“Eggs existed long before chickens,” according to On Food and Cooking: The Science and Lore of the Kitchen by Harold McGee. “The first eggs were released, fertilized, and hatched in the ocean. Around 250 million years ago, the earliest fully land-dwelling animals, the reptiles, developed a self-contained egg with a tough, leathery skin that prevented fatal water loss. The eggs of birds, animals that arose some 100 million years later, are a refined version of this reproductive adaptation to life on land. Eggs, then, are millions of years older than birds. Gallus domesticus, the chicken more or less as we know it, is only a scant 4 or 5 thousand years old.”

The entire time from ovulation to laying is about 25 hours. Then about 30 minutes later, the hen will begin to make another one.

Shell eggs can be infected with Salmonella enteritidis (SE), in several ways:

• SE can contaminate eggs during their development inside the hen’s reproductive tract, before the shell even forms around the yolk and egg white.
• As the egg exists the hen’s body, it passes through the same canal where feces are excreted, potentially exposing the shell to the bacteria.
• After eggs are laid, bacteria can also enter through the pores of the shell.

To reduce the risk of contamination that may affect safety and degrade quality, most eggs undergo a washing process at the plant; this is often followed by a sanitizing step.

Researchers say that, if present, the SE can be in the yolk or “yellow” or the albumen “egg whites” or on the egg shell.

What Safe Handling Instructions Are on Egg Cartons?
All packages of raw shell eggs not treated to destroy Salmonella must carry the following safe handling statement:

SAFE HANDLING INSTRUCTIONS: To prevent illness from bacteria: Keep eggs refrigerated, cook eggs until yolks are firm and cook foods containing eggs thoroughly.

Everyone is advised against eating raw or undercooked egg yolks and whites or products containing raw or undercooked eggs. This includes “health food” milk shakes made with raw eggs, Caesar salad, Hollandaise sauce and any other foods like homemade mayonnaise, ice cream or eggnog made from egg ingredients that are not thoroughly cooked.

Infants, young children, older adults, pregnant women and people with weakened immune systems are particularly vulnerable to SE infections. A chronic illness weakens the immune system, making the person vulnerable to foodborne illnesses.

However, in-shell eggs that have been pasteurized  may be used safely without cooking.

Federal and state governments, the egg industry and the scientific community are working together to solve the problem. Involved government agencies include: USDA's Food Safety and Inspection Service (FSIS), Agricultural Marketing Service (AMS), Agricultural Research Service (ARS) the Animal and Plant Health Inspection Service (APHIS), the U.S. Food and Drug Administration (FDA) and State departments of agriculture.

Many government agencies cooperate to ensure the safety of shell eggs from farm to table.

USDA Agencies

Agricultural Marketing Service (AMS)

• AMS is responsible for the Shell Egg Surveillance Program that assures eggs in the marketplace are as good as, or better, than U.S. Consumer Grade B quality standards. AMS conducts inspection of handlers four times each year and hatcheries annually to ensure conformance with these requirements. Eggs exceeding the tolerance for checks or loss must be diverted from the marketplace for further segregation or processing.
• AMS also administers a voluntary egg-quality grading program for shell eggs paid for by processing plants.
• The USDA grade mark on egg cartons means the plant that processed the eggs is following USDA's sanitation, good manufacturing processes and eggs have been graded for quality by a USDA grader.
• AMS prohibits the repackaging of eggs previously shipped for retail sale that were packed under its voluntary grading program.

Animal and Plant Health Inspection Service (APHIS)

• APHIS conducts activities to reduce the risk of disease in flocks of laying hens.
• APHIS administers the voluntary National Poultry Improvement Plan (NPIP), which certifies that poultry breeding stock and hatcheries are free from certain diseases. Participation is necessary for producers that ship interstate or internationally.

Food Safety and Inspection Service (FSIS)

• FSIS verifies shell eggs packed for the consumer are labeled “Keep Refrigerated” and stored and transported under refrigeration.
• USDA also educates consumers about the safe handling of eggs. FSIS has developed several English and Spanish publications on egg safety and uses a variety of networks (such as the USDA Meat and Poultry Hotline, podcasts, various social media platforms, blogs and USDA cooperative extension agents) to get this information to consumers.

Agricultural Research Service (ARS)

• USDA also carries out food safety research through ARS and through a program administered by USDA's National Institute of Food and Agriculture (NIFA).
• In 2005, ARS established the Egg Safety and Quality Research Unit at the Russell Research Center in Athens, GA, to expand egg safety and egg processing research.

National Agricultural Statistics Service (NASS)

• USDA collects processing and production data information for the economic analysis of the egg products industry through NASS.

