Question

Which among the following reactions is not favoured by an increase in the moisture content of the milk powder?

• A. Enzymic reaction
• B. Caking
• C. Microbial growth
• D. Fat oxidation

Hint:

While enzymatic activity, caking, and microbial growth increase with moisture, lipid oxidation rates are higher in extremely dry milk powders due to the absence of a protective water monolayer.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The physical and chemical stability of spray-dried milk powder during storage is influenced by its moisture content and water activity (\(a_w\)).
Moisture content affects different degradation pathways in different ways.

Step 2: Detailed Explanation:
Let us evaluate how increasing moisture content affects each reaction:
Enzymic reactions: Water acts as a solvent and a reactant for enzyme-catalyzed reactions.
Increasing moisture content increases water activity, which promotes enzymatic activity.
Caking: Elevated moisture content decreases the glass transition temperature (\(T_g\)) of amorphous lactose.
When the storage temperature exceeds \(T_g\), lactose transitions from a glassy to a rubbery state and crystallizes, leading to caking.
Microbial growth: Bacteria, yeasts, and molds require free water to grow.
Increasing moisture content increases water activity, promoting microbial growth.
Fat oxidation: The relationship between lipid oxidation and water activity (\(a_w\)) is U-shaped.
At very low water activities (\(a_w < 0.1\)), lipid oxidation is rapid because there is no water to hydrate catalysts or form hydrogen bonds with hydroperoxides.
As moisture increases to the monolayer level (\(a_w \approx 0.2\) to \(0.3\)), water molecules form a protective barrier that reduces oxidation rates.
Therefore, increasing moisture content within typical storage ranges does not favor lipid oxidation.

Step 3: Final Answer:
Consequently, fat oxidation is the reaction that is not favored by an increase in moisture content within typical storage ranges.