The egg is the best, most complete and densest source of nutrients for humans after breast milk. Like breast milk, the egg is the food source for chick embryos until hatching. The perfect balance and diversity of its nutrients, together with its high digestibility (only a small part of the protein content can remain unassimilated, especially if the egg is eaten raw) and its affordable price, have put the egg in the spotlight as a staple food for humans.
The egg is a complete source of protein, and its content in its components, egg white and yolk, is different. This can be explained by the fact that egg yolk and egg white are made up of distinct tissues. Egg yolk is essentially of hepatic origin, while egg white is synthesized and secreted after ovulation of the mature yolk in the hen’s oviduct. Basically, the chick is formed from the yolk, and the egg white is its food until it hatches.
Egg white
It is mostly water, 88%, and proteins, 10% (150 proteins in total, of which 50% ovalbumin). The content of lipids and carbohydrates is negligible. The white makes up half the volume of the egg, including the shell. The egg white contains mostly water-soluble vitamins (B2, B3 and B5, but also significant amounts of vitamins B1, B6, B8, B9 and B12), the fat-soluble ones being absent, due to the low content in lipids.
Egg allergy, which can occur in early childhood, is mostly caused by the egg white. Egg allergy usually develops in the first five years of life, with 50% of children outgrowing egg hypersensitivity by age three. Fortunately, in most cases, the prevalence of egg allergy decreases with age and usually resolves by school age.
Egg yolk
It is also mostly made up of water, 55%, lipids, 27%, and proteins, 15%. The total lipid (fat) in the whole egg is between 8-11%, completely concentrated in the yolk. It is a complete source of fat-soluble vitamins (A, D, E, K), being among the few foods that contain vitamin D. And the water-soluble vitamins (B1, B2, B5, B6, B9, and B12) are mostly concentrated in the yolk, the only vitamin absent being vitamin C. In addition to these vitamins, eggs are a major source of choline, an essential macronutrient for the liver, nerves, muscle activity, normal brain development, maintaining good energy levels and maintaining a healthy metabolism. Choline is essentially concentrated in the yolk (68% in the yolk versus 1% in the egg white). Hard-boiled egg is the second major source of choline after beef liver.
Cholesterol content
In 1968, the egg industry had to contend with recommendations from the American Heart Association that encouraged people to eat fewer than three whole eggs per week, arguing that high dietary cholesterol equated to blood cholesterol and therefore greater risk of cardiovascular diseases. These recommendations affected not only the egg industry, but also partly influenced people’s eating habits, depriving them of an accessible food of great nutritional interest.
By 1995, there was a concerted effort to unify all U.S. national dietary guidelines and to support egg rehabilitation research. Half a century of research has now demonstrated that egg consumption is not associated with increased health risk and that it is worth incorporating such a product into our diet due to its high nutrient content and its many bioactivities.
Starting from 2017 the restriction of consuming only 2-3 eggs per week has been lifted. It has been shown that the bad cholesterol contained in eggs is not absorbed, because the egg also contains the “antidote”: lecithin, choline and vitamin B3. Only people with hypercholesterolemia, diabetes and hypertension (estimated at 15-25% of the population) should pay attention to the number of eggs consumed, but the consumption of eggs should not be abandoned, because the balance of benefits versus risks clearly leans in favor of the benefits. If meat consumption were to be given up completely, the consumption of two eggs a day could ensure the intake of complete proteins for the body, as well as vitamins and micronutrients.
As for the oxidation of cholesterol in the egg, it only happens in industrial conditions, when the eggs are intensively processed at high temperatures and with the help of chemical compounds. When we cook eggs at home, in whatever form, cholesterol oxidation does not occur. Exception, the microwave oven.
Factors affecting egg quality
Chickens can be raised in 3 ways: free-range (“country” chickens), organic (fed only organic feed, which does not contain animal by-products, synthetic fertilizers, pesticides, other chemical additives or genetically modified food) and industrial (chickens raised in cages and fed everything that is not allowed in raising organic chickens).
Because free-range birds have access to grass, insects and worms in addition to their basic diet, the content of some micronutrients in eggs can vary slightly. For example, free-range farming results in significantly higher total tocopherol, alpha-tocopherol and lutein content compared to industrial and organic farming, respectively, when hens are fed a similar conventional diet. In contrast, no significant differences were observed in total lipid and sterol content. However, eggs from conventional systems generally contain more carotenoids and vitamins due to the possibility of including chemical additives in the diet, knowing that such a practice is not carried out in organic systems.
In parallel, since the hen’s immune system is likely to be more affected by the presence of environmental microbes in free-range systems, an increase in the immunoglobulin Y content of the egg yolk (initially to provide some passive immunity to the chicks, thus similar to maternal colostrum for infants) is also likely to increase. Furthermore, the antimicrobial capacity of egg white can also be slightly modulated when hens are exposed to environmental microbes. In conclusion, rearing laying hens in free-range systems can improve the overall antimicrobial potential of eggs.
Egg quality is also influenced by the age of the hens, their breed, their health, season and freshness.
How can we determine the freshness of eggs
Place the eggs in a bowl with cold water. Carefully. If the egg sits on the bottom of the bowl, it is fresh. If it rises from the larger side, it’s a few days old, but still fresh. If it floats, the egg is old. The explanation is that the air pocket increases as time passes, so the ability to float also increases. Although this method can tell you whether an egg is fresh or old, it does not tell you whether an egg is spoiled or not. An egg may sink and be spoiled, while an egg that floats may still be good to eat.
The best way to tell if an egg is good to eat is to crack it, smell it and take a good look. If it has dark spots on the yolk it is spoiled. If the egg white has lost its viscosity and has become liquid, it is old. If the yolk becomes flat and breaks quickly it is old. If it smells bad it is spoiled.
How can we preserve eggs?
Fresh eggs that have not been washed have a long shelf life due to the protective layer that covers the egg. It provides a protective barrier over the porous shell of the egg to keep harmful bacteria out of the egg. Removing this layer makes the eggs spoil much faster. It is recommended to wash the egg only before use. Eggs are considered “fresh” for up to 28 days if they are stored correctly and not washed.
Refrigerating eggs or storing them in a cool place is one of the most common ways to extend the shelf life of fresh eggs. Fresh eggs that are kept in a carton and kept on the coldest shelf in the refrigerator, not in the egg compartment on the fridge door, will last up to 5-6 weeks. They can last longer if the humidity is high to prevent too much evaporation. Fresh, unwashed eggs that are stored at -1 degree Celsius (30 F) with 85% humidity can last up to 9 months. Once the egg contents are removed from the shell, its shelf life decreases rapidly. Leftover egg whites can be stored in an airtight container for up to 4 days, while leftover yolks can be stored in cold water in an airtight container for up to 2 days.
Eggs can be frozen. We can freeze whole eggs, without the shell, beaten like for an omelette, in bags or in the ice compartment. Or separately, the whites beaten, and the yolks powdered with a little salt or sugar, it depends on what we want to use them for. I froze bags of 6 egg whites and bags of 6 yolks, for cakes. Or bags of 4 whole eggs, whites and yolks, for the omelette.
Conclusions
For centuries, eggs have been considered a food of high nutritional value for humans and are widely consumed around the world. Their consumption is expected to increase continuously in the future, given the increasing number of Western consumers who are starting to adopt a meat-free (vegetarian) diet or who are significantly reducing their meat intake, as the daily consumption of eggs successfully replaces the nutrients in meat.
