it's still safe to eat.

Check the Pack Date

Near the expiration date, you'll also notice a three-digit code on the carton. This is the pack date and it's typically right around the plant number, which starts with the letter "P."

The pack date uses the Julian calendar starting with 001 as Jan. 1 and Dec. 31 as 365, except in leap years. Use the chart above to quickly translate the numbers on your carton. You can store fresh eggs in their cartons in the refrigerator for up to five weeks beyond this date.

Do a Sniff Test

You might be able to tell if your eggs have gone bad just by that rank smell that's coming from your refrigerator. But the best way to do a sniff test it to crack an egg into a bowl and look at it for an unusual appearance or a bad odor, suggests the USDA. A spoiled egg will smell bad whether it's raw or cooked. If it looks and smells normal, it's OK to use.

Do an Egg Float Test

When an egg is freshly laid it has no air cell inside. But as it cools, a pocket of air forms typically in the large end of the egg between the cell's shell membranes. As the egg gets older, the yolk absorbs liquid from the egg white. As moisture and carbon dioxide begin to evaporate through the pores in the egg, more air penetrates the shell, allowing that air cell to grow.

To figure out how old an egg is, you can measure the air pocket by doing an egg float test. Drop a raw egg into a glass of water.

• If it stays on the bottom of the glass horizontally, it's very fresh.
• If it's not so fresh, it will slightly tilt in a semi-horizontal position.
• If it's old and stale, it will float to the top in a vertical position.