Milk and color experiment: the magic of an invisible emulsion
Discover why milk is a natural emulsion with this magical experiment. Fun activity to observe how soap breaks down fat.
In this post, we are going to explore one of the lesser-known and perhaps most surprising properties of milk, the fact that it is a natural emulsion. With the help of food colouring and some liquid soap, we are not only going to observe what happens when the soap comes into contact with the milk and the colours disperse all at once, but we will also discover how this so-called magic milk experiment works.
It is an ideal activity to awaken the scientific curiosity of children (and why not, also adults). That white liquid you may have in your refrigerator, whether it is whole, skimmed or lactose-free, hides a complex structure that is worth knowing.
No matter where in the world we live, milk is our first food, and our bodies are designed to feed on breast milk from birth. It is not surprising, then, that at some point in history, we began consuming the milk of other mammals. From the analysis of organic residues on prehistoric ceramics at several archaeological sites in Europe and Southwest Asia, we know that their consumption began about 9000 years ago.
Over time, some humans developed the ability to process lactose beyond infancy, an ability that most people lose as they grow older. It is currently estimated that approximately 30% of the global population tolerates lactose in adulthood, especially in countries in Northern Europe and some traditional cattle-raising communities in Africa and Asia. In addition, milk from other mammals is not the same as human milk: its protein, fat and sugar content, for example, is different.
Milk may seem like a simple liquid, and although we know that it has a sweet, neutral taste and a characteristic whitish colour, it is not just colored water. In fact, it has no molecule or pigment that directly produces that colour; instead, its appearance is the result of the dispersion of fat and protein particles in suspension (we’ll talk more about this in another post). In reality, milk is an emulsion, like mayonnaise, and if you want to know more about what an emulsion is, you can read this other experiment:
👉 https://blog.mealnstock.com/mayonesa-casera-vs-comprada-comparacion
So, milk is an emulsion?
Yes, milk has a high water content (almost 90%), but it also contains fats or lipids (what is the difference? Good question for another post). As we know, water and lipids do not mix, and fat floats because it has a lower density than water. However, in the case of milk, we don’t see a visible separation between the water and the fat it contains; instead, it looks like a homogeneous substance. This is because milk is a natural emulsion of the “oil-in-water” type – that is, small droplets of fat suspended in water – and because of this, the mixture looks fluid and stable.
Besides water and fat (mostly triglycerides), milk contains other ingredients such as lactose, which is a type of sugar, as well as proteins, minerals and vitamins. What allows milk to appear uniform and stable is a tiny structure of nanometric size, that is, much smaller than anything we can see under a microscope. To give you an idea, a nanometer is almost a hundred thousand times smaller than the thickness of a human hair or about a thousand times thinner than a bacterium. This structure is called MFGM ( Milk Fat Globule Membrane).
The MFGM membrane acts like small fluffs stuck to the fat particles, preventing them from binding to each other and floating to the surface. This membrane is made up of special compounds called phospholipids, sphingolipids and glycoproteins, which act as a natural barrier between water and fat.
It has also been observed that some milk proteins, such as caseins, contribute to the stability of the emulsion, even though they are not a direct part of the membrane. Thanks to this invisible organization, milk not only looks white and fluid, but also plays important roles in fat digestion and infant nutrition, especially in the early stages of life.
What happens to milk when we apply heat?
Thanks to the interaction between fat globules and milk proteins (casein and whey), milk has a high heat tolerance. This means that, if we heat the milk slowly to reduce it, the emulsion does not break. The MFGM membrane remains stable, while the whey proteins react to heat and change the milk’s texture. As the temperature rises, these proteins begin to clump together and thicken the mixture. The result is a creamier milk, slightly more yellow in colour, but still a stable emulsion. This is the basis for products such as evaporated milk.
We have already seen that milk is an emulsion, thanks to the MFGM membrane, and that this membrane remains intact even when we heat the milk. But what happens if something external attacks this membrane?
Here is a step-by-step activity for you to try at home.
Activity: Breaking the emulsion, the real magic milk experiment
This fun and visual experiment allows us to easily see something that is not normally seen with the naked eye: the invisible structure of milk. Here is the step-by-step activity for you to try at home, and the explanation of the magic milk experiment.
Materials:
- 3-4 tablespoons (45-60 ml) whole cow’s milk
- 4-5 drops of food colouring (you can use several colours)
- 1 tablespoon liquid soap (can be dishwashing or hand soap)
- 1 flat-bottomed plate
- 2-3 cotton swabs (Q-tips)
Procedure:
Step 1. Pour the milk into the plate
You only need to cover the bottom with a thin layer of milk.
Step 2. Add the food coloring
Place three or four drops of each color near the center of the plate.
Step 3. Dip the swab in soap
All you need is a swab tip lightly moistened with liquid soap.
Step 4. Touch the milk with the swab
Place the tip of the soapy swab in the center of the dish, where the colour drops are. Do not remove it until it stops flowing. Observe the reaction! You can test in different spots or with new swabs moistened with soap.
Questions for discussion
- What happened when we touched the swab?
- Why did the colours start to move just at that moment?
- What happens if we do not add colourant? Does it make any difference?
- Do you think the effect would be the same with vegetable milk or skim milk?
- What happens if instead of cow’s milk we use vegetable or skim milk?
What do we learn from this experiment?
Soap (or liquid detergent) contains a substance called surfactant, and it is a very special molecule, one part of the molecule “loves” water (it is hydrophilic), and the other part “loves” fat (it is lipophilic). It is a kind of “invisible force” that holds the water together and also holds the fat droplets in the milk together.
Two phenomena occur at the same time when we add soap to milk: on one hand, the soap breaks the surface tension of the liquid, causing an immediate movement on the surface that pushes the food coloring in different directions; on the other hand, it interferes with the MFGM membrane, which is responsible for keeping the fat suspended in water and for allowing milk to behave like a stable emulsion. As this membrane weakens and breaks, the fat droplets are released and begin to spread out, creating the colourful swirls on the surface of the milk. It’s not magic; what we’re observing is how a natural emulsion transforms when its invisible structure is broken apart.
