Eggs, rich with protein, fats, and natural lecithin, are a staple when it comes to both baking and cooking. As an integral part of custards, doughs, sweets, and an assortment of non-Western desserts, eggs represent a versatile ingredient with a wide range of roles-whether that's as a main flavor or a structural component.
There are many varieties of eggs availble when baking with eggs, but most recipes will call for large AA eggs. However, there are so many types of eggs: medium, large, jumbo, brown, and even duck and quail eggs; to make maters more complicated they share enough similarities to yield similar ingredients, but with slight differents in texture or taste.
The egg is mostly composed of two components: the white and the yolk. The white is very protein dense and contains lots of the protein known as albumen. The yolk, on the other hand, is more full of lipids and phospholipids (which allow for mixing of fats and water).
In a meringue, only the egg whites are used. Whites are rapidly whisked (either by hand or with an electric mixer). As a result, the proteins unfold in a process called denaturation, thus exposing the pieces of the protein to the air which is rapidly being introduced via mixing. Some parts of the protein prefer exposure to air while others dislike it; the result is that the egg whites will form the air bubbles thatt constitute the foamy meringue. However, a meringue composed of just egg whites will often collapse before reaching the stable structure commonly used for baked goods. For this reason, sugar and small quantities of acid are introduced to the mixture to help keep the meringue stable. However, the yolk must be absent for a meringue to form. The presence of fats hinders the denaturation of albumen by coating the proteins and preventing them from opening up. For this reason, it is important to make a meringue with clean non-plastic tools that are grease-free.
Egg yolks, with their high fat content, serve a much different role. Egg yolks make silky sauces and thick custards by exploiting the emulsifying properties of egg yolks. When added to a mix of water and fat, egg yolks allow the two to combine, creating a thicker sauce with more body as the fats and water molecules form a more cohesive, uniform substance. However, when heating egg yolks one must be careful to do so gently. Adding egg yolks directly to hot liquids will yield scrambled eggs rather than a thick sauce. Instead, try tempering the egg before adding it in. Heating the mixture to too high of a temperature can also cause the sauce to "break"; the sauce will appear fractured as fats and water begin to reseparate when the temperature reaches a certain point. Egg yolks also generally contain more fat, which allows for moister, richer baked goods. For example, some cookie dough recicpes call for an extra yolk to create chewier cookies an many brownie recipes will use additional egg yolks for increased fudginess.
Finally, egg yolks can be used as leaveners (though they are much less potent than typical chemical leaveners like baking soda or baking powder). Eggs do not provide baked goods with rise via chemical reactions, but through physical incorporation of air. Beaten eggs (whites or whole eggs) take on air when the air is introduced. The rest of the ingredients are mixed in. While baking, these air pockets can expand, thus helping baked goods rise.
