The Chemistry of Eggs & Eggshells

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As the chemistry of chocolate is a topic that’s been pretty much exhausted on the site (see here, here, here, here, and here), for the Easter weekend we’re instead homing in on the ‘egg’ side of Easter Eggs. For such a simple staple of the kitchen, the chemistry of eggs is surprisingly complex. Here we take a brief look at their composition, and also at some chemistry tips that can help with cooking them!

The Egg’s Shell

It makes sense to start from the outside of the egg and work our way in, so let’s begin with the egg’s shell. It’s made primarily from calcium carbonate, the chemical compound which also makes up the majority of sea shells, as well as chalk and limestone. Nanoparticles of calcium carbonate are arranged into ordered crystals by proteins, eventually forming the calcite mineral that makes up the shell. The shell isn’t actually completely solid – it has thousands of tiny pores, around 9,000 on average, which allow gases to pass in and out. As we’ll see later, this can have implications for cooking.

The colour of egg shells can also vary; chicken eggs tend to be somewhere on a colour spectrum between white and brown, but the eggs of other avian species can also encompass blue or green hues. This colouration is due to the deposition of pigment molecules on the eggshell whilst it is being formed in the chicken’s oviduct. Some of these pigments, such as protoporphyrin, the pigment that gives shells a brown colour, are breakdown products of haemoglobin, the oxygen-carrying compound found in blood. Others, such as oocyanin which gives blue and green colours, are side-products from the formation of bile. White egg shells have an absence of pigment molecules.

The Egg White

Once inside the egg, we first come to the egg white, or albumen. The egg white is formed of a number of different layers, and is in fact mostly composed of water (90%). A range of proteins make up the majority of the remaining 10%, serving a number of varying purposes. Some, such as ovalbumin, are thought to provide nourishment for the developing chick, whilst also blocking the action of digestive enzymes. Another, conalbumin or ovatransferrin, binds iron atoms tightly, both to prevent their use by bacteria and hence help prevent infection, but also to ensure a supply of iron for the developing chick. Finally, one of the most important proteins in the albumen in terms of the egg white’s consistency is ovomucin. This protein helps to thicken the egg white and give it its gloopy consistency.

The Egg Yolk

The egg’s yolk is made up of a number of spherical compartments. Unlike the egg white, which contains very little fat, the yolk contains a significant amount of fatty acids such as oleic acid, palmitic acid, and linoleic acid, as well as high level of cholesterol. It also contains fat soluble vitamins (A, D, E, and K).

The colour of the yolk is a consequence of the presence of two chemical compounds: lutein and zeaxanthin. These are both compounds known as xanthophylls, and can also be classed as carotenoid compounds; they are hence members of the same chemical family to which beta-carotene, the chemical that gives carrots their orange colour, belongs. A chicken’s feed can influence the colour of the yolk, and for this reason beta-carotene-containing substances, or even marigold petals, can be added to chicken feed to enhance the colour. Interestingly, including the main colour-creating compounds in red peppers, capsanthin and capsorubin, in chicken feed can cause the yolks to appear a deep orange or even red.

Cooking Eggs

So, now we know a little about what eggs are made of, what’s actually going on when we cook one? This is largely a consequence of what happens to the proteins in the egg when we heat them. To begin with, in a raw egg the long protein molecules are individually coiled up. However, as we heat them, these chains begin to unfold, a process known as ‘denaturing’. Once unravelled, the proteins form a three-dimensional network, trapping water into tiny pockets in the network, and causing solidification. The egg white changes from being transparent to opaque due to the clustered protein network deflecting light instead of allowing it to pass through.

In terms of the compounds that give eggs their flavour, remarkably few studies have been carried out to gauge which compounds make important contributions. However, we do know that hydrogen sulfide is the most important contributor to the characteristic eggy smell. This compound is created as sulfur-containing proteins in the egg white react during cooking. The longer an egg is cooked, the greater the amount of hydrogen sulfide that is produced, and older eggs also produce it in higher amounts when cooked. Spoiled eggs smell unpleasant because of even higher levels of hydrogen sulfide.

Hydrogen sulfide can also play a part in the appearance of a green layer around the egg yolk that can sometimes occur. This is due to the reaction of the gas with iron in the egg yolk to create iron sulfide, which forms the green layer. Often prolonged heating of the egg during cooking can cause this; though it is harmless, it can be prevented by cooling the eggs quickly after cooking by immersing them in cold water.

When hard-boiling eggs, how old the egg is can influence how difficult the egg shell is to peel off. This is due to the change in alkalinity of the egg white over time. As the egg shell contains thousands of pores that allow carbon dioxide gas to diffuse out of the egg, the egg white’s pH rises from around 7.6 to around 9.2 after around a week of storage. The cooked albumen adheres more strongly to the inside of the shell at the lower pH, meaning fresh eggs make for a more frustrating egg-peeling experience. By contrast, it’s much easier with an older egg.

To combat this, if you’re making boiled eggs using fresh eggs, it’s suggested that baking soda can be added to the water in which the eggs are being cooked to make it more alkaline – though this can also make the eggs taste a little more sulfury, and whether it actually makes any real difference is disputed. Adding salt to the water can help if the egg springs a crack during cooking, as egg white solidifies more quickly in salted water, though again there’s not really any evidence that it makes the eggs easier to peel.

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References & Further Reading