Other Government Agencies

FSIS/FDA Cooperation

• FSIS and the FDA share authority for egg safety and work together on solving the problem of SE in eggs.
• FSIS and the FDA are working to strengthen the Food Code and to encourage its adoption by States and local jurisdictions.

U.S. Food and Drug Administration

Egg Safety Rule (codified at 21 CFR part 118) requires virtually all egg producers with 3,000 or more laying hens to implement measures to prevent SE from contaminating eggs on the farm. These measures include controlling rodents, flies and other pests; purchasing chicks and pullets from suppliers who control for Salmonella in their flocks; implementing a biosecurity plan; and cleaning and disinfecting poultry houses. The rule also requires refrigeration of eggs on the farm and during transportation and environmental testing at certain time points in the laying hens’ production cycle.

• These facilities must register with FDA and are required to maintain written plans outlining their SE prevention practices.

State Agriculture Departments

• State agriculture departments monitor compliance with the official U.S. standards, grades and weight classes for egg packers who do not use the USDA/AMS shell egg grading service.

State and Local Health Departments

• State and local health departments monitor retail food and foodservice establishments for compliance with state and local health department requirements.

Candling is the process of using light to determine the quality of an egg. During candling, eggs travel along a conveyor belt and pass over mechanical sensors integrated with computerized systems to segregate defective eggs. Most egg packers use automated mass-scanning equipment to detect eggs with cracked shells and internal defects. Manual scanning involves passing the eggs over a light source from above or below where the defects become visible and the defective eggs are then segregated. Hand candling— holding a shell egg directly in front of a light source — is done to spot check and determine accuracy in grading. Advanced technology, utilizing computerized integrated cameras and sound wave technology, can also segregate eggs.

Shell eggs can be pasteurized by a processor if the FDA has accepted the process for Salmonella destruction. Pasteurized shell eggs are available at some grocery stores. Since they have been treated to destroy all viable Salmonella, they are not required to be kept refrigerated, but doing so will help retain quality. The equipment to pasteurize shell eggs isn't available for home use and it is very difficult to pasteurize shell eggs at home without cooking the egg.

Always purchase eggs from a refrigerated case. Choose eggs with clean, uncracked shells. Don't buy out-of-date eggs. Look for the USDA grade shield or mark. Graded eggs must meet standards for quality and size. Choose the size most useful and economical for you. Refrigerate shell eggs as soon as possible after purchase.

Inspection for wholesomeness is mandatory, but grading for quality is voluntary. If companies choose to have their eggs graded, they pay for this USDA service. The USDA grade shield on the carton means that the eggs were graded for quality and checked for weight (size) under the supervision of a licensed USDA grader. USDA monitors compliance with quality standards, grades and weights. Some state agencies may monitor quality at retail. All cartons normally bear a grade and size but may not have the USDA shield.

What Are Egg Grades?
There are three consumer grades for eggs: U.S. Grade AA, A and B. The grade is determined by the interior quality of the egg and the appearance and condition of the eggshell. Eggs of any quality grade may differ in weight (size).

U.S. Grade AA eggs have thick, firm whites; high, round yolks that are practically free from defects; and clean, unbroken shells. Grade AA and Grade A eggs are best for frying and poaching where appearance is important.
 

U.S. Grade A eggs have characteristics of Grade AA eggs except that the whites are “reasonably” firm. This is the quality most often sold in stores.

U.S. Grade B eggs have whites that may be thinner and yolks that may be wider and flatter than eggs of higher grades. The shells must be unbroken but may show slight stains. This quality is seldom found in retail stores because they are usually used to make liquid, frozen and dried egg products.

Size tells you the minimum required net weight per dozen eggs. It does not refer to the dimensions of an egg or how big it looks. While some eggs in the carton may look slightly larger or smaller than the rest, it is the total weight of the dozen eggs that puts them in one of the following classes:

Egg processors typically print dates commonly called “Code Dates” on cartons for rotating stock or controlling inventory. “EXP,” “Sell By,” and “Best if Used Before” are examples used for code dating. Code dates on USDA graded eggs is optional; however, if they are used, certain rules must be followed.

If an expiration date is used, it must be printed in month/day format and preceded by the appropriate prefix. “EXP,” “Sell By,” and “Not to be sold after the date at the end of the carton” are examples of expiration dates. Expiration dates can be no more than 30 days from the packing date.

Another type of code dating used indicates the recommended maximum length of time that the consumer can expect eggs to maintain their quality when stored under ideal conditions. Terminology such as “Use by”, “Use before”, “Best before” indicates a period that the eggs should be consumed before overall quality diminishes. Code dating using these terms may not exceed 45 days including the packing date.

