anatomy of an egg
The egg is a versatile ingredient that can be incorporated into dishes or prepared on its own. It has a cylindrical shape with four main parts: shell, shell membrane, yolk and albumen. The shell is made of calcium carbonate and is fragile. The shell membrane is the thin, skin-like layer underneath the shell. The yolk is the yellow portion of the egg and the albumen, also called egg white, is the clear portion of the egg. The yolk is anchored to the middle of the albumen by two strands called chalazae.
As the eggs are laid, they are covered with a cuticle, a layer of liquid that dries to protect the egg from contamination. In the United States, eggs are washed to clean the shell, but this process damages the cuticle. As a result, eggs must be refrigerated to prevent bacteria from getting in.
Eggs are considered a nutrient-dense food, which means they have a high amount of nutrients and relatively few calories. They are an excellent source of riboflavin and selenium, minerals with antioxidant properties to help protect cells from damage. Eggs are a good source of protein needed to build and repair tissues and muscles. They contain trace amounts of several vitamins and minerals for optimal health. Eggs contain a high amount of cholesterol and should be eaten in moderation.
Types and Varieties
The USDA oversees the inspection of eggs and provides a voluntary egg-quality grading program. Egg cartons containing eggs graded for quality by the USDA will bear the USDA shield.
- Grade AA recognizes the highest quality. The eggs have a thick, firm yolk, dense white, and clean, unblemished shell. These eggs are best for frying and poaching where appearance is important.
- Grade A eggs are similar to grade AA eggs, but with a less dense white and a larger air pocket.
- Grade B eggs have a flatter egg yolk, watery white and a larger air pocket. These eggs are often used to make liquid, frozen and dried egg products.
- Pasteurized eggs have been heated to a specific temperature for a specific period of time to kill bacteria that can cause foodborne illness. They are a good option for preparing egg dishes that are not fully cooked. The egg white will be slightly milky, but taste is not affected.
- Organic eggs require strict feeding of hens with certified organic feed, as well as outdoor access, but exact standards are not defined.
- Free-range and cage-free are loose terms that mean the hens must have outside access, but it is not specified how much space, for how long, and how often access must be given.
Selecting and Storing
- Store eggs in the original carton in the refrigerator. For best flavor and quality, use within one week of purchase. They can, however, be kept for two to three weeks past the “sell by” date.
- Inspect eggs for breaks and cracks in the shell. Do not eat eggs that have a foul odor or a pinkish egg white when cracked.
- Leftover egg whites and egg yolks should be tightly covered with plastic wrap before being refrigerated, and should be used within two to four days.
- Frozen egg products should be thawed in the refrigerator and stirred thoroughly before use. Dried eggs should be stored in a cool, dry place and refrigerated once they have been reconstituted.
Healthy Ingredient Contribution
Cholesterol: One serving of eggs provides 71 percent of the daily recommended value of cholesterol, a type of fat that is used to develop crucial hormones, found in the egg yolk. Too much cholesterol can clog blood vessels, reducing blood flow and causing damage to the heart and brain.
Selenium: Eggs offer 22 percent of the daily recommended value of selenium, a trace mineral that helps regulate inflammation. It also works as an antioxidant to protect cells from damage.
Riboflavin: One serving of eggs contains 15 percent of the daily recommended value of riboflavin, an essential vitamin that aids in transforming proteins, carbohydrates and fats in food into energy. It helps protect the body from free radicals.
Protein: Eggs provide 13 percent of the daily recommended value of protein, an important macronutrient needed to build and repair tissues and muscles. It plays a role in the production and action of enzymes, involved in most body functions.
Vitamin B12: One serving of eggs offers 9 percent of the daily recommended value of vitamin B12, essential for a healthy nervous system. It is needed to convert carbohydrates into glucose for energy production.
Phosphorus: Eggs provide 9 percent of the daily recommended value of phosphorus, a mineral needed to form healthy bones. It plays an important role in how the body stores and uses energy and helps reduce muscle pain.
Lutein and zeaxanthin: The egg yolk contains phytochemicals, natural pigments that give the yolk its color and act as antioxidants. Lutein and zeaxanthin help prevent eye disease, including macular degeneration and cataracts.
Values from NutritionData.com based on eggs, whole, cooked, hard-boiled, 50 grams (large)
- Shell color is determined by breed of hen and does not affect taste or nutrition. Generally, hens with white ear lobes lay white eggs, and hens with red ear lobes lay brown eggs.
- The yolk color is related to the hen’s diet. White corn will result in a lighter egg yolk than yellow corn.
- Many countries do not wash eggs, which means that the intact cuticle protects the egg from bacteria. Some countries vaccinate hens against salmonella.
- Egg shell can be ground and sprinkled over food for added calcium.
- Hens are omnivores, meaning they eat both meat and plants.
- An old egg will have a large air pocket and will float in water.
About this article
Ingredient of the Month is produced by ACF’s Education Department every month as a tool to help chefs educate children and families on healthy eating and nutrition through the Chef and Child Foundation. Tools for additional classroom education are available free for download at http://www.acfchefs.org/CCF.