First up - what is an egg?
Here, we’re talking about chicken eggs, specifically what’s inside them: a baby-chicken starter kit, complete with everything needed to get from conception to hatching – which takes around 21 days.
If the egg is fertilized, the chick grows from the ‘germinal disc’; a small dot on the surface of the yolk that contains the cell’s nucleus and DNA (sometimes this disc is visible, and looks like a little spot of blood).
The yolk and white provide nutrition for the growing chick. The yolk, a nutritious meal for the embryo and the white, a drink and simple snack. But when they egg isn’t fertilized, instead they provide nutrition for us humans.
From pale yellow to deep orange, the colour of an egg yolk depends almost entirely on what the chicken has been eating. Colour pigments from the chicken’s food get into the egg yolk, and that distinctive yellow comes from plant pigments called xanthophylls, particularly a yellow-orange one called lutein.
But lutein-looking eggs aren’t necessarily an indicator of a plant-heavy diet; this plant pigment is artificially added to commercial chicken feed to obtain the dark yolk colours that consumers go for.
So while many consumers associate dark yolks with better nutrition, and a chicken’s diet does impact their egg quality, yolk colour is no guarantee of a healthy egg.
The yolk is one massive single cell and is the most nutrient-dense part of the egg. It contains about 26 per cent fat, 16 per cent protein, and 4 per cent carbohydrates, with the remaining 53 per cent made up of water and other good bits, like vitamins and minerals.
The egg white
The white also called the albumen – is mostly water (around 90 per cent) and protein (around ten per cent).
In cooking, egg whites and yolks perform pretty different tasks
Yolks are great emulsifiers: they can blend substances that don’t usually mix, like water or vinegar and oil or fat, making them central ingredients in baking and in sauces like mayonnaise and hollandaise.
Egg whites are multitaskers: they form stiff peaks for meringue, give structure to quiches and flans, bind and leaven batter – the list goes on. Below, we’ll look at how cooking and whisking brings about physical changes in egg white proteins that make them so valuable in the kitchen.
Looking closer at egg white proteins
Proteins are made of long chains of amino acids that fold in unique three-dimensional shapes.
Egg white proteins are globular, curled and folded up into roughly spherical shapes held together by weak chemical bonds – the scientific drawings of globular proteins look a bit like tangled balls of Christmas ribbon.
When raw, these protein balls float freely around in the surrounding water, giving the egg white a runny consistency.
Heat ‘em up
Poach an egg, and the white will quickly turn from clear and runny to white and firm. Heat energy agitates the egg-white proteins, making them bounce around and hit water molecules and other proteins.
These collisions break the weak bonds that held the protein globs curled up, allowing the amino acid chains to partly unwind – a process called ‘denaturing’.
When these uncurling proteins bump against one another, new and stronger chemical bonds form between them. As these proteins join together in an interconnected web, the egg white congeals into a solid.
Here’s a really basic analogy: lots of people are standing in a room holding their own hands (the curled proteins), and are able to move around freely. When they let go and grab the hands of those around them, they make an interconnected web and are fixed in place.
It’s this web that gives cooked egg white its firm shape, which is why eggs are so good at binding ingredients together.
Beat ‘em up
When you whisk egg whites, the same thing happens: proteins denature, uncurl and bond together in a web. But in this case, you’re using physical force to unwind the proteins.
The addition of air bubbles into this stretchy egg matrix means egg whites can expand to 8 times their original size. When whisked egg whites are folded gently into meringue, mousse, and cake batters or tipped out of cocktail shakers, the trapped air gives the finished product a light, fluffy texture.
How much heat does an egg need to cook?
While you might not be able to cook an egg on the pavement, it doesn’t take a huge amount of heat to denature egg proteins.
Depending on cooking time, egg whites turn that jelly-like consistency at around 60 degrees C, and firm up at around 65. Egg yolks behave differently, starting to harden at 62 degrees and setting at around 70.
Some advice for delicious eggs: keep the heat low and don’t cook for too long. As more and more proteins denature the egg becomes firmer, and more water is pushed out. Overcooking makes egg whites rubbery and yolks chalky and dry – a far cry from a good egg.