Milk has been one of the most beloved dairy products for thousands of years. By definition, it is produced by the mammary glands of mammals. More than six billion people across the globe consume milk and milk products because of their impressive nutritional profile.
Milk contains a wide variety of nutrients like proteins, fats, vitamins, minerals, healthy fats, and antioxidants. These nutrients can benefit the body in multiple ways –
• Bone and brain health
• Muscle building and weight loss
• Heart health and regulation of blood pressure
In this article, we will study the chemistry of milk and the most searched questions related to it-
• Is milk a colloid?
• What is a colloid and its classification?
• What are the properties of colloids?
• Is milk heterogeneous or homogenous?
• What is the stability of the colloidal system in milk?
So, Is milk a colloid? Yes, milk is a colloid. It has tiny globs of butterfat suspended throughout the mixture. It is an example of liquid in a liquid colloid (commonly known as an emulsion).
In general terms, pure indicates ‘free of adulteration’’ however, for a chemistry learner, pure means a substance composed of a single type of particle.
Generally, most of the matter around us exists not as a pure substance, but as a mixture. A mixture is a combination of more than one type of pure form of matter. Based on the nature of the components of the mixture, it is of three types –
• Solution
• Colloid
• Suspension
What is a Colloid?
A colloid is a mixture that exists as a dispersion of microscopic insoluble constituents of one substance in another. Examples – milk, fog, smoke, dust, sprays, cheese, foam; jelly, etc.
A colloid has a dispersed phase and a dispersed medium as its primary components.
The solute-like component of the colloid that is dispersed is known as the dispersed phase whereas the component of the colloid in which the dispersed phase gets suspended is called dispersing medium.
Classification of Colloids:
Based on the physical state (solid, liquid, gas) of the dispersing medium and dispersed phase, colloids can be classified as follows
| Dispersed phase | Dispersing medium | Type of Colloid | Examples |
|---|---|---|---|
| Solid | Gas | Aerosol | Smoke, automobile exhaust |
| Liquid | Gas | Aerosol | Fog, clouds, mist |
| Gas | Liquid | Foam | Shaving cream |
| Solid | Liquid | Sol | Milk of magnesia, mud |
| Liquid | Liquid | Emulsion | Milk, face cream |
| Gas | Solid | Foam | Foam, rubber, sponge, pumice |
| Solid | Solid | Solid sol | Colored gemstone, milky glass |
| Liquid | Solid | Gel | Jelly, cheese, butter |
| Hydrophilic colloids | Hydrophobic colloids |
|---|---|
| Hydro means water and philic means love of so, these colloids are water-loving | Hydro means water and phobic means afraid of so, these colloids are not water-loving |
| Their particles show attraction towards water molecules | Their particles show repulsion from the water molecules |
| Reversible sols | Irreversible sols |
| Strong interaction between the dispersion medium and dispersed phase | Weak interaction between the dispersion medium and dispersed phase |
| Surface tension is generally lower than that of the medium | Surface tension is as same as that of the medium |
| Viscosity is higher than that of medium | Viscosity is as same as that of medium |
| Colloidal particles are visible under an electron microscope | Colloidal particles are visible only under an ultramicroscope |
| Examples: Agar, gelatin, pectin, etc | Examples: Gold sols, clay particles, etc |
Based on the properties of the sol particle, colloids can be classified as follows-
| Multimolecular Colloids | Macromolecular Colloids | Associated Colloids (Micelles) |
|---|---|---|
| When a large number of atoms or smaller molecules of dispersed media [having a diameter of < 1nm] during the dissolution accumulated together to form particles of colloidal dimensions, the resulting colloidal solutions are known as multimolecular colloids. | When the macromolecules (having high molecular masses) are dispersed in a suitable dispersion medium to form particles of colloidal dimensions, the resulting colloidal solutions are called macromolecular colloids. | Certain strong electrolytes at higher concentrations exhibit colloidal properties and molecules of dispersed phase form micellar structures. |
| Lyophobic and unstable in nature and particles are bonded together by weak Van der Val forces. | Lyophilic sols are generally macromolecular in nature. | This particular concentration at which colloids form micelles is known as critical micellar concentration. |
| Examples: Gold sol, Sulphur sol | Examples: starch, proteins, gelatin, cellulose, nucleic acids, polyethylene, polypropylene, synthetic rubber | Examples: soaps and synthetic detergents |
Properties of Colloids:
A colloid is an example of a heterogeneous mixture
You must also check out the article on is milk a homogeneous or heterogeneous mixture.
