Question

Which of the following milk constituents scatter/reflect light for milk to appear whitish?
(A) Fat
(B) Lactose
(C) Casein
(D) Whey proteins
Choose the correct answer from the options given below:

• (A) and (B)
• (A) and (C)
• (B) and (D)
• (C) and (D)

Hint:

Light scattering requires particles of sufficient size.
Fat globules (emulsion) and casein micelles (colloid) are the only particles in milk large enough to scatter light, which is why skimmed milk (lacking fat) looks less white and more bluish-translucent.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The optical properties of milk (such as its white, opaque appearance) are determined by how light interacts with the various constituents suspended or dissolved in the liquid.
When light hits suspended particles that have a different refractive index than the surrounding medium, the light is scattered and reflected in all directions, making the liquid appear white.

Step 2: Detailed Explanation:
Let us analyze the state of the different milk constituents:
- Lactose and whey proteins are in true solution or highly dispersed states.
Because of their extremely small molecular size, they do not scatter visible light significantly.
- Milk fat exists as an emulsion of fat globules (ranging from \( 0.1 \) to \( 15\ \mu\text{m} \) in size).
These globules are large enough to scatter and reflect all wavelengths of visible light.
- Casein exists as highly hydrated colloidal aggregates called casein micelles (ranging from \( 30 \) to \( 300\text{ nm} \) in size).
These micelles are also large enough to scatter light (Tyndall effect).
Together, the light-scattering effects of the emulsified fat globules and the colloidal casein micelles prevent light from passing directly through the milk, giving it its characteristic white, opaque appearance.
Therefore, fat and casein are the primary constituents responsible for the white appearance of milk.

Step 3: Final Answer:
The correct option is 2, which includes (A) and (C).