Why Does a Raw Egg Turn Solid When You Boil It?
– How Proteins Unfold and Tangle
Crack open a raw egg and out flows a clear, runny liquid. Heat it in hot water for a few minutes, though, and it becomes a white, solid egg that can never turn raw again. You haven't mixed anything in. You haven't added any chemicals. So why does heat alone cause such a dramatic change?
When you crack a raw egg into a frying pan, the almost-clear egg white spreads out in a slippery pool. Put it over the heat and within tens of seconds the clear parts turn white and opaque, and firm up.
Cooling it doesn't undo the change. Even in ice water, the set white will never go back to runny raw egg.
Why does heat alone cause a change that can't be reversed?
Raw egg white and yolk hold proteins folded into set three-dimensional shapes, dissolved in water.
When heated, the proteins unfold (denaturation), and the unfolded strands tangle into a network (coagulation).
Let's follow this change, unfolding and tangling, step by step.
An egg is a water solution of folded proteins
Egg white is nearly 90% water, and most of the rest is protein. The yolk also contains plenty of protein, along with fats.
A protein is a molecule made of small building blocks called amino acids, linked in a long chain. The chain doesn't run straight. It is folded up in a complicated but fixed three-dimensional shape. A raw egg stays clear and runny because each protein keeps this folded shape and floats separately in the water.
Heat makes proteins unfold
A protein's folded shape is held together only barely, by fairly weak forces (such as hydrogen bonds). Heat makes the molecules vibrate harder, and these weak forces are overwhelmed, so the folds come undone. This is called denaturation.
The unfolded proteins tangle and set
Once the folds come undone, parts that were hidden inside are exposed on the outside. The exposed parts readily bind to one another, and the unfolded proteins tangle together one after another. This is called coagulation.
The tangled proteins form a fine, net-like structure. The water that was already there gets trapped in this net, which gives the springy, almost solid texture. That is the secret behind the distinctive feel of a boiled egg white.
The proteins simply unfolded and tangled.
Once set, it never goes back
Tangled proteins are a state in which a huge number of molecules have bonded to one another again in a complicated way. Even when cooled, they don't return by themselves to the original state of each one neatly folded. It may feel a bit like trying to flatten a sheet of paper you've crumpled into a ball. Denaturation and coagulation are, in general, changes that can't be undone.
The "onsen egg" makes use of this temperature difference
Egg white and yolk contain different kinds of protein, so they start to denature and set at different temperatures. The yolk's proteins begin to set at a fairly low temperature, whereas the main proteins of egg white don't set firmly until the temperature is higher than the yolk needs.
The onsen egg (a Japanese hot-spring egg) exploits this difference. A set yolk with a still-soft white, the opposite of an ordinary soft-boiled egg, is possible because it cleverly uses the gap between the temperatures at which these two kinds of protein start to set. In the fold-out section below, we check with numbers how big that gap actually is.
Eggs are said to sometimes contain bacteria that can make you ill if the egg is not cooked enough. Cooking eggs thoroughly (especially when they are eaten by small children, older people or pregnant women) is regarded as the basic precaution.
What you can check for yourself
- Put only the egg white, separated from the yolk, in a clear glass dish
- Heat it a little at a time, in a safe way, such as a microwave or a water bath (in a microwave, beware of bursting, and heat in short bursts while checking)
- Watch the clear egg white turn white and opaque
- Check that the part that has changed colour never goes back to liquid
Clear turns white because the unfolded, tangled proteins now scatter light. You can see with your own eyes that denaturation is under way.
Summary
A raw egg sets when heated because the proteins inside it unfold from their fixed three-dimensional shapes (denaturation), and the unfolded strands tangle into a network (coagulation). This change is in general irreversible. Because egg white and yolk proteins start to set at different temperatures, you can even make an onsen egg, with a texture unlike an ordinary boiled egg.
Setting an egg takes no magic.
Heat, plain energy, is what reshapes the proteins.
By the way, why does a raw yolk stay near the middle of the shell before it is heated? This article explains.
For those who want to know more: terms, numbers and links to textbooksFrom middle-school science to topics under active research, with the level of each part marked
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Chemistry"
- High school+High-school "Biology", or advanced or column material in textbooks
- UniversityUniversity specialist content (food chemistry) not taught in high school
- ResearchTopics researchers are still investigating, not yet settled even at university level
Middle schoolTerms: the words behind how an egg sets
- Protein: a main molecule of living bodies, made of linked amino acids.
