Eggs are one of the four staple ingredients in conventional cake baking (along with flour, fat and sugar). Understanding the composition and functionality of the egg can help you make better decisions as a baker. Such as when to use whole eggs, when to use just the whites or just the yolks and how many eggs to use in a cake recipe.
By throwing a spotlight on eggs and their role in baking, we can also begin to understand how our cakes will be different if we omit the eggs – for example in vegan / plant-based baking.
THE COMPOSITION OF AN EGG
Eggs are made up of an outer shell, the white (albumen) and a yolk. That funny stringy white bit you will find when you crack an egg is called the chalaza. Its job is to hold the yolk in position in the middle of the egg.
The yolk of an egg is high in both protein and fat. It is yellow in colour, although the depth of colour depends on the diet of the chicken (and, some say, its level of happiness). Egg yolks can add to the richness of a bake. Pâte sucrée, for example, (an enriched pastry dough) is typically made with egg yolks.
The white of an egg consists mostly of water (around 85%+) and protein. In its raw state it is clear and fluid. When it is cooked it turns white and solidifies (unless you don’t cook it properly, in which case it is snitty. And I have yet to find *anyone* who likes a snitty egg) 🙂
The thicker the egg white, the fresher the egg is. And so conversely, the more fluid the white, the older the egg).
The shell of an egg is the inedible part. It is fragile and pesky bits of it are prone to landing in the eggs when you crack them. Always crack your eggs into a separate bowl rather than directly into your creamed cake mixture. It is much easier to fish stray bits of shell out from just the eggs than once they’ve been added to the batter.
Eggs are also porous, which is why you should never store eggs next to anything with a strong flavour or odour as they will take on those flavours.
THE ROLE OF EGGS IN BAKING
So now that we’ve established what an egg is made up of, we can turn to the nitty gritty of what an egg does when we use it in our baking.
When it heats up, the protein in egg coagulates (changes from a liquid to a semi solid state). This helps to give structure in baked goods. It is why eggs are such a valuable addition to the gluten-free baker’s pantry of baking ingredients. The protein in eggs goes some way to compensate for the lack of gluten (which is made up of two proteins) in our flours.
If you add too much egg to a cake recipe, the texture may become dry, tough or slightly chewy. The quantity of egg must therefore be balanced with the quantity of fat and sugar in your cake recipe (both of which help to tenderise the crumb) to prevent a tough cake that nobody will come back for seconds of!
The yolk of an egg contains a natural emulsifier (lecithin). This helps to bind fats and liquids together (not the best of bed fellows when left alone).
When you add eggs to a creamed mixture of butter and sugar, you are adding a lot of water (remember: the majority of egg white is water). Fat and water naturally repel each other. The lecithin in the yolk helps to emulsify (bring together into a smooth and stable mixture) the fat and water.
Eggs, when beaten into the fat and sugar of a cake mix, trap bubbles of air. These bubbles expand when subjected to the heat of an oven. And this helps cakes to rise.
Some recipes call for you to separate the yolks and whites before beating the whites to stiff peaks and then adding to your cake batter. When doing this, be sure to fold the beaten egg whites slowly and gently. You want to knock out as little of the air beaten into them as possible.
The browning process (Maillard reaction) is triggered when proteins are exposed to heat. And because eggs contain protein, they contribute to this browning process. Think, for example, of the golden colour that adding an egg wash to the top of your pie crust results in!
Eggs contribute to the overall flavour profile of a cake. The flavour comes from the yolks, though, not from the whites. A classic fat, flour, sugar, egg-based cake recipe will taste very different to an egg-free / plant-based / vegan sponge cake.
Eggs consist mostly of water. So they contribute to the overall moisture content of your cake. If you decide to enrich your cake by using just the yolks instead of whole eggs, you will need to adjust the amount of liquid in the recipe (for example, by adding milk) to compensate for the liquid the egg whites would have contributed.
The quirky nature of eggs is that they not only contribute moisture in baking, but they can also dry things out! Cakes using only the whites can have a tendency to be dry because of the high water content (which evapourates during baking) and the absence of fat. The recipe will therefore need adjusting to compensate, typically with a high ratio of sugar (which holds on to the moisture in a cake).
Eggs are a great culinary thickener when heated/cooked. Think lemon curd, pouring custard and set custard tarts. The more egg you add to something in this way, the thicker and more ‘set’ the result.
The fat content of the egg yolk contributes to the overall tenderness of the cake crumb. It is also why adding extra egg yolk to a cookie recipe will result in a softer, chewier cookie.
The white of an egg does not contain any fat and so cakes made with whites only rely on higher quantities of other ingredients (fat and sugar) to tenderise the crumb.
The yolk of an egg will add some yellowy colour to your cakes. That is why if you want a whiter cake crumb, you will need a cake recipe that uses egg whites only.
The same is true if you want to colour your cake sponges. Take blue, for example. The yellowy hue of a cake containing egg yolks makes it difficult to achieve a true blue colour because yellow and blue make green!
Eggs help to ‘shorten’ cakes and bakes because of their fat content. This essentially means that eggs help to tenderise a bake and create a delicate crumb. Think of brioche, which is oh so light and soft. It has a much higher egg content (and butter!) than a ‘lean’ dough.
Some people keep their eggs in the fridge, others keep them in the cupboard or on their kitchen bench. They should be stored below 20 degrees centigrade*.
If you store your eggs in the fridge, always lift them out in advance of any baking. They need to be at room temperature when you are ready to use them.
In the UK, eggs come in four different sizes*. These are:
These weights include the shell of the egg.
*Source, The British Egg Industry Council
I hope you have found this spotlight on eggs helpful in understanding the role they play in baking. Baking without eggs (or with egg replacers / substitutes) isn’t something I have addressed in this article as it warrants a spotlight of its own. Given the many and varied roles that eggs play in baking, simply omitting them from a recipe could lead to any number of cake fails. So this is a topic to tackle on another day!
If you enjoyed this little dive into the science of baking, you may also find my spotlight on sugar and its role in baking an interesting read 🙂