Every egg has an air cell. It is just under the shell, typically on the large end of the egg, formed by the inner and outer membranes. When a warm freshly-laid egg begins to cool, the egg contents contract and the two shell membranes separate to form a pocket of air, the air cell at the egg’s large end. As the egg ages, moisture and carbon dioxide pass through the shell pores and air enters, increasing the size of the air cell in the space between the two membranes. The air cell is visible in the flattened end of a peeled, hard-boiled egg.

It is not true that freshness can be judged by placing an egg in salt water. Depending on the amount of salt added to the water, the density of water will increase to an amount greater than the density of an egg, allowing the egg to float regardless of air cell size.  A very fresh egg can have a large air cell that will allow it to float.  Air cell size is one of many criteria used to determine egg grade. 

An “expiration” or “sell by” date on some egg cartons helps to ensure that the eggs are fresh by informing the grocery store not to sell the eggs after the marked date.  An expiration date on the carton is not required but, if one is used, it can be no more than 30 days after the eggs were packed.  Since the packer or retailer may choose a date under 30 days, your retailer can give you more complete information about how many days a “sell by” or “expiration date” allows after packing.

Some cartons show a Julian date on the short side of the carton.  The Julian date is the day the eggs were packed – starting with 001 as Jan 1 and ending with 365 for December 31.  For example, eggs packed on June 15 will be marked 166.  Some other egg packers print an open “use by” date – July 15, for example – right on the eggshell itself.  If properly refrigerated, shell eggs will keep with insignificant quality loss for at least 4 to 5 weeks after the Julian or pack date.  If there is no Julian or pack date, using your eggs within 3 weeks of purchase will allow for the possibility that your eggs may have been temporarily warehoused by the retailer before you bought them.

What difference does time make?  As eggs age, the whites thin and the yolks flatten.  This means that the eggs will spread more in a pan if you fry them and there will be more “angel wings” of white in the water if you poach them.  Because the yolk membranes also weaken with age, the yolks may break whether you want them to or not.

For recipes where shape isn’t important, particularly when whites and yolks are beaten together, you can still use the eggs.  The weakening of the yolk membrane, however, makes it easier for bacteria – if they’re present – to reach the nutritious yolk.  So, to prevent the possibility of foodborne illness, it’s best to use older eggs in fully cooked items, such as quiches, stratas and baked goods.

The egg white, or sometimes called albumen, is more opalescent than truly white when in the raw state. The cloudy appearance comes from carbon dioxide. As the egg ages, carbon dioxide escapes, so the egg white of older eggs is more transparent than that of fresher eggs. The albumen of in-shell pasteurized eggs will also appear cloudy due to some albumen proteins denaturing, or unfolding, during the pasteurization process.

Eggs can be frozen, however, not in the shell. 

The moment an egg is laid, physical and chemical changes begin to conspire against freshness and food safety. Warm temperatures encourage those changes, so newly laid eggs must be gathered frequently and refrigerated quickly.

Refrigerate raw shell eggs in their cartons in the main compartment of the refrigerator and not on the door where the temperature can fluctuate. Keep eggs away from any meats that might drip juices or any produce that might come into contact with eggshells. Refrigerated raw shell eggs will keep without significant quality loss for about 4 to 5 weeks beyond the pack date (the date the egg was placed in the carton, expressed as a Julian date) or about 3 weeks after you bring them home.

What difference does time make?

As eggs age, the whites thin and the yolks flatten.  This means that the eggs will spread more in a pan if you fry them and there will be more “angel wings” of egg white in the water if you poach them.  Because the yolk membrane also weakens with age, the yolks may break whether you want them to or not  – making it difficult to cleanly separate egg yolk from egg white. The egg white of older eggs will not form as stable a foam for recipes such as angel food cake and meringue cookies.

For recipes calling for whole eggs, you can still use the eggs.  The weakening of the yolk membrane, however, makes it easier for bacteria – if they’re present – to reach the nutritious yolk.  So, to prevent the possibility of foodborne illness, it’s best to use older eggs in fully cooked items, such as quiches, stratas and baked goods.