- Denaturation: the unfolding of a protein's folded shape, caused by heat and other things.
- Coagulation: unfolded proteins tangling together and setting solid.
High schoolChecking with a formula: how much heat does it take to warm egg white?
Egg white is mostly water. Let's calculate how much heat it takes to warm egg white from fridge temperature to the temperature at which its proteins start to set.
Heat needed = mass × specific heat × temperature change
| Mass | As an example, take the white of one egg: 30 g (0.03 kg) |
| Specific heat | We use the specific heat of water, about 4.18 J/(g·°C), as a rough guide for egg white |
| Temperature change | From fridge temperature (about 5°C) to 80°C as a typical guide: 75°C |
Strictly, egg white's specific heat differs from water's because of the proteins and other content, but since it is nearly 90% water, we use the value for water as a guide.
| Mass × specific heat | 30 × 4.18 = 125.4 |
| Then multiply by the temperature change | 125.4 × 75 = 9405 |
| Heat needed | About 9405 J (about 9.4 kJ) |
So just warming the white of one egg to the temperature where it starts to set takes about 9.4 kilojoules of heat. In reality, the process of unfolding the proteins is said to use still more heat energy.
Egg white and yolk start to set at different temperatures. As a typical guide, the yolk starts to set at around 68°C, while the main proteins of egg white don't set firmly until around 80°C.
| Difference between the temperatures at which white and yolk start to set | 80 − 68 = 12 |
| Temperature difference | About 12°C |
Heating for a long time in this gap of about 12°C is thought to produce the onsen egg texture: a yolk that has set while the white stays soft.
* The specific heat and setting temperatures are typical guide values for understanding the mechanism. Actual values vary with the type of protein, the heating time and differences between eggs.
High school+Heat is not the only thing that causes denaturation
Protein denaturation is known to be caused not only by heat but also by acids, alcohol and strong salt. For example, soaking an egg in vinegar can make the proteins denature and start to set without any heating. In each case, whether by heat or something else, the basic idea is the same: the weak forces that hold the folds in place are overcome in some way.
UniversityDifferent proteins denature at different temperatures
Egg white contains several kinds of protein, including ovalbumin, and each is said to begin denaturing and setting at a different temperature. Some denature at fairly low temperatures, others need higher ones. This spread in when each kind starts to set is thought to make the change in an egg's texture as it heats a continuous, smooth one rather than a simple single step.
ResearchWhat is still unclear
- How precisely egg texture can be controlled through fine combinations of heating temperature and time is a topic still being researched in cooking science (low-temperature cooking, molecular gastronomy and so on). Professional chefs and researchers are pursuing their own combinations of temperature and time.
- Egg substitutes made from plant-based ingredients, which try to reproduce denaturation and coagulation behaviour like that of egg proteins, are also being developed. How closely they can match real egg in the way they set and in texture when heated is regarded as a research challenge in food science.
- The detailed molecular-level process by which proteins denature and coagulate (which parts unfold first, and in what order) is also said to be under continuing study in structural biology.
Even the boiled egg you see almost every day holds deep questions from protein science.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Middle school | Science: living things and matter | Basic terms: protein, denaturation, coagulation |
| High school | Basic Chemistry: heat calculations | Finding the heat needed from specific heat; the difference in setting temperatures |
| High school+ | Biology: protein structure | Causes of denaturation other than heat (acids, alcohol and so on) |
| University | Food chemistry | Different denaturation temperatures for different egg-white proteins |
| Research | Cooking science and food science (under research) | Precise control in low-temperature cooking, developing egg substitutes, structural analysis of denaturation |
- Explanations of egg-white protein denaturation and coagulation in food chemistry textbooks.
- Explanations of specific heat and heat calculations in basic chemistry textbooks.
- Explanations of yolk and white setting temperatures and the principle of the onsen egg in cooking-science materials.
- General-audience explanatory material from the Ministry of Health, Labour and Welfare (厚生労働省) on hygienic handling of eggs (「卵の衛生的な取り扱い」).
- Research reviews on developing plant-based egg protein substitutes in food science.
* Values such as specific heat and setting temperatures are typical guides, and actual values vary with conditions.
* This article is a general-audience science explanation. For safe handling and cooking guidelines for eggs, please check information from public bodies such as Japan's Ministry of Health, Labour and Welfare.