They’re an everyday miracle of chemistry, and delicious into the bargain. As Nathan Kilah explains, there’s a lot of science packed inside the shell. This article was originally published in the Cosmos Print Magazine, December 2022.
There are few foods as versatile as the humble egg. These nutrient-filled packages are excellent scrambled, poached, or fried, and are essential in cakes, puddings, soufflé, quiche, ice creams, sauces, drinks, cosmetics, vaccines and more. But what is it about the humble egg that makes it such an adaptable ingredient and scientifically useful object?
Chicken or egg?
First thing first: the egg definitely came first. The emergence of amniote eggs some 330 million years ago marked a significant evolutionary development as both the water and nutrients were available in the one package. This broadened the possible habitat of egg-laying creatures to varied terrestrial environments. Chickens are relatively recent, evolving around 60,000 years ago and were only domesticated 3000 years ago. The complexity and efficiency of the egg can’t be understated: the developing chick makes excellent use of the contents, especially as there is no way for wastes (other than carbon dioxide) to escape the shell.
Sturdy capsule: it takes a fair bit of force to break an egg. Here, normal force at failure (breaking point) is shown as a function of eggshell radius. Each circle is centred on the mean breaking force and the radius is proportional to the standard deviation for each given type of egg. The force increases with egg size. Source: The Royal Society.
Let’s get cracking
The egg shell is primarily made of the calcium carbonate mineral calcite, bound and shaped by the inclusion of proteins and sugars. The combination of inorganic crystals and organic proteins give incredible microstructural and mechanical strength to protect the embryo from physical damage.
The surface of the egg is covered with a very thin layer of proteins, which keep bacteria outside the shell while allowing gases to permeate through thousands of surface pores. These pores run through vertically aligned crystals that make up the shell. The inner surface of these calcite crystals are covered in bumps that help to connect the shell to the two membrane layers inside. The membrane is lifted off the shell at the air cell (or space), and you might be familiar with the “float test” as a marker for the safety of an egg. The air cell gets larger as the egg dehydrates, so while this test will help you tell a new egg from an older egg, it doesn’t indicate bacterial spoilage. In fact, soaking eggs tends to destroy the thin protective protein layer, making it easier for bacteria to enter the shell – so if you do test an egg, cook it soon after.
Source: The Royal Society.
The white stuff
The white of the egg (the albumen) is a solution of proteins in water. Proteins are exquisitely complicated molecular origami but can rapidly change their properties if this folding is disrupted. This is certainly true for ovalbumin, the major protein in the egg white, as it readily unfolds (known as denaturing) when shaken or whipped, as the proteins trap air bubbles. These foams can be stabilised in meringues, or function as a leavening agent in foods like biscuits, souffles and cakes.
The pH of the albumen is neutral (pH 7.6) at the time of laying, but rapidly rises to alkaline (pH 9.7) in just a few days through the release of carbon dioxide. This change in pH of the albumen is thought to influence the ovomucin gel, resulting in a thinning of the white and weakening the adhesion of the membrane to the shell. These two properties make old eggs harder to poach but much easier to peel when boiled.
Goog inside: the anatomy of an egg.
Outside: The shell’s cuticle is its protective coating, blocking pores to hinder microbes and preserve freshness. An egg’s shell forms about 9–12% of its total weight. Two shell membranes – inner and outer – surround the albumen and provide a barrier against bacteria.
Inside: Fresh laid eggs are warm. As they cool, the contents contract and the inner and outer shell membranes separate to form the air space. The chalazae are ropey strands of egg white that secure the yolk. The more prominent the chalazae, the fresher the egg. The egg white, known as the albumen, accounts for most of an egg’s liquid weight, about 67%. The white is separated from the yolk by the chalaziferous layer and the vitelline membrane. Yolk makes up about 33% of an egg’s weight and contains all its fat and roughly half its protein. On the yolk’s surface is a small spot (2–3mm across); this blastoderm is where the sperm enters the egg. The embryo develops from this disk, and develops in a fertilised egg in the nucleus of pander.
Have your cake and emulsify it too
The proteins of the albumen are connected to the yolk through the chalazae, twisted bungy-cord-like structures. This shock-absorbing scaffold helps keep the yolk centred within the egg white, with a clockwise twist at the air cell end and a counter-clockwise twist at the pointed end.
