Do you know why this layer is formed on the surface of milk (or on the hot cup of tea you drink)?
I, as a child, had a misconception that this thin film is formed on the surface of milk when it is kept to cool down (after boiling it for some time).
But the reality is a bit different.
I am going to tell you why (and how) a thin film is formed on the surface of the tea you drink.
And…
How this phenomenon is related to the boiling of an egg.
Disclaimer: As usual, to understand my explanation behind such daily life phenomenon you do not need to be perfect in science or need a proper scientific background. Let you be a descendant of Louis Pasteur or Milton Friedman, you can give it a read.
My mom often scolds me to drink the cup of coffee or tea before a thin skin is formed on the surface. This signifies that I should drink that hot beverage made from dairy milk before it gets cold. I have also seen many people hate this Milk Skin and remove it before having a sip of tea (or coffee).
Before I tell you the exact reason behind this daily life phenomenon, you will need to know some basic chemistry of milk.
- Milk is a colloidal solution composed of:
1. Water
2. Fats
3. Proteins
4. Lactose
5. Minerals
(I mentioned it as a colloidal solution because, at the microscopic level, the particles present in milk are way too large (since humongous organic molecules like proteins and fats are present in the solution). These large particles scatter the light passing through it. This makes the solution look non-transparent (unlike transparent solutions like sugar in water).
Proteins, Lactose and Minerals are together present as Milk Solids in the milk.
This chart will give you an idea of the composition of milk:
Milk is popular for the proteins present in it. This is why it is present in the common diet of everyone.
- Proteins present in milk are:
1. Casein
2. Whey Protein
In cow’s milk, the composition goes like this:
Casein (80%) & Whey Protein (20%).
I thought of providing the percentage along with the protein composition just to inform you about an amazing fact… that the protein composition of human milk is almost opposite to that of a cow.
That is:
Casein (30%) & Whey Protein (70%).
- Whey is a mixture of these three proteins:
1. Beta-Lactoglobulin
2. Alpha-Lactalbumin
3. Bovine Serum Albumin
Now that you know the composition of milk, I am free to answer the question:
Why does Milk Skin form on milk on heating?
We know that proteins are present in milk. (Casein & Whey Protein)
And Whey Protein consists of individual protein molecules like Beta-Lactoglobulin.
These humongous individual protein molecules are made (chained) from small and cute molecules called Amino Acids.
See this amino acid as an example:
Structure of Proline
Cute and little molecules like these combine by peptide bonds and form long chains to make humongous proteins like this:
In a protein molecule, the hydrogen atoms of the amino acids exhibit attractive force on the atoms of neighbour amino acids (of the same protein). They form a weak bond called Hydrogen Bond.
Always remember, if a phenomenon happens in chemistry that doesn’t follow your common sense, it is always the damn hydrogen bond playing games in the background.
Lol.
These forces are present inside a single molecule of protein, so these are Intramolecular Forces.
Due to these intramolecular forces between the amino acids of the protein, it folds and becomes an independent entity.
Like this:
When you heat this protein, those weak hydrogen bonds between amino acids break. No intramolecular forces are present.
Then, the protein is unfolded like this:
Coming to the proteins inside milk.
Before heating, all protein molecules are present independent of each other… Folded around their own body.
When you heat the milk, proteins present in the mixture of Casein and Whey unfold. (As heat breaks the hydrogen bond between the atoms of every protein). We call this Denaturation of Proteins.
On further heating, a certain energy is provided for the formation of bonds between proteins. We call this as the Bond formation enthalpy since it is the energy required or released when a certain bond is formed. This leads to the formation of attractive forces between the molecules, which we say Intermolecular forces.
In a simple language: Instead of the hydrogen atoms forming bonds inside a single protein itself, each protein starts forming bonds with other protein chains present around.
That is, when heat energy exceeds the Bond formation enthalpy, proteins start forming bonds with other protein chains like this:
This way, each protein links with other neighbour proteins and form a strong network. This network of protein coagulates the fat molecules and holds them inside it and then it rises to the top of milk.
On reaching the surface, water from this protein-network (with coagulated fat) is evaporated. This, in turn, solidifies the network and creates a layer on the surface of the milk.
We call this as the Milk Skin (for everyone to understand) or Lactoderm (To sound like an intelligent scientific civilization).
The skin formation is visible when the heated milk is kept for cooling. This makes a common misconception among people that Milk Skin forms when heated milk is cooled (like the childhood Sunny).
In reality, Lactoderm formation starts while heating itself, but the networks of proteins move randomly due to reasons like:
1. Due to heat, the protein molecules have high kinetic energy and hence move randomly at high speeds.
2. Convection currents.
3. Manual stirring by a human, who is boiling the milk.
After cooling, these protein networks and coagulated fats combine together well and form a visible layer.
When it comes to the solidification of an egg after boiling… the reason is the same.
An egg contains Proteins in the transparent liquid called Egg White and Fats in the yellow part called Yolk.
On heating, the proteins present in the Egg White form a network (just like the case of milk) and it solidifies.