There are few foods, if any, as versatile as eggs. They can be boiled, fried, be part of a pancake, waffle, omelet, fluffy foam, custards, mayonnaise, cakes, meringues, used in baking etc. The possibilities seem endless. In this article, I will go through some basic information about eggs and what makes them so special in cooking. There are many different kinds of eggs, but in this article, I will focus on eggs from chickens.
An egg is actually a cell made for reproduction, however, the eggs we usually eat are non-fertilized eggs, meaning they can´t give rise to an offspring. A chicken can lay around 250-290 egg per year. The egg mainly consists of a white (~2/3 of the weight), a yolk (~1/3 of the weight) and a shell.
Some eggs contain a “blood spot” which is due to a ruptured blood vessel during egg development, it has nothing to do with the egg being fertilized or not. This red spot is not harmful nor does it affect the taste.
Egg allergy is more common in children and is usually resolved with age (68% of 16-year-olds outgrew egg allergy). Furthermore, the heating egg can make the egg less allergenic by changing the structure of the allergenic protein (denaturing it) (Patel and Volcheck, 2015). The yolk of the egg seems to be less allergenic than the white.
The quality of the egg deteriorates faster at room temperature than at colder temperature, so keeping them in the fridge is recommended if you want to prolong the shelf life. Also important on how to store eggs, “shaking” eggs will make the egg white thinner and therefore storing them in the door of the refrigerator is not optimal.
Additionally, if you don’t want to risk the egg absorbing odors and loosing moisture, keep them in an airtight container (this can, however, increase the stale flavor of the eggs themselves).
If an egg is cracked it should be consumed or can be stored frozen (they can survive several months in an airtight container in the freezer. Crack the shell, otherwise it will be cracked by the expansion of the egg). Freezing eggs will affect the egg somewhat, e.g. the egg whites won´t foam that well, the yolk can become pasty (due to protein aggregation) and difficult to mix with other ingredients (to combat this, mix the yolk with salt, sugar or acid before freezing).
With age, the egg becomes more and more alkaline. The reason for the increase in alkalinity is the escape of carbonic acid from the white and the yolk through the pores in the eggshell. The increase in alkalinity has effects on the color of the white (which is a bit cloudier when fresh and more acidic, due to clusters of albumen (proteins) at this pH. At higher pH, they repel each other). An older egg also changes somewhat in consistency. The white becomes more runny with time due to a smaller proportion of thick albumen compared to thin, from 60% and 40% to 50 % and 50 %, respectively) making the egg float out more in the pan when frying. The yolk swells (due to higher osmotic pressure which leads to an increase in water with time).
How you can figure out if an egg is fresh or old and why
To figure out if you have an old egg or not you can put it in a container of water. The older the egg the higher it floats. This is because eggs become less dense with time, due to a loss of moisture and therefore an increase in the air cell (small pocket of air in the wide end of the egg).
There is no nutritional difference between an unfertilized egg and a fertilized egg. As you can see in the table, the yolk contains most of the calories and fat, while the white is mostly water and protein. Have you read my previous article about the bioavailability of protein in cooked vs raw eggs? If not, I suggest you do. We are actually not able to absorb and utilize as much protein from raw eggs as cooked eggs. Read the article to learn how this is relevant and can affect us.
Table 1. Nutrient content of Whole Egg, raw, fresh, and Egg white, raw fresh and Egg yolk, raw, fresh (“Food Composition Databases Show Foods — Egg, whole, raw, fresh,” n.d.).
When you cook a raw egg, the protein (which is mostly compact and separate) in the egg unfold due to the heat and bind to each other creating a 3D structure. The water in the egg will now be divided among the protein and can’t move as readily as before. This is the reason why an egg goes from a liquid to a moist solid. In addition, the densely clustered protein molecules deflect light making the egg opaque instead of the transparent character of a raw egg.
Acid, salt and beating eggs affect the egg in a similar way, by increasing the bonds between protein. The consistency of the egg after cooking is also affected by the freshness of the egg, a fresh egg needs less cooking than an old for any given consistency. This is due to the most plentiful egg white protein, ovalbumin, and due to its heat resistance increasing with age. Eggs can be overcooked giving them a rubbery consistency. This is due to the protein binding too hard to each other which makes the water escape the protein network. Eggs start denaturing at around 73°C/165°F (coagulate at ~80°C/180°F), only the white at around 63°C/145°F (coagulation ~66°C/151°F), the yolk at around ~65°C/150°F (coagulation at ~70°C/158°F). The white always set before the yolk, making sunny-side up fried eggs, baked eggs, and poach eggs possible. Adding ingredients to the eggs will affect the coagulation, usually increasing the temperature at which coagulation occurs (liquids and sugars by diluting the protein).