Temperature fluctuation is critical to safety. To reduce Salmonella growth, eggs gathered from laying hens should be refrigerated as soon as possible. After eggs are refrigerated, they need to stay that way. A cold egg left out at room temperature can sweat, facilitating the movement of bacteria into the egg. In addition, any Salmonella in or on the egg will multiply at room temperature. Refrigerated eggs should not be left out more than 2 hours.

No. It's not necessary or recommended for consumers to wash eggs. This increases the risk of contamination because the wash water can be “sucked” into the egg through the pores in the shell. Government regulations require that egg processors carefully wash and sanitize USDA-graded eggs using only compounds meeting FDA regulations for processing foods and at temperatures that prevent wash water from being “sucked” into the egg.

The process of hard-cooking eggs leads to a thin layer of water forming on the cooked egg surface under the shell. The presence of the water provides a favorable environment for bacterial growth, including those which lead to spoilage. Hard-cooked eggs should be refrigerated within 2 hours of cooking and used within a week.

At the store, choose Grade A or AA eggs with clean, uncracked shells in refrigerated display cases. Any bacteria present in an egg can multiply quickly at room temperature. When purchasing egg products or substitutes, look for containers that are tightly sealed.

Take eggs straight home and store them immediately in the refrigerator set at 40 degree F or below. Keep them in their carton and place them in the coldest part of the refrigerator, not in the door.

Bacteria can enter eggs through cracks in the shell. Never purchase cracked eggs. If eggs crack during hard cooking, they are safe. Remember that all eggs should be thoroughly cooked.

Proper refrigeration, cooking and handling should prevent most egg-safety problems. Persons can enjoy eggs and dishes containing eggs if these safe handling guidelines are followed:

• Wash utensils, equipment, and work areas with hot, soapy water before and after contact with eggs.
• Don't keep eggs out of the refrigerator more than 2 hours.
• Cook dishes containing raw eggs immediately or refrigerate and cook within 24 hours.
• Cook casseroles and other dishes containing eggs to a safe minimum internal temperature of 160 degrees F. Use a food thermometer to be sure.
• Always cook eggs until both the white and yolk are firm.
• Serve cooked eggs and dishes containing eggs immediately after cooking or place in shallow containers for quick cooling and refrigerate at once for later use. Use within 3 to 4 days.

Sometimes eggs are decorated, used as decorations and hunted at Easter. Here are some safety tips for Easter eggs.

• Dyeing eggs: After hard cooking eggs, dye them and return them to the refrigerator within 2 hours. If eggs are to be eaten, use a food-safe coloring. As with all foods, persons dyeing the eggs should wash their hands before handling the eggs.
• Decorations: Italian Easter bread is decorated with dyed, cooked eggs in the braided bread. After baking, serve within 2 hours or refrigerate and use within 3 to 4 days.
• Blowing out eggshells: Because raw eggs may contain Salmonella, use caution when blowing out the contents to hollow out the shell for decorating, such as for Ukrainian Easter eggs. Use only uncracked and refrigerated eggs. To destroy bacteria on the egg surface, wash the egg in hot water and then rinse in a solution of 1 teaspoon liquid chlorine bleach per half cup of water. After blowing out the egg, refrigerate the contents and use within 2 to 4 days.
• Hunting Eggs: We do not recommend eating hard cooked eggs after hiding or playing with them, because bacteria picked up on the shells can be transferred to the edible part of the egg.

No. The breed of the hen determines the shell color. Nutrient levels are not significantly different in white and brown shell eggs.

Araucana chickens in South America lay eggs that range in color from medium blue to medium green. Nutrition claims that araucana eggs contain less cholesterol than other eggs haven't been proven.

No. There is no benefit in eating fertilized eggs. There is no nutritional difference in fertilized eggs and infertile eggs. Most eggs sold today are infertile; roosters are not housed with the laying hens. Fertile eggs with cell development, which is detected during the candling process, are removed from commerce.

Sometimes, but not usually. Variation in egg color is due to many factors.

• Blood spots are caused by a rupture of one or more small blood vessels at the time of ovulation. It does not indicate the egg is unsafe.
• A cloudy white (albumen) is a sign the egg is very fresh. A clear egg white is an indication the egg is aging.
• Pink or iridescent egg white (albumen) indicates spoilage due to Pseudomonas bacteria. Some of these microorganisms—which produce a greenish, fluorescent, water-soluble pigment—are harmful to humans.
• The color of yolk varies in shades of yellow depending upon the hen’s diet. If she eats plenty of yellow-orange plant pigments, such as from marigold petals and yellow corn, the yolk will be a darker yellow than if she eats a colorless diet such as white cornmeal. Artificial color additives are not permitted in eggs.
• A green ring on a hard-cooked yolk can be a result of overcooking as the sulfur and iron compounds in the egg react on the yolk's surface. The green color can also be caused by a high amount of iron in the cooking water. Scrambled eggs cooked at too high a temperature or held on a steam table too long can also develop a greenish cast. The green color is safe to consume.

