Eggs are among the most nutritious foods on earth and can be part of a healthy diet. However, they are perishable just like raw meat, poultry, and fish. Unbroken, clean, fresh shell eggs may contain Salmonella Enteritidis (SE) bacteria that can cause foodborne illness. While the number of eggs affected is quite small, there have been cases of foodborne illness in the last few years. To be safe, eggs must be properly handled, refrigerated, and cooked.

What is the History of the Egg?

Eggs existed long before chickens, according to On Food and Cooking: The Science and Lore of the Kitchen by Harold McGee. These all-in-one reproductive cells, incorporating the nutrients to support life, evolved about a billion years ago. The first eggs were hatched in the ocean. As animal life emerged from the water about 250 million years ago, they began producing an egg with a tough leathery skin to prevent dehydration of its contents on dry land. The chicken evolved only about 5,000 years ago from an Asian bird.

How Often Does a Hen Lay an Egg?

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

How Does Salmonella Infect Eggs?

Bacteria can be on the outside of a shell egg. That’s because the egg exits the hen’s body through the same passageway as feces is excreted. That’s why eggs are washed and sanitized at the processing plant.   Bacteria can be inside an uncracked, whole egg. Contamination of eggs may be due to bacteria within the hen’s ovary or oviduct before the shell forms around the yolk and white. SE doesn’t make the hen sick. It is also possible for eggs to become infected by Salmonella Enteritidis fecal contamination through the pores of the shells after they’re laid.  

What Part Carries Bacteria?

Researchers say that, if present, the SE is usually in the yolk or "yellow." However, they can’t rule out the bacteria being in egg whites. So everyone is advised against eating raw or undercooked egg yolks and whites or products containing raw or undercooked eggs.  

Should eggs be eaten Raw?

No one should eat foods containing raw eggs. This includes "health food" milk shakes made with raw eggs, Caesar salad, Hollandaise sauce, and any other foods like homemade mayonnaise, ice cream which the egg ingredients are not cooked.

Why Should Eggs Be Refrigerated?

Temperature fluctuation is critical to safety. With the concern about Salmonella, 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 growth of bacteria. Refrigerated eggs should not be left out more than 2 hours.  

Should You Wash Eggs before Storage?

No. When the egg is laid, a protective coating is put on the outside by the hen. 

Why Do Hard-Cooked Eggs Spoil Faster than Fresh Eggs?

When shell eggs are hard cooked, the protective coating is washed away, leaving bare the pores in the shell for bacteria to enter and contaminate it. Hard-cooked eggs should be refrigerated within 2 hours of cooking and used within a week.  

Safe Storage

Make sure eggs  refrigerated after purchase in the store. Any bacteria present in an egg can multiply quickly at room temperature.

Bringing Eggs Home from the Store

Take eggs straight home and store them immediately in the refrigerator set at 5 °C or below. Keep them in their carton and place them in the coldest part of the refrigerator, not in the door. Don’t wash eggs. That could remove the protective  coating on the shell and increase the potential for bacteria on the shell to enter the egg.  

Is It Safe to Use Eggs That Have Cracks?

Bacteria can enter eggs through cracks in the shell. Never purchase cracked eggs. However, if eggs crack on the way home from the store, break them into a clean container, cover it tightly, keep refrigerated, and use within 2 days. If eggs crack during hard cooking, they are safe.  

How Are Eggs Handled Safely?

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 hands, 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. 
• Raw eggs and other ingredients, combined according to recipe directions, should be cooked immediately or refrigerated and cooked within 24 hours. 
• 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 1 to 2 days.  

What is the Cholesterol Content of Eggs?

The most recent studies  show that  cholesterol content of one large egg to be 213 milligrams. All of the cholesterol is in the yolk.

An egg  is a good source of complete protein, vitamins A and B-12, riboflavin, folacin, iron, zinc, and phosphorus.  

Does the Color of the Shell Affect the Egg’s Nutrients?

No. The breed of the hen determines the color of her eggs.

Is the Appearance of Eggs Related to Food Safety?

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 in the yolk 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.   

The color of yolk varies in shades of yellow depending upon the diet of the hen. 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 is a result of overcooking, and is caused by sulphur and iron compounds in the egg reacting 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.  

How Do Time and Refrigeration Affect Egg Quality?  

The egg, as laid at 105 °F, normally has no air cell. It forms as the egg cools, usually in the large end of the egg, and develops between the two shell membranes. The air cell is formed as a result 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.  

What Does It Mean When an Egg Floats in Water?

An egg can float in water when its air cell has enlarged sufficiently to keep it buoyant. This means the egg is old Crack the egg into a bowl and examine it for an off-odor or unusable 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.  

Safe Cooking Methods

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.