Why Does an Egg Yolk Stay in the Middle, Even When You Shake It?
― Near-Identical Density and Two "Cords" Hold It in Balance
You put a carton of eggs in your bicycle basket and ride home over a bumpy road. Every stop at a red light brings more rattling, and by the time you get home you are a little worried: "Has the yolk slid to one side?" Yet when you crack the egg, the yolk is usually still round and sitting near the middle. There is a good reason for that.
You are walking home a bit briskly, with a carton of eggs in a shopping bag. The bag swings at every red light, and sometimes it bounces over a kerb.
If it were a plastic bag of water, a good shake would push the contents to one side. So what about the yolk inside an egg? When you actually crack one open, the yolk is usually still round and close to the middle. It does not move as easily as a liquid would.
You cannot see inside until the shell is broken, yet the yolk always seems to settle in much the same place. It is as if something inside the shell were keeping it in position.
Only Two Reasons Keep It in the Middle
Yolk and white are, in fact, almost equally "dense". The force trying to lift the yolk and the force trying to sink it nearly cancel out, so the force pushing the yolk around is tiny to begin with.
Inside the white are two twisted white cords linking the yolk to the two ends of the egg. These are the chalazae. They keep the yolk from wobbling by holding it close to the middle.
Together, these two things mean the yolk is not moved by a mild knock. Let's take them one at a time.
Reason 1: Yolk and White Are Almost Equally Dense
"Denseness" here is what science calls density: how heavy something is for a given volume. Make an oil-and-vinegar salad dressing, for example, and after a while it separates neatly into layers. Oil is much lighter than water (the main ingredient of vinegar), meaning its density is much lower.
So what about the yolk and white inside an egg? The density of the yolk is said to be nearly the same as that of the white. Values vary a little between sources, but figures of about 1.030 [g/mL] for the yolk and about 1.036 [g/mL] for the thick white around it are commonly used as a guide. The difference is only about 0.006.
With densities this close, the upward push and the downward pull of weight almost cancel. Unlike the oil in dressing, the yolk does not clearly rise or sink.
Crack the shell and you will notice a sticky white right around the yolk and a runnier white outside it. The sticky one is called thick albumen and the runny one thin albumen. The thick albumen acts like a wall close to the yolk, while the thin albumen spreads along the inside of the shell.
Reason 2: Two "Cords" Called Chalazae
Density alone is not enough, though. Even with similar densities the force is not zero, and shaking creates small forces. The chalazae take up those forces.
A chalaza is a white, thread-like spiral that links the yolk to each end of the egg. It is made of twisted proteins from the white and, like fishing line, pulls the yolk taut from both sides.
The twist matters too. A single straight cord would let the yolk spin sideways. With two twisted cords fixed at both ends, the yolk finds it hard either to slide sideways or to rotate.
A Trick for a Neatly Centred Boiled Yolk
When making ramen eggs or marinated eggs with a nice cross-section, people sometimes say, "For the first few minutes of boiling, keep rolling the egg gently with chopsticks." Why roll it rather than simply boil it?
This is also thought to be linked to the yolk's tendency neither to float nor to sink. Raw, the yolk is already roughly in the middle, held by the chalazae. During the first few minutes of boiling the white has not yet set, and slowly rotating the egg is said to help the yolk stay more evenly at the centre. Once the white has fully set, the yolk is fixed wherever it happens to be. That is why rolling only during the first few minutes is said to matter (we explain how the white sets in more detail in this article).
What You Can Check in Your Kitchen
- Fill a glass or bowl with water and gently lower in a raw egg
- If it sinks and lies on its side, it is a fresh egg; if the round end lifts and it tilts or stands up, it has been around a little longer
At the round end of an egg there is a small air cell between the inner membranes. The shell has countless tiny pores you cannot see, and water is said to evaporate through them bit by bit. The older the egg, the bigger the air cell, the more buoyancy it gives, and the higher the round end rises. This is a different mechanism from the yolk's position, but the same idea of floating and sinking explains it. Incidentally, in older eggs the chalazae are said to slacken and the white to thin, so the yolk drifts off-centre more easily. "The fresher the egg, the rounder and more central the yolk" is not an illusion.
Summary
The yolk resists drifting from the middle because two things work at once: (1) yolk and white have almost the same density, so the force trying to move the yolk is small in itself; (2) two twisted cords, the chalazae, hold against that small force.
The yolk is not held by a strong force.
It is simply moored, gently, in a place where there is hardly any force to move it.
For those who want more ― terms, formulas and links to textbooksFrom middle-school science to active research, each part is labelled with its level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Physics" or "Basic Chemistry"
- High school+High-school "Biology", or textbook advanced material and sidebars
- UniversityUniversity-level specialist subjects (food science, animal science) not taught in high school
- ResearchTopics researchers are still investigating, not taught even at university as settled fact
Middle schoolTerms: words used inside an egg
- Density: weight per unit volume. Here it is the yardstick for comparing how "dense" yolk and white are.
