Question

__________ is the primary protein of milk, comprising approximately 80% of the milk protein.

• A. Lactose
• B. Serum
• C. Protease
• D. Casein

Hint:

Milk protein = Casein (80%) + Whey (20%). Always remember this ratio for exams.

Answer 1

The Correct Option is D

Concept: Milk proteins are broadly classified into two major groups: casein proteins and whey (serum) proteins. Casein is the dominant protein fraction in milk and accounts for approximately 80% of the total protein content, while the remaining 20% consists of whey proteins.

Step 1: Understanding composition of milk proteins.
Milk contains:
• Casein proteins $\approx 80%$
• Whey (serum) proteins $\approx 20%$ Casein exists in the form of micelles, which are colloidal aggregates that give milk its white appearance.

Step 2: Role and properties of casein.
Casein:
• Is responsible for the nutritional value of milk protein
• Precipitates at its isoelectric point (around pH 4.6)
• Plays a major role in cheese formation

Step 3: Evaluating each option.
• Option (A): Lactose — This is a carbohydrate (milk sugar), not a protein.
• Option (B): Serum — Refers to whey fraction, but it constitutes only about 20%.
• Option (C): Protease — An enzyme, not a structural milk protein.
• Option (D): Casein — Main protein comprising about 80% of milk protein. Correct.

Step 4: Final reasoning.
Since the question specifically asks for the primary protein making up about 80% of milk protein, casein clearly satisfies this condition. Final Conclusion:
Casein is the primary protein of milk, forming approximately 80% of total milk protein. Hence, the correct answer is option (4).