Molecular Gastronomy

The chemistry behind baking

The Science Behind Eggs

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Depending on the recipe, most baked goods require the use of eggs. Believe it or not, eggs serve a crucial role in many recipes, but cooking the perfect egg often proves to be a difficult task. This one simple ingredients has a series of characteristics, which can very depending on the method used to cook the eggs.

Factors that affect the cooking time of eggs:

Age of  the eggs: There is a specific bonding between the inner membrane of the eggs and the egg whites. This bond can be specifically be seen in fresh eggs from a farm, which is why they are often difficult to boil/cook. These type of eggs can be best used for frying. On the other hand, this bond eventually breaks down overtime, making it easier to boil/cook eggs found at a supermarket.

Protein Bonding Temperature: Egg whites contain certain proteins, that can bond together. These bonds affect the appearance and structure of the egg, as these bonds can lead to the rubbery texture of the egg white that can sometimes be found in hard-boiled eggs. Between 30-140°F, the proteins of the egg whites expand. Above 140°F, the proteins will bond and after 155°F, the proteins will solidify. At around 180°F, the proteins will bond together, giving the opaque and firm characteristics of the egg white. Any temperature above 180°F may lead to the release of hydrogen sulfide, which is responsible for the smell of rotten eggs, as well as the dark green-grayish compound located between the egg white and yolk if overcooked.

Egg Yolk Temperature: The egg yolk is mainly composed of fatty acids, cholesterol, and some proteins. Because of this, different temperatures affects its performance. An egg cooked at any temperature below 145°F will have no affect on the yolk. Once reaching a temperature around 160°F, the yolk will become firm but it will still retain its bright color. Any temperature over 170°F will cause the yolk to turn a pale yellow, and it will have a crumbly consistency. This will result in its chalky texture and it will also release ferrous sulfide, which is also responsible for the smell of rotten eggs.

Altitude: Altitude also plays a key role in cooking an egg as altitude affects the boiling temperature of water. According to the U.S Department of Agriculture (USDA), the boiling temperature at a point above 2,000 feet is around 208°F, lower than the ground level boiling temperature of 212°F. Due to this difference, it will take more time to properly cook an egg at higher levels than that of lower levels.

On the other hand, there are also many ways to prepare eggs, with each method having its own scientific background to it.

Science Behind Various Methods of Preparing Eggs:

Heating: When heating eggs, the egg-white proteins move around and collide with water molecules. Due to this, weak bonds may break, causing the egg white proteins to uncurl and collide with other proteins that have uncurled as well. This results in new chemical bonds, connecting different proteins to one another. The breaking of bonds and formation of new bonds allow for the egg white proteins to form a series of interconnected proteins. These bonds are responsible for the ability of the egg whites to develop a rubbery texture, which was previously discussed in the “Protein Bonding Temperature” paragraph located earlier in this post.

Beating/whipping: Beating or whipping eggs exposes air bubbles to egg whites. This exposure allows the unfolding of the egg proteins just as heating would unfold these proteins. Egg-white proteins consist of both hydrophilic and hydrophobic amino acids. In other words, some amino acids are attracted to water, while others are repelled by it. Prior to uncurling, the hydrophobic amino acids are located in the center away from the water, while the hydrophilic amino acids are located closer to the water. When the egg-white protein encounters an air bubble, part of the protein is exposed to both air and water. This causes the protein to uncurl so the hydrophilic and hydrophobic amino acids can be located in its desired respected area. This allows the amino acids to bond with each other, creating a series of bonds that hold the air bubbles in space. When heating the air bubbles, the gas inside them expands. Often, the area around the bubble solidifies, and the structure usually does not collapse when the bubbles burst. The protein that lines the outside of the air bubbles is known as lecithin. This is what prevents them from collapsing when baking. This allows for the consistency found in a soufflé or meringue. The more whipped the egg whites are, the more stiff they will become. On the other hand, unbeaten egg whites often allow the lecithin as a binder, which hold the cake together.

Here’s an interesting fun fact: There is a myth stating that copper bowls are better for whipping eggs. There is some actual scientific support to this myth, proving that it is true.  The copper ions from the bowl combine with conalbumin, one of the proteins found in eggs. This combination forms a bond that is stronger than the protein itself, making it less likely for the egg-white proteins to unfold. The copper could also react with sulfur-containing groups on other proteins found in eggs, making the egg proteins even more stable. If a copper bowl is not used, ingredients such as cream of tartar or vinegar can be used to produce a similar effect.

Mixing: Many recipes call for the mixing of oil-based and water-based liquids. However, these two are immiscible and do not interact with one another. Because of this, egg yolks are often used to create an emulsion. Egg yolks contain a number of emulsifiers, with some of them being hydrophobic and others being hydrophilic. Because of this, thoroughly mixing egg proteins with oil and water will allow part of the protein to attract the water and another part to attract the oil.

Just like egg-whites, egg yolks contain lecithin. The lecithin, which is a phospholipid, also acts as an emulsifier. Due to its structure (see figure below), it has a hydrophilic head and a hydrophobic tail. The tail gets attracted to the oil, while the head gets attracted to the water.

These important characteristics of egg proteins play a crucial role in making foods such as mayonnaise, which require the mixing of water and oil.

On a side note, eggs can also be used as moisteners (instead of using water) and a good source of fat and amino acids. It can also be used as a glaze as a source of protein for the Maillard Reaction.

So next time you bake, don’t forget to note the complex structure and characteristics of an egg…it is because of this that eggs are used in various methods of cooking. Not only does an egg serve for taste, but it also plays a vital role in the texture or appearance of certain kinds of food.

Author: Erica Rowane Bautista



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