- Chalaza: the two white, twisted, cord-like structures linking the yolk to the ends of the egg.
- Thick albumen / thin albumen: the sticky white around the yolk and the runny white outside it.
- Air cell: the small gap inside the shell at the round end of the egg. It grows over time.
- Germinal disc (blastodisc): a small round mark on the yolk's surface, the entry point of what would become a chick. It is present whether or not the egg is fertilised.
Middle schoolHigh schoolChecking with a formula: how small is the force the chalazae must hold?
Hearing that the densities are "almost the same" does not tell you how small the force really is. The calculation gives a clear number.
Net force = (white density − yolk density) × yolk volume × gravitational acceleration
| Net force | The leftover force the chalazae must support [N] |
| White density, yolk density | Both about 1.03 [g/mL], close to water |
| Yolk volume | About 17 [mL] for a size L egg |
| Gravitational acceleration | 9.8 [m/s²] |
The key point is that only the density difference matters. If yolk and white had exactly the same density, the net force would be zero. In reality a very slight difference remains, so a very slight force remains.
| Density difference, yolk and white | 1.036 − 1.030 ＝ 0.006 [g/mL] |
| Convert to SI units (kg/m³) | 0.006 × 1000 ＝ 6 [kg/m³] |
| Net force (buoyancy minus weight) | 6 × 9.8 × 0.000017 ≒ 0.001 [N] |
| Convert to gram-force (gf) | (0.001 ÷ 9.8) × 1000 ≒ 0.1 [gf] |
The result is a very small value, 0.1 gram-force (gf). Next to the weight of the yolk itself (about 17 gf), it is a tiny remainder.
0.1 gf is about one-tenth the weight of a 1-yen coin (1 g). That is all the chalazae have to hold.
Put another way, the buoyancy of the white already carries more than 99% of the yolk's weight. The chalazae only hold the small remainder. That is why such thin cords can keep the yolk near the middle.
High school+UniversityThe yolk corrects its own orientation
So far we have looked at why the yolk does not slide sideways. The yolk has another interesting property. On its surface is a small round mark called the germinal disc, and whichever way you lay the egg, the disc is said to end up almost always facing up.
The likely reason lies in the yolk's internal structure. At the centre of the yolk is a slightly less dense region that forms a narrow channel leading to the germinal disc. This is called the latebra. Being lighter than its surroundings, the latebra is thought to act like a float, slowly turning the yolk so that its lighter side (the disc side) points up.
This self-righting is much like a lopsided object slowly settling into one orientation in water. The chalazae are thought to support the yolk from the sides too, so that it does not turn too far and this orientation is kept.
ResearchWhat is still not well understood
- The exact time the yolk takes to turn its disc upward, and the size of the forces that set its rotation speed, have not been measured precisely. That is because it is hard to watch the inside of a yolk without breaking it.
- It is well known from experience that chalazae weaken in older eggs, but there is limited data showing which protein changes cause this, or how it depends on time and storage temperature. Freshness measures such as the Haugh unit (calculated from the height of the white) are widely used, but there is still no easy way to measure the strength of the chalaza itself.
- How much transport vibration damages the cords and membranes that hold the yolk is a topic still being studied in food engineering and animal science. How much vibration eggs can withstand during long-distance transport and automatic sorting is also a concern at production sites.
Even the yolk you see in the kitchen every day is still an object of research in the finer details of how it moves. Being familiar is not the same as being understood.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: density / buoyancy | Density of yolk and white; the forces that lift and sink |
| High school | Basic Physics: balance of forces / Basic Chemistry: density and mixtures | The net-force calculation; thinking in density differences |
| High school+ | Biology: development and organ formation (sometimes a sidebar in textbooks) | Germinal disc; the yolk's self-rotation |
| University | Food science, animal science (egg quality evaluation) | Latebra; chalaza strength; freshness by Haugh unit |
| Research | Egg quality evaluation, food engineering (unresolved) | Quantifying chalaza deterioration; resistance to transport vibration |
| ― | Household wisdom | Centring a boiled yolk; the sink test for freshness |
- Explanatory materials on egg structure (shell, shell membrane, air cell, albumen, chalaza, yolk) from bodies such as the Japan Egg Association (日本卵業協会).
- Stadelman, W.J. & Cotterill, O.J. (eds.), Egg Science and Technology (on the density of albumen and yolk, and chalaza structure).
- Haugh, R.R. (1937), A New Method for Determining the Quality of an Egg, U.S. Egg and Poultry Magazine (the original source for judging freshness by albumen height).
- Romanoff, A.L. & Romanoff, A.J., The Avian Egg (on yolk structure and the positional relationship of latebra and germinal disc).
- Various public databases of food composition and nutrition, for guide values of egg density and composition.
* Values such as density and volume vary with egg size, individual eggs and sources. This article gives commonly used guide values.
* This article is a general-audience science explainer. Figures such as egg density and volume vary between sources and are given as guide values for understanding the mechanism.