Why Eggs Turn Solid When You Cook Them

Egg white is about 90 percent water, and nearly all the rest is protein: long chains of amino acids folded into compact balls and dissolved in the water. In the raw egg the balls float past each other freely, which is why the white pours. The folds are held in place by weak bonds that need only a little energy to break.

Heat supplies that energy. As the temperature rises the folded proteins shake themselves loose and unravel into long strands, a change called denaturation. The strands then bump into each other and bond, forming a tangled three-dimensional mesh that traps the water inside it. Liquid becomes gel, and with more heat the mesh tightens into a firm solid. The change is a network forming, not water disappearing, which is why a perfectly cooked egg is still moist.

An egg contains several proteins and each unfolds at its own temperature. In the white, ovotransferrin starts to set at about 62 degrees Celsius, while ovalbumin, the most abundant white protein, needs about 84 degrees. That is why a white goes milky first and firm only later. The yolk's proteins set between about 65 and 70 degrees.

Those numbers explain the cook's whole repertoire. A soft-boiled egg is one pulled from the water when the white has passed 62 degrees but the yolk has not reached 65. A Japanese onsen egg, held at 63 to 65 degrees for an hour, has a yolk that is barely set and a white that is soft and custardy, a combination impossible to get by boiling. A poached egg works because the water is kept just below the boil, so the outside sets before the inside overcooks.

Keep heating and the mesh keeps tightening. It squeezes out the water it was holding, which is why an overcooked scrambled egg is both rubbery and sitting in a puddle. A hard-boiled egg cooked too long develops a green-grey ring around the yolk, which is iron sulphide: sulphur released from the white's proteins meeting iron in the yolk. It is harmless, and it does not happen if the egg is cooked for the right time and cooled quickly.

The trick behind good scrambled eggs is the same physics in reverse. Low heat, constant stirring and removal from the pan while they still look slightly under-done let the mesh form gently and hold on to its water. The eggs finish cooking on the plate.

Heat is not the only thing that unfolds a protein. Acid does it too, which is why lime juice turns raw fish opaque in ceviche and why a splash of vinegar in poaching water helps the white set faster. Mechanical force does it as well: whisking egg whites unfolds the proteins and traps air between them, and the unfolded strands then bond around the bubbles to make a foam stable enough to hold peaks. Meringue is that foam, dried by gentle heat.

Custards use the same setting proteins diluted with milk or cream, and the dilution shifts the setting temperature upward and the texture towards silk. A creme caramel sets at around 80 degrees; a few degrees more and the mesh tightens too far and the custard curdles, which is why custards are baked in a water bath that cannot exceed 100 degrees. The temperatures worth remembering:

• •62 degrees: egg white begins to set
• •65 to 70 degrees: yolk sets
• •80 degrees: custard sets, and curdles not far above
• •84 degrees: egg white fully firm
• •Prolonged boiling: water squeezed out, grey ring forms

Anyone who has boiled eggs straight from the farm knows they can be maddening to peel. The reason is chemical. A freshly laid egg white is only mildly alkaline, but carbon dioxide escapes through the thousands of tiny pores in the shell over the following days, and the white becomes noticeably more alkaline. At the lower pH of a very fresh egg, the white sticks firmly to the membrane lining the shell, so pieces of white tear away with it. Eggs a week or more old usually peel far more cleanly.

The same pores explain the familiar float test. As an egg ages, moisture and gas escape and air seeps in to enlarge an air cell at the broad end. A fresh egg sinks and lies flat in a bowl of water, an older one tilts or stands on its end, and a very old one floats. The test tells you about age, not directly about safety, which depends on storage and on whether bacteria such as Salmonella are present. Lowering eggs into already boiling water also helps, because the outer white sets before it can bond to the membrane.

Water boils at 100 degrees Celsius only at sea-level air pressure. Higher up, the pressure is lower, so water molecules escape into vapour more easily and the water boils at a lower temperature. At around 3,000 metres it boils at roughly 90 degrees, and in high cities such as La Paz in Bolivia it boils lower still. A pan can bubble as fiercely as you like, but the water will not get hotter than its boiling point. That is why a recipe timed for sea level leaves eggs underdone in the mountains, and why cooks there add several minutes.

The setting temperatures earlier in this article show why the effect matters. Near the summit of Mount Everest water boils at around 70 degrees. That is hot enough to set the yolk and the first proteins of the white, but well short of the 84 degrees needed to firm the white fully, so the white sets into a soft, tender gel rather than the firm white of a sea-level hard-boiled egg. A pressure cooker does the reverse. By trapping steam it raises the pressure and lets water reach about 120 degrees, which speeds up cooking but makes eggs easy to overcook.

To tell a boiled egg from a raw one without cracking it, spin each on a table. The boiled egg turns smoothly and quickly, because it rotates as one solid object. The raw egg spins slowly and wobbles, since the liquid inside lags behind the shell and drags on it. Now stop each egg briefly with a fingertip and let go at once. The boiled egg stays still, but the raw egg starts turning again, because the liquid inside kept swirling while the shell was held. A hard-boiled egg spun fast on its side can even rise up onto its end, a puzzle that physicists analysed in the early 2000s.

Denaturation is usually described as irreversible, and in the kitchen it effectively is. In 2015, though, a team of chemists in the United States and Australia showed that one protein from boiled egg white could be refolded. They first dissolved the cooked white with a chemical called urea, which loosens the tangled mesh, then used a rapidly spinning device that stretched the protein chains in thin films of liquid, helping them return to their natural folded shapes. The aim was not to restore breakfast but to find faster, cheaper ways of recovering useful proteins in industry and research, where misfolded proteins are a costly problem.

Eggs solidify because heat unfolds their proteins and the strands bond into a mesh that traps the water. White and yolk set at different temperatures, which is what makes soft-boiled, poached and custard possible, and pushing past those temperatures tightens the mesh until the egg turns rubbery.