Question

Which of the following constituents mainly contributes to depression in freezing point of genuine milk

• A. Fat
• B. Casein
• C. Lactose
• D. Beta Lactoglobulin

Hint:

Freezing point depression is determined by the molarity of the dissolved solute. Because lactose is the most abundant solute dissolved in milk serum, it exerts the largest influence on the freezing point of milk.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The freezing point of milk is a colligative property. Colligative properties depend solely on the total number of solute particles (osmotic concentration) in a given volume of solvent, rather than the chemical identity or molecular size of those particles.
The average freezing point of genuine bovine milk is approximately \(-0.525\ ^{\circ}\text{C}\) to \(-0.560\ ^{\circ}\text{C}\).

Step 2: Detailed Explanation:
The constituents of milk are present in different states of dispersion: solution (dissolved), colloidal suspension, and emulsion.
- Fat: Milk fat exists as a coarse emulsion of relatively large globules, which does not contribute to the osmotic pressure or the freezing point depression.
- Proteins (Casein, \(\beta\)-lactoglobulin): These large macromolecules are present in colloidal suspension. Because of their high molecular weight, their molar concentration is extremely low, making their contribution to the freezing point depression negligible.
- Soluble Constituents: Only the dissolved, low-molecular-weight molecules and ions contribute significantly to colligative properties.
- Lactose (a soluble disaccharide) and soluble mineral salts (especially sodium, potassium, and chloride ions) are the primary components dissolved in the aqueous phase of milk.
- Lactose alone accounts for approximately 55% of the total freezing point depression of genuine milk, while the soluble mineral salts contribute most of the remaining 45%.
Therefore, lactose is the constituent that mainly contributes to the depression of the freezing point of genuine milk.

Step 3: Final Answer:
The constituent that mainly contributes to the depression of the freezing point of milk is lactose.