Question

The specific gravity of milk is lowered by the addition of -

• A. Skim milk
• B. Water and cream
• C. Sugar
• D. Salt

Hint:

Remember this simple rule for lactometer readings (which measure specific gravity): Adding water lowers the reading. Removing fat (making skim milk) increases the reading. Adding both water and removing fat can be done to try and normalize the reading, which is a sophisticated form of adulteration.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Specific gravity (SG) is the ratio of the density of a substance to the density of a reference substance, usually water. The specific gravity of milk is determined by its components. Adding a substance with a lower SG will decrease the overall SG, while adding a substance with a higher SG will increase it.
Step 2: Key Formula or Approach:
We need to compare the specific gravity of the additives with the specific gravity of normal milk.
Average SG of whole milk: \(\approx\) 1.028 - 1.032
SG of water: 1.000
SG of milk fat (cream): \(\approx\) 0.93 (less dense than water)
SG of Solids-Not-Fat (SNF): \(\approx\) 1.6
SG of skim milk (high in SNF, low in fat): \(\approx\) 1.036 (higher than whole milk)
Step 3: Detailed Explanation:
Let's analyze the effect of each addition:
(A) Skim milk: Has an SG of \(\approx\)1.036, which is higher than whole milk. Adding it would increase the SG.
(B) Water and cream: Water has an SG of 1.000, and cream (fat) has an SG of \(\approx\)0.93. Both are significantly lower than the SG of milk. Adding either or both will lower the SG of the mixture. This is a common form of adulteration.
(C) Sugar: Dissolving sugar in milk increases the concentration of solids, thereby increasing its density and SG.
(D) Salt: Similar to sugar, adding salt increases the dissolved solids and thus increases the SG.
Step 4: Final Answer:
The addition of water and cream will lower the specific gravity of milk.