Spin the egg on the side, if it spins fast it´s boiled. If it spins more slowly and kind of wobbles, it´s raw (the liquid inside counteracts the movement in a way).
The taste of the egg usually increases with age. Interestingly, the storage condition and the age of the egg can affect the flavor of the egg more than the hen´s diet.
There are many compounds in an egg contributing to its flavor. One of the most notable might be hydrogen sulfide, H2S. H2S can in large amount be unpleasant. It mainly forms in the white (due to albumen (proteins) unfolding and freeing sulfur which reacts with other molecules/atoms when above 60°C/140°F).
The age of the eggs affects the production of H2S, with older eggs with higher pH increasing the production.
If you store a cooked egg H2S can actually escape and ameliorate the H2S flavor and smell with time.
Don´t boil eggs in turbulent water. This will increase the risk of breaking the shell.
Don’t have a too high temperature. This will coagulate the outer white and make it rubbery but leave the white less cooked. Instead use water a bit colder than bubbling water, a bit less than boiling, for soft cooked eggs. Cook for around 5-6 min for a semi-liquid yolk and firm white. For hard-boiled eggs, you can use even colder water, around 80°C/190°F but cook it for longer, 10-15 min.
Old eggs are usually easier to peel than fresh eggs (the albumen are more strongly attracted to the shell at lower pH. Older eggs have higher pH). You can add baking soda to alkalinize the eggs and make them easier to peel, however, this increases the sulfur flavor (alkaline environment liberates H2S).
Sometimes you can observe a greenish tint on the surface of the yolk. No worries, they are still fine and edible. The reason for the color is the formation of ferrous sulfide, a molecule of iron and sulfur, when eggs are heated (higher temperature and longer cooking increases this formation). The iron from the yolk reacts with the sulfur from the white. A more alkaline egg (eg an older egg) is at higher risk of this discoloration due to less sulfur bound to albumen. In other words, to avoid this non-dangerous but less visually appealing color, use fresh eggs and don’t overcook them.
There are tricks to minimize the eggs floating out all over the pan. Use fresh eggs. As mentioned above the egg white in the old eggs are thinner. Don´t use a too hot skillet which will increase the risk of the eggs loosing too much water and becoming rubbery. A temperature of 120°C/250°F is good for cooking a tender fried egg. With a higher temperature, you can get more flavor and brown, crisp surface but lose the tenderness.
Egg whites foam when whisked because the whisking causes air to mix with the egg white creating a mass of bubbles. The bubbles are relatively stable due to the surface tension being reduced by non-water molecules and the proteins increase the stability of the foam. When whisking you´re not only introducing air into the egg whites but your also making the proteins unfold and bond to each other. This strengthens the walls of the bubbles.
Water doesn´t foam because the surface tension is too strong and the bubbles therefore collapse. You can strengthen an egg foam by adding flour, gelatin, sugar (will, however, delay foaming and lower the volume if mixed in before) or chocolate for example. This thickens the foam. Heating egg white foam will reinforce the bubble walls by further unfold and coagulate protein, making the foam more stable and also dry, due to water being evaporated. Egg yolk, fat or oil, and detergent will interfere with the foaming by disturbing the protein bonding and compete for a place between the water and the air in the bubbles. However, fat and yolk can be mixed in with a finished foam, as in soufflés.
Egg yolks are very rich in proteins and also phospholipids (emulsifiers which can both mix with water and fat) but still can’t foam. This is due to the lower water content and the water being bound to other molecules. Also, the proteins are very stable. However, you can combat this by adding some water and heating the yolk slightly (making the proteins less stable). This will make the yolk easier to foam but still not as easy as egg whites.
Lecithin, a mixture of emulsifying lipids (fats), are abundant in the egg yolks, ~10 %. Eggs are therefore great in mixtures of fat and water, like cake batter and Hollandaise sauce, and allows for mixing between fat and water without it separating from each other, like trying to mix water with olive oil for example.
Written by: Fredrik Wernstål
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