The yolk starts its development early: 3000 small ova are already present in each fluffy female chick. Once the hen reaches sexual maturity, each ovum undergoes a 10-day development prior to being released into the oviduct, where it is rapidly covered in secretions that form the egg white. Shell membrane fibres are produced and then the egg enters the shell gland, where over about 20 hours the shell is created and the pigments form on the final layers, adding colour. Finally the egg rotates, ready to be laid.
What’s in the box? The basic components of an egg’s edible parts – the white (a) and yolk (b) – are shown above. Water, protein and fat dominate. Note that yolk analysis includes both the yolk and vitelline membrane that contains it. Source: Anses-Ciqual (French agency for Food, Environmental and Occupational Health and Safety).
Modern breeds of chicken that lay multiple eggs in a week have many swelling ova at any one time. The yolk is made up of fat molecules closely surrounded by proteins to keep them suspended in water. “Designer” eggs fortified with vitamins and omega-3 fatty acids have been trialled, but the transfer of these nutraceuticals from feed to yolk is variable and can shorten an egg’s shelf life.
One of the key culinary roles of the yolk is to stabilise emulsions. The yolk’s phospholipids (sometimes referred to as lecithin) are key to this property, as they are composed of long fatty chains that are excellent at interacting with other oily molecules, and positively and negatively charged groups that can interact strongly with water.
What’s best: home laid or commercial? It depends
Although free range chickens generally lay more nutritious eggs than their caged counterparts, a recent study has highlighted the potential risks of raising chickens and eating eggs in urban environments. The study examined the lead content of the urban soil, and the corresponding blood lead levels in chickens foraging on that soil, and in their eggs. The lead levels were up to 40 times higher in home garden eggs than the commercial free range equivalent, and at a level higher than the currently regulated soil lead limits. Home chicken keepers: get your soil tested!
These dual properties mean they can act as a bridge between fat and water to keep them separated and stable. Classic examples of yolk-based emulsions include mayonnaise – an emulsion of oil and vinegar – and hollandaise sauce, where heated whipped yolk is used to stabilise an emulsion of butter.
Cakes also benefit from the properties of these emulsions as the phospholipids hold onto water to maintain a tender and moist crumb.
The glorious golden colour of egg yolks (colour wheel, right) comes from a combination of structurally related antioxidant compounds. These are abundant in common forage herbs, and backyard chooks confined to smaller runs can be given leafy vegetables such as silverbeet and kale to ensure their eggs display the lovely colours of lutein and zeaxanthin. Chef Dan Barber has pushed the colour of his flock’s yolks to red by feeding them a massive amount of capsanthin-rich dried red pepper (no chickens were harmed). The insipid, pale appearance of cage eggs is a sign of their grain-based diet.
Composite material
Egg shells are an example of a composite material: where two or more materials are combined to give a material with enhanced properties. You can test this by dissolving the calcium carbonate with vinegar to make a bouncy egg, or by roasting a blown egg shell to remove the proteins and make a brittle shell. Many modern materials make use of the properties of composites by taking a fibrous material and surrounding it with a solid matrix, for example fibreglass, in which glass threads strengthen plastic resins. Modern aviation requires materials that are light, strong and stiff, and composite materials of carbon fibres and irreversibly hardened thermosetting polymers are used extensively. The downside is that composite materials tend to be more expensive, and they are much more difficult to recycle at the end of their life.
The final key transformation that eggs can undergo is gelling, where heating causes the proteins to start to coagulate. The coagulating properties are highly useful for fried or boiled eggs, or when using egg to bind hamburgers, or as an egg wash when crumbing a schnitzel.
An astonishing number of cuisines make use of eggs using an equally astonishing diversity of cooking techniques. So whenever you next poach, fry, or boil an egg, take some time to enjoy its wonderful chemistry.
Pigment of imagination
Egg pigmentation varies widely across the avian world. Chicken eggs in commercial production are normally white or brown. White eggs highlight the underlying colourless calcium carbonate of the shell, while the brown colour comes from the pigment protoporphyrin IX laid down on the surface of the egg. The brown pigment is the same molecule that’s in the haem group of haemoglobin in blood, but without the iron that gives the bright red colour. It is only added at the final step of the egg-laying process, so the inside of a brown egg shell remains white. A genetic mutation in certain breeds of chicken from South America has given rise to blue coloured eggs, where a horseshoe shaped molecule called biliverdin gives a lovely pale blue colour. Unlike the surface addition of protoporphyrin IX, the blue pigment is added throughout the shell process. Chicken breeders have mixed and matched these coloured egg breeds to make all sorts of colours from blue to green to brown, including trend colours like avocado and kalamata.