Salmonella are destroyed when hard-boiled eggs are fully coagulated and thoroughly cooked, however, hard-boiled eggs can spoil more quickly than raw eggs. After cooking, cool hard-boiled eggs quickly under running cold water or in ice water. Avoid allowing eggs to stand in stagnant water. Refrigerate hard-boiled eggs in their shells promptly after cooling and use them within 1 week.

How should you keep Salmonella from spreading in your kitchen? 

Cleanliness throughout the kitchen and in every step of food preparation is important for food safety. Wash hands, utensils, equipment and work surfaces thoroughly in hot soapy water for at least 20 seconds before and after you handle high-protein foods, such as eggs, as well as produce and other raw foods.

To wash hands properly, wet your hands with running water and lather them with soap for a full 20 seconds. Then rinse and dry with a clean towel or paper towel.

To clean and sanitize your surfaces, you’ll want to start with warm, soapy water and a clean paper towel or dish rag to wash the surfaces.  Next, you’ll need to sanitize the surfaces to kill any remaining bacteria. Many different sanitizers can be used: an easy homemade version is to make a solution of 1 tablespoon of liquid chlorine bleach per gallon of water, or you can use a commercial sanitizer or sanitizing wipe. Follow the label instructions on commercial sanitizers to determine whether you need to rinse food preparation areas after use. Washed surfaces should be dry before sanitizing.

Since retail eggs are washed and sanitized under prescribed conditions before packing and incorrect home procedures might infect the contents, avoid washing eggshells at home.

Separate raw meat, fish, seafood, poultry and eggs from other foods, especially foods that are ready-to-eat. Refrigerate raw shell eggs in their cartons in the main compartment of the refrigerator and not on the door where the temperature can fluctuate. Keep eggs away from any meats that might drip juices or any produce that might come into contact with eggshells. Cover or wrap well any egg mixtures or leftover cooked eggs dishes before refrigerating.

Wash thoroughly with hot soapy water any bowls, pans, blenders, mixers, or other container which has held a raw egg mixture before you use it again for more eggs or another food. Do the same with any container that has held raw meat, fish, seafood or poultry. Also use separate cutting boards for raw meat, fish seafood, poultry and other foods, particularly cooked and ready-to-eat foods. Thoroughly wash and sanitize work surfaces, cutting boards and utensils, such as knives and beaters, after each use.

When you break or separate eggs, it’s best to avoid mixing the yolks and whites with the shells. Rather than broken shell halves or your hands, use an inexpensive egg separator or a funnel when you separate eggs to help prevent the introduction of bacteria. Also use a clean utensil to remove any bits of eggshell that accidentally fall into an egg mixture and do not use empty eggshells to measure other foods.

Cooking to proper temperatures brings eggs and other foods to a temperature high enough to destroy bacteria that causes food borne illness.

For food safety, instant thermal kill of Salmonella occurs when the egg reaches a temperature of 165°F. Thermal kill of Salmonella occurs when at temperature of 160°F is maintained for 2 minutes.  All leftover foods must be reheated to an internal temperature of at least 165°F. Test with an instant read thermometer. Toss any egg-containing leftover after 3 days

Refrigerate is the last food safety tip. Rapid growth of bacteria can occur between 40 and 140°F. Cold temperatures keeps bacteria from growing to large enough numbers to cause illness. Keep shell eggs, egg mixtures and egg-containing leftovers refrigerated at 40°F or below when not being cooked or eaten. Broken out eggs and pooled eggs should be used quickly and not stored for long periods in the refrigerator. For all perishable foods, including eggs and egg-containing dishes, allow no more than 2 hours at room temperature for preparation and serving, 30 minutes to 1 hour when its 85°F or hotter.

Promptly after serving, refrigerate any leftovers containing eggs and consume within 2 to 3 days. Thoroughly reheat leftovers to an internal temperature of 165°F.  Without tasting them, discard any egg-containing leftovers that have been refrigerated more than three days.