Question

Given below are two statements:
Statement (I) : Lactose in freshly prepared spray dried skim milk powder is crystalline.
Statement (II) : Lactose in spray dried skim milk powder, upon exposure to air, undergoes crystallization, which causes caking of powder.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both Statement (I) and Statement (II) are true.
• B. Both Statement (I) and Statement (II) are false.
• C. Statement (I) is true but Statement (II) is false.
• D. Statement (I) is false but Statement (II) is true.

Hint:

To prevent caking in milk powders, keep them in moisture-barrier packaging to maintain the lactose in its stable, amorphous glass state.
Once the powder absorbs moisture and the temperature exceeds its glass transition point (\(T_g\)), crystallization and caking are inevitable.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Lactose is the primary carbohydrate in milk and skim milk powder.
During spray drying, moisture is evaporated extremely rapidly.
This rapid transition prevents lactose molecules from organizing into an ordered crystalline structure, affecting the physical state of the sugar in the final powder.

Step 2: Detailed Explanation:
Let us analyze Statement (I): "Lactose in freshly prepared spray dried skim milk powder is crystalline."
This statement is incorrect.
During the spray drying process, concentrated milk is atomized into a chamber of hot air, evaporating water in milliseconds.
Because the drying occurs so quickly, the lactose molecules do not have enough time to align and form a regular crystal lattice.
As a result, the lactose in freshly prepared spray-dried skim milk powder is trapped in an amorphous, metastable glass state (amorphous lactose).
Now let us analyze Statement (II): "Lactose in spray dried skim milk powder, upon exposure to air, undergoes crystallization, which causes caking of powder."
This statement is correct.
Amorphous lactose is highly hygroscopic and readily absorbs moisture from the surrounding air.
When skim milk powder is exposed to humid air, the absorbed water acts as a plasticizer, lowering the glass transition temperature (\(T_g\)) of the amorphous lactose below the storage temperature.
This increases molecular mobility, allowing the lactose molecules to reorganize and crystallize into \(\alpha\)-lactose monohydrate.
As these crystals form, they release the excess absorbed moisture.
This released water dissolves adjacent soluble powder particles, forming liquid bridges.
As drying continues, these liquid bridges solidify into solid crystalline bridges, binding the powder particles together into hard, insoluble lumps.
This process is known as caking.

Step 3: Final Answer:
Statement (I) is false, but Statement (II) is true.