The egg, as laid at 105 degrees F, normally has no air cell. As the egg cools, an air cell forms usually in the large end of the egg and develops between the two shell membranes. The air cell is formed because of the different rates of contraction between the shell and its contents.

Over time, the white and yolk of an egg lose quality. The yolk absorbs water from the white. Moisture and carbon dioxide in the white evaporate through the pores, allowing more air to penetrate the shell and the air cell becomes larger. If broken open, the egg's contents would cover a wider area. The white would be thinner, losing some of its thickening and leavening powers. The yolk would be flatter, larger and more easily broken. The chalazae (kah-LAY-zuh), the twisted cord-like strands of egg white that anchor the yolk in the center of the white, would be less prominent and weaker, allowing the yolk to move off center. Refrigeration slows the loss of quality over time.

An egg can float in water when its air cell has enlarged sufficiently to keep it buoyant. This means the egg is of poor quality, but it may be perfectly safe to use. Crack the egg into a bowl and examine it for an off-odor or unusual appearance before deciding to use or discard it. A spoiled egg will have an unpleasant odor when you break open the shell, either when raw or cooked.

Many cooking methods can be used to cook eggs safely including poaching, hard cooking, scrambling, frying and baking. However, eggs must be cooked thoroughly until yolks are firm. Scrambled eggs should not be runny. Casseroles and other dishes containing eggs should be cooked to a safe minimum internal temperature of 160 degrees F. Use a food thermometer to be sure the eggs have reached the correct temperature.

Egg mixtures are safe if they reach 160 degrees F, so homemade ice cream and eggnog can be made safely from a cooked egg-milk mixture. Heat it gently and use a food thermometer.

• Dry meringue shells are safe, as well as divinity candy and 7-minute frosting, made by combining hot sugar syrup with beaten egg whites. Avoid icing recipes using uncooked eggs or egg whites.
• Meringue-topped pies should be safe if baked at 350 degrees F for about 15 minutes. Chiffon pies and fruit whips made with raw, beaten egg whites cannot be guaranteed to be safe. Instead, substitute pasteurized dried egg whites, whipped cream or a whipped topping.
• Use pasteurized eggs or egg products when preparing recipes that call for using eggs raw or undercooked.
• To make a recipe safe that specifies using raw eggs, heat the eggs separately from all other ingredients in a liquid form over low heat, stirring constantly, until the mixture reaches 160 degrees F. Then combine it with the other ingredients and complete the recipe.
• To determine doneness in egg dishes such as quiche and casseroles, the center of the mixture should reach 160 degrees F when measured with a food thermometer.
• Eggs and egg dishes, such as quiches or soufflés, may be refrigerated for serving later but should be thoroughly reheated to 165 degrees F before serving.

The pH of the albumen (egg white) affects the ability to easily peel eggs. When an egg is laid, the albumen has a neutral pH (around 7.4). As an egg ages, the pH of the albumen increases to approximately 9. The increase in pH reduces the attraction of the outer (thin) albumen to the inner shell membrane allowing for easier peeling of the hard cooked egg. For this reason, older eggs make better candidates for hard cooking.

These Chinese eggs are not really 1,000 years old, but are somewhere between a month and several years old. The egg is not retained in its original state, but rather converted into an entirely different food, probably by bacterial action. They are exempt from inspection and grading by USDA, but imported products may be subject to other USDA and FDA regulations. Hulidan, Dsaudan and Pidan are types of thousand-year-old Chinese eggs.

Hulidan results when eggs are individually coated with a mixture of salt and wet clay or ashes for a month. This process darkens and partially solidifies the yolks and gives the eggs a salty taste.

Dsaudan eggs are packed in cooked rice and salt for at least 6 months. During this time, the shell softens, the membranes thicken and the egg contents coagulate. The flavor is wine-like.

Pidan, a great delicacy, is made by covering eggs with lime, salt, wood ashes and a tea infusion for 5 months or more. The egg yolks become greenish gray and the albumen turns into a coffee-brown jelly. Pidan smell ammonia-like and taste like lime.

Pickled eggs are hard-cooked eggs marinated in vinegar and pickling spices, spicy cider or juice from pickles or pickled beets. Studies done at the American Egg Board substantiate that unopened containers of commercially pickled eggs keep for several months on the shelf. After opening, keep refrigerated and use within 7 days. Home-prepared pickled eggs must be kept refrigerated and used within 7 days. Home canning of pickled eggs is not recommended.