• The size of the suspended particles in a colloid can range from 1 to 1000 nanometres (10-9 meters). They are too tiny that it is not easy to see them with naked eyes
• Colloids are big enough to scatter a beam of light through them. So, the path of light is visible in a colloid
• Colloids are stable as the particles do not settle down when left undisturbed
• The separation of colloidal particles cannot be done by the process of filtration. It requires a special technique known as centrifugation
Is Milk a Colloid?
Milk consists of several compounds like proteins and fats. The small globules of fat and proteins float around freely and independently throughout the liquid.
They repel each other and do not settle after standing due to the (usually negatively) charged particles and make milk a colloidal solution.
What Type of Colloid is Milk?
Milk is an example of liquid in a liquid colloid, known as an emulsion. An emulsion is defined as a mixture of two or more immiscible liquids. In milk emulsion, milk fat is mixed with water.
Another example of emulsion includes water + oil
Is Milk a Solution?
| Solution | Colloid |
|---|---|
| The size of the particles is less than 1nm. | The size of the particles is 1-1000nm. |
| Homogenous in nature | Heterogeneous in nature |
| True solutions do not show the Tyndall effect | Colloids show the Tyndall effect in their colloidal solution |
| Filtration is used to separate particles | Filtration cannot be used for the separation of particles |
| Example: Sugar solution in water | Example: Starch dissolved in water |
As discussed above, milk consists of water and small globules of fats and proteins that float throughout the mixture.
Unlike true solutions, it is heterogeneous in nature. In fact, particles are big enough to scatter a beam of light through them and show the Tyndall effect. It follows the properties of a colloid.
Hence, Milk is not a solution.
Is Milk a Suspension?
| Suspension | Colloid |
|---|---|
| The size of the particles is more than 1000 nm. | The size of the particles is 1-1000 nm. |
| Particles can be seen by the naked eye. | Particles are small enough to be individually seen by the naked eye. |
| Particles are not stable as they settle down after some time. | Particles are stable as they do not settle down after some time. |
| Example: Mixture of chalk and water | Example: Milk and blood |
If you examine a drop of milk under the microscope, tiny globs of proteins and fats suspended throughout the liquid can be seen.
Moreover, unlike suspension, they do not settle down after standing indicating that milk is a colloid.
Stability of the Colloidal System in Milk
The stability of the colloidal system is the capability of the particles to remain suspended in the solution at equilibrium. The dynamics of aggregation and sedimentation phenomenon disturb the stability of the colloid.
Milk is a colloidal solution in which tiny fat globules repel each other due to the presence of identical and equivalent negative charges. They do not aggregate together to form larger particles. They just float throughout the liquid freely without clumping.
However, when the pH of milk changes or becomes acidic (for eg, by adding lime juice to boiling water), milk protein (casein and others) molecules begin to attract one another. This leads to the formation of “curdles” or lumps or long chains.
Further, these long chains of protein molecules get separated from water molecules and form a sediment, known as “chhena” or “paneer”.
Must Read: pH of Milk – Acidic or Alkaline
Conclusion
A mixture is composed of two or more substances, mixed in any proportion. It can be of three types- true solution, suspension, and colloids.
A true solution is a homogenous mixture. It has two components – solute and solvent. The size of the particles of the solution is ˂ 1nm so they cannot be seen by naked eyes and do not show the Tyndall effect. They are stable in nature.
Suspensions are heterogeneous mixtures in which one substance is soluble in another. The size of the particles of suspension is very large so they can be easily seen by naked eyes. They are unstable in nature so they settle down when left undisturbed
Colloids are heterogeneous mixtures. The size of particles is small enough to be visible by the naked eye; however, are large enough to show the Tyndall effect. There are two components in the colloid – dispersed phase and dispersed medium.
Milk is an example of a colloidal solution. It has floating small particles of fat and protein globules which show the Tyndall effect as well as do not get settled down after some time. So when you put milk in the fridge overnight, its composition remains as such. No fat blobs get settled down. They stay where they are.
