What do the formulas of most cake batters, mayonnaise, French buttercream, and ice cream have in common? They all use eggs, specifically egg yolks. You may ask yourself why a simple ingredient like eggs are used in such a wide variety of foodstuffs, and the answer would be because of a category of chemicals in the yolk, named lecithin.
Figure 1. Labelled diagram of a chicken egg.
Lecithins are compounds found in the egg yolk. They are amphipathic, which means they have both hydrophilic (water loving) and lipophilic (fat loving) functional groups. Compounds that express this behaviour can be used as emulsifiers, or surfactants. Emulsions are mixtures of two or more immiscible fluids: fluids which do not form a stable mixture (eg. water and oil). Emulsifiers help stabilize these mixtures by lowering the surface tension between the interfaces.
Figure 2: Labelled projection of lecithin.
In a mixture of just oil and water, the two fluids will separate, eventually forming two distinct layers if left undisturbed. This is because the oil molecules will bind to each other. The water molecules also tend to bind to other water molecules. As we can see from this Fig 2, lecithin molecules contain a phosphate group, the hydrophilic site that binds to the water phase in an emulsion. The two lipophilic fatty acids chains bind to the fats or oils in a mixture. Lecithin binding to both oil and water stabilizes the emulsion and slows the separation of the two phases. Lecithin has a high emulsifying power; the typical dosage level required ranges from 1-10% of the fat, depending on the type of emulsion desired. Water in oil emulsions typically requires less lecithin, while oil in water emulsions typically requires more.
Figure 3.1 (Left): Oil and water combined, with two distinct layers formed.
Figure 3.2 (Right): The mixture immediately after agitation. A cloudy mixture can be seen.
Figure 4.1. (Left) The mixture from Fig 3, allowed to sit, undisturbed for five minutes, the layers can be seen forming.
Figure 4.2.(Right) The addition of a drop of egg yolk to the mixture from Fig 3. This mixture was allowed for sit for five minutes. The mixture still remained cloudy, and has not started to separate compared to Fig 4.1.
The food products named earlier involve the mixing of an oil based ingredient and a water based ingredient, which are immiscible. The thorough mixing of egg yolks will help to stabilize and emulsify these mixtures.
Let us look at the procedure of a sugar batter cake for example. Initially, fat and sugar are creamed together, incorporating air cells. Eggs are then added in additions, scraping the bowl between additions. As the eggs are blended in, the hydrophobic site on the lecithin molecules in the yolk are starting to bond to the fat in the bowl. After this step, flour and the remaining liquid (water or milk), is incorporated into the batter. After the liquid is added, sufficient mixing with the lecithin will promote even fat distribution in the batter. The hydrophilic phosphate group from the lecithin molecules will start to bond to the liquid, adding stability to the batter. This will allow the batter to not split, and will also help the batter hold the air cells from the initial creaming process.
Knowing a reason why we use eggs in baking is important. It allows us, as bakers to gain an appreciation of the discovery of these functions, and gives us the information to potentially further optimize formulas.
Thank you for reading, and I hope you have gained an appreciation for the simple, and delicious egg.