Baking Science I – The Egg

I remember when I was a little girl and just learning to bake with my Mom, how magical it was that you could put a bunch of ingredients into a bowl, mix them, bake it, and have the whole thing turn into a cake.  As I grew a little older, I stopped wanting the magic and I started wanting the science.  I wanted to know why butter melts as it grows warmer, but an egg turns solid as it gets hotter.  I wanted to know why you put flour into a cake when it always makes the batter taste worse and why my randomly dumping ingredients together never produced a tasty result.  Recently, I’ve started researching the science behind baking and it’s been incredibly illuminating.  Baking is not a mysterious magic trick, but a fantastically precise art.  Learning these basics has transformed baking into a wonderful sort of edible alchemy.

For the first section, I wanted to start with the ingredient I was first, and most, curious about: the egg.

I.  The Anatomy

So, everyone knows an egg has two parts: yolk and egg white (and there’s also the shell, of course).  But that’s not really true.  An egg is far more complex than you may think.  There is a yolk, an egg white, and a shell, but there’s also a chalaza, middle albumen, vitelline membrane, nucleus of pander, germinal disk, yellow yolk, white yolk, air cell, and several more.  Prepare to get science-y.

There is a really great (and very in-depth) article on the anatomy of the egg, but here’s the quick summary:

Albumen (aka Egg White) – provides additional nutrients for the embryo.  It’s an excellent source of riboflavin and protein.  Egg whites are composed overwhelmingly of water, with the rest made up of proteins.  They contain little to no fat nor carbohydrates.  Since they contain so much protein, it is possible to whip them into a stiff foam by breaking and releasing the proteins (more on cooking foams later).

Chalaza – this is the thick, twisted, white mass found in the egg white.  It helps keep the yolk centered and is a good indicator of freshness; the more twisted the chalaza, the fresher the egg.

Yolk – feeds the developing embryo.  Some of you may not know this, but yolks are an excellent source of vitamins and minerals (including vitamins A, D, E, and K).  The yolk also contains a fair amount of protein.  However, the yolk is where all of the egg’s fat and cholesterol can be found.  According to the USDA, one standard egg yolk contains an average of 210mg cholesterol and 4.5g of fat.  It also contains an emulsifier, lecithin, which helps thicken and bind non-combining ingredients (like oil and water).

Air Cell – forms when the insides of an egg contract as they cool after being laid.  Air cell size is used to grade eggs; the smaller the air cell, the fresher the egg.

II. Baking:

If you have baked, you have probably baked with eggs.  But, if you’re like me, you didn’t know exactly what those eggs did.  I understood that egg whites make something fluffier and egg yolks make it moister, but I didn’t know how or why.  Now I do, and here’s a brief explanation of how it works:

Eggs are often used as binding agents in cooking (due to the function of lecithin as an emulsifier).  They can be used to make the consistency of a mixture more even and help ingredients like fats and water combine smoothly.  Egg whites are used as leavening agents; when heated, the proteins inside break open and expand, causing the pastry to rise.  However, they also push out moisture, which can leave the pastry dry.  You can also break open and release the proteins by beating or whipping the egg whites, which creates a foam and can be used to add airiness to a pastry.  Egg yolks are used to moisten and combine baked goods; they add richness help create a smooth texture.  Egg yolks can leave your pastry too wet, however, if the correct proportions are not used.

III. Tips:

Peeling Eggs – to more easily peel boiled eggs, run them under cold water and start peeling from the air pocket

Preventing Eggs from Cracking – to prevent eggs from cracking while boiling, prick a small hole in the air pocket. Eggs crack because the air becomes heated, expands, and pushes at the shell, cracking it.  You can also let the eggs come to room temperature and, most importantly, place them in the water before you turn on the heat; do not place eggs straight into boiling water.

Judging Freshness – heavier, fuller eggs are fresher because, as eggs age, water evaporates through their porous shell.  Therefore, in water, an old egg stands on end, a rotten one floats, and a fresh egg sinks. A small air chamber is another sign of a fresh egg; to test, hold an egg up to the light and look for the air cell.  When cracked, the egg white should be thick, solid, and slightly cloudy.

Whipping Eggs – when whipping eggs into a foam, make sure to start with room temperature eggs.  This makes it easier to release the proteins and helps them create a stronger structure.


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