Hello Lovelies! I'm back again with the next instalment on the science of baking. You might remember that I mentioned in this post about the different types of flour used in baking and the way gluten affects the finished result of your bakes. Then we looked at the different types of sugar available and how the type of sugar you use will make a difference to your baking.
This week we are looking at eggs.
What do eggs do in baking?
In addition to their nutritional value, eggs can provide structure, leavening, richness, colour, and flavour to baked products. The height and texture of baked goods is determined by the balance between eggs and flour which provide strength, and sugar and fat which add tenderness. Because eggs become firm when heated (coagulation) they set the structure of cakes, cookies, and other baked items; the same property makes it possible for them to thicken sauces, pies, and casseroles. In addition, lightly beaten eggs, either whole or separated can be brushed onto the surface of breads, cookies, and biscuits to give them a pretty shine.
Recipes often specify that eggs should be at room temperature when added. This is because adding cold eggs to recipes with a high fat content can re-harden the fat, making the batter appear curdled or lumpy. Recipes that involve beating eggs or egg whites, with or without sugar, also usually specify room temperature eggs as the eggs whip up to a greater volume when they have had a chance to warm up a bit.
In addition to baking powder and baking soda, whisked egg whites can act as a leavener as they trap air bubbles in the batter, so when whisked egg whites are folded into a batter, they hold in the air and help the baked goods rise. When combined with the flavour and structure provided by the yolks, because of their fat content, this adds a richness and will ensure your baking has a great flavour and texture.
Did you know?
Although most people will discard or compost them, egg shells are actually edible. Sometimes they are blended into nutritional supplements or shakes as they contain calcium. They are also sometimes added to chicken feed and some people actually eat the ground up powdered egg shells either on their own or sprinkled onto cereals etc.
The technical bit
Crack an egg open and you'll see the large, transparent egg white with the round, yellow yolk in the middle. The white is made almost entirely of proteins and water; the yolk, on the other hand, is packed full of nutrients, vitamins, and fats. Look closely and you'll also see a thin white strand floating somewhere in the mix. This is called the chalaza and it anchors the yolk to the white and to the inside of the shell, keeping the yolk suspended.
A large egg (still in its shell) weighs about 2 ounces; that's about 1 ounce for the white, 2/3 ounce for the yolk, and a fraction of an ounce for the shell itself. Inside the shell, a large egg has about 2 parts white to 1 part yolk. Not all eggs are created equal, though, and there can still be some small variance between eggs even within the same size category. On average, though, large eggs will all be roughly the same.
Some recipes may specify eggs by weight instead of volume, calling for something like 4 ounces of egg whites instead of 2 egg whites. In these cases, it's a good indication that precise measurements are important for this recipe, and so it's best to weigh the eggs. At a pinch, however, you can use the general weights for whites and yolks given above.
The Role of egg yolks : fat
Recipes that use just the yolk of an egg typically do so for the yolk's fat content and emulsifying abilities. The fat gives baked goods an extra-rich flavour and a velvety texture. The yolk also has the unique ability to bind liquids and fats together, creating an emulsion that prevents them from separating. This emulsion process helps create a more homogenous mix of ingredients — aiding in an even distribution of liquid and fats throughout a recipe for smooth batters and satiny custards. When yolks are heated, the proteins they contain unfold and gel together. This is a delicate situation; too much heat and the proteins will gel too much and turn curdled and grainy, but when warmed gently over a low heat, egg yolks have a great ability to thicken products like sauces and custards. Again, their emulsifying properties enhance and thicken cream when they are cooked together.
The role of egg whites : foam
When egg whites are used alone, they perform an entirely different role from the yolks, especially when whipped up. Whipping egg whites means incorporating millions of little air bubbles within the white. This creates a fairly stable foam that can be used to make everything from a soufflé to meringue. To help stabilize egg whites even further, we can add acidic elements, like cream of tartar and lemon juice. One of the best ways to use whipped egg whites is as a natural leavening agent in something like a delicate cake or a soufflé. In the heat of the oven, the air trapped in the foam starts to expand, causing the recipe to rise without the need for things like yeast or baking soda. In fact, whipped egg whites are how many classic baked goods, like sponge cakes, got their lift in the days before baking soda and baking powder.
Egg whites can also be whipped with sugar to make meringue, which can then be baked into crunchy meringue kisses, made into billowy pavlovas, or dolloped on top of desserts like baked Alaska. Sugar makes whipped egg whites incredibly stable — you can even pipe it using a piping bag into stars, kisses, and toadstools.
While the idea of "light" and "airy" whipped egg whites might make you think that more is better, it is possible to overdo it. Over-whipped whites will become clumpy, grainy, and difficult to fold into your batter. Also, using too many whites in a batter can end up making the final product dry.
The role of whole eggs : fat and foam
When you use whole eggs in a recipe, you get some of the best properties of both the yolk and the white. While whole eggs aren't quite as good as straight yolks at creating an emulsion, they are still excellent binding agents, especially in cakes, cookies, and other baked goods. Eggs also firm up and solidify when heated, giving crucial structural support to delicate desserts and pastries. At the same time, eggs make baked goods more tender, creating light textures, soft breads, and delicate crumbs. When mixed with sugar (like in a cake or cookie batter), eggs help trap and hold air — not quite as well as whipped egg whites, but enough to give the finished product some lightness and lift. The combination of eggs and sugar also adds a great deal of moisture and flavour to a recipe. Yolks and whites can also be used separately in the same recipe. This truly is the best of both worlds, with the yolks providing richness to a base or batter while the whipped whites do their job of lightening and leavening. Chiffon cakes and soufflés are good examples of this kind of recipe.
Now that you've learned quite a bit about eggs, lets look at the best way to crack them. The baking pros always crack their eggs by rapping them on a flat surface or the inside wall of a mixing bowl. This avoids small bits of jagged shell getting forced inside the egg, which can puncture the yolk and leave tiny, annoying bits of shell to fish from the bowl. If you do get a bit of shell in your bowl, use a larger piece of the shell left in your hand to scoop it out; the sharp edge of the egg shell will break the surface tension and make it easier to scoop out the stray shell. Another trick is to wet the end of your finger with water and you should be able to slide the piece of shell out of the bowl. Make sure your hands are perfectly clean first of course!
It's also best to crack eggs into a smaller bowl, separate from your actual mixing bowl. This makes it easier to scoop out the stray bits of shell, and also helps prevent having to start completely over if you accidentally break a yolk that you needed left whole — throwing away one broken egg is better than tossing a whole mixing bowl of ingredients.
But what about eggless cakes, I hear you ask. Well, yes, it is entirely possible to make delicious cakes without the use of eggs but the science behind these is a bit different, and something to be covered another time.
As you can imagine, I get through a lot of eggs each week making scrummy celebration cakes so if you have a celebration coming up, why not get a free, no obligation quote for a show stopping cake? You can contact me here, email firstname.lastname@example.org or private message the Facebook page. And if you can't eat eggs for dietary or religious reasons, never fear as I also have a great range of egg free cakes!
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Related: The Science of Baking - Flour Power! The Science of Baking - The Sweet Stuff!