1. Mayonnaise Is Mostly Tiny Oil Drops Held Together by Egg Yolk
You cannot make mayonnaise by simply mixing oil and vinegar together, because oil and water do not mix. Instead, egg yolk is used. The components in egg yolk help the oil and vinegar blend together smoothly. This action is called "emulsification" [1].
Inside emulsified mayonnaise, the oil becomes tiny droplets about 2 to 10 micrometers (μm) in size. These droplets are scattered inside the water-based liquid like vinegar. Since 1 μm is one-thousandth of a millimeter, these drops are very small, between 0.002 and 0.01 mm [2]. In traditional mayonnaise, oil makes up about 65 to 80% of the mix, while egg yolk is about 5 to 8% [2]. A review paper in English also states that the ratio of oil droplets is around 70 to 80% [3].
This means mayonnaise is not just "mixed oil and vinegar." It is closer to "tiny oil drops packed tightly into the small gaps between a little bit of water." If there are so many particles, why does it hold its shape?
2. How Packing Spheres Together Explains Why Mayonnaise Is Thick
If you pack spheres of the same size randomly, they fill about 63.7% of the space (a ratio of 0.637). Even if packed in the neatest possible way, the maximum is about 74.0% (a ratio of 0.74048) [3]. These are theoretical numbers for same-sized spheres, and gaps always remain between them.
It is thought that mayonnaise has so many oil drops that they push against each other. However, the sphere packing numbers refer to volume ratios, while Japanese standards often use weight ratios (like 65% or more). Because the measuring units are different, you cannot compare these numbers directly. Real mayonnaise has drops of varying sizes, and the drops can squash and change shape. This means real mayonnaise can sometimes exceed the theoretical limits. Think of this as a rough guide to understanding "packed tight" [3].
Experiments changing the oil ratio show interesting results. In studies of mayonnaise-like emulsions with different oils, the thickness (viscosity) changed greatly when the oil volume reached about 0.55. Above 0.6, it suddenly became much thicker [4]. When there are few particles, the mixture flows easily. But once a certain ratio is passed, it becomes hard to move all at once.
In mayonnaise, particles push against each other to keep the shape. So, does yogurt also rely on crowded particles?
3. How Milk Turns into Yogurt When Bacteria Make Lactic Acid
Milk is runny, but it sets firm when lactic acid bacteria work on it. The bacteria produce lactic acid in the milk, which causes the milk protein (casein) to clump together. These clumps are called "curds" [7]. It is explained that this curdling by acid creates the smooth texture of yogurt [8].
Why does it set with acid? In milk, casein particles have a negative electric charge on their surface. This makes them repel each other and stay separate. When acid increases and the pH gets close to casein's isoelectric point (about pH 4.6), this charge is canceled out, and the proteins stick together [9]. Normal milk has a pH of about 6.6, so when lactic acid lowers it enough, the milk sets. Note that 4.6 is a rough guide; the final value in actual yogurt is not known from these materials.
The connected casein forms a three-dimensional net that traps water inside [9]. Mayonnaise uses "crowded particles," while yogurt uses a "net." We can see these are opposite ways of holding shape.
4. Why Clear Water (Whey) Appears on Top of Yogurt
When you take yogurt out of its container, clear liquid may pool on the surface or in dents. This is "whey," which is the water part of milk. If you shake it or tip it over and the shape breaks, whey comes out. As fermentation continues, the curds can shrink slightly, squeezing out this clear whey [7]. This separation of water is called "syneresis."
It is thought that whey appears because the net shrinks and pushes out the water inside. Whey is simply the water separated from the yogurt [7].
In low-fat yogurt, this separation of water is known as a quality issue. It is known that in yogurt with reduced milk fat, whey can separate, or the texture can become grainy and lose its smoothness [10]. Even for the same "yogurt," the way the net forms changes depending on how it is made, affecting the feel in your mouth. The amount and type of polysaccharides (EPS) made by lactic acid bacteria are also said to affect how the net forms [10].
5. A 2025 Study Linking Mayonnaise-Like Materials to Glass
Materials made of soft particles packed randomly and tightly, like mayonnaise, are called "soft jammed solids." They have properties in between liquid and solid (viscoelasticity) [6].
On January 10, 2025, the University of Tokyo and Kyushu University announced joint research. Their results were published in the science journal Nature Physics. They measured the thickness and elasticity of a high-density emulsion with oil drops about 0.5 μm in diameter when deformed slowly. The measurements matched theoretical predictions quantitatively [6]. Note that this 0.5 μm size was for the experiment, which is different from the oil drops in store-bought mayonnaise (2 to 10 μm).
The research focused on "anomalous viscous loss," where thickness suddenly increases when deformation is very slow. It was shown that this relates to low-frequency vibrations usually seen in glass (boson peak vibrations). This is said to be a step toward understanding solids with disordered structures [6]. Mayonnaise is not "glass," but the way its structure is disordered is similar.
Foods in the kitchen connect to basic problems about materials. The mysteries of everyday things are becoming entry points for research.
6. Comparing How Mayonnaise and Yogurt Hold Their Shape in the Fridge
This is an observation you can do today in your fridge. Always do this with an adult and a clean spoon.
Yogurt Whey: Scoop plain yogurt with a spoon and look at the dent left behind. After a while, clear water may pool there. The same yogurt might release more water if it has been shaken. Imagine how the net shrinks [7].
Mayonnaise Flow: Take a little with a spoon and tilt it slowly. It should not flow immediately but move in a sticky way. You only see that it resists flowing; you cannot see the movement of the oil drops. It is thought that crowded particles pushing against each other are involved [4].
After observing, serve with a clean spoon, put it back in the fridge, and eat it soon. Do not put it in the freezer.
If you want to learn more, the announcement from the University of Tokyo and Kyushu University (reported by source [6]) explains why they study mayonnaise-like materials. For details on the yogurt net, explanations from manufacturers are available [7].