Question

In freshly made milk powder, lactose is non-crystalline and exists in the same equilibrium mixture of \(\alpha\)- and \(\beta\)- lactose as existed in milk prior to drying, which is _________

• A. 1.0 : 1.68
• B. 1.0 : 2.2
• C. 2.2 : 1.0
• D. 1.0 : 1.42

Hint:

Remember:
- In liquid milk, the equilibrium ratio of \(\alpha\)-lactose to \(\beta\)-lactose is \(1.0 : 1.68\).
- Rapid spray-drying "freezes" this exact ratio into an amorphous lactose glass.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Lactose is a disaccharide that exists in two isomeric forms in solution: \(\alpha\)-lactose and \(\beta\)-lactose.
These two anomers undergo mutarotation in water until they reach a stable thermodynamic equilibrium, which depends on the temperature.

Step 2: Detailed Explanation:
Let us analyze the state of lactose in liquid milk and during the drying process:
- In liquid milk at room temperature (around 20°C), \(\alpha\)-lactose and \(\beta\)-lactose exist in a dynamic equilibrium.
This equilibrium mixture consists of approximately 37.3% \(\alpha\)-lactose and 62.7% \(\beta\)-lactose.
- Calculating this ratio:
\[ \text{Ratio of } \alpha \text{ to } \beta = \frac{37.3}{37.3} : \frac{62.7}{37.3} \approx 1.0 : 1.68 \]
This means there is significantly more \(\beta\)-lactose than \(\alpha\)-lactose in normal liquid milk.
- During the manufacture of milk powder by spray drying, the concentrated milk is rapidly atomized into hot air.
The moisture evaporates extremely quickly (within milliseconds).
Because the water is removed so rapidly, the lactose molecules do not have time to align and form regular crystal structures.
Instead, the lactose is "frozen" into an amorphous, non-crystalline glass state.
This amorphous glass preserves the exact molecular distribution of \(\alpha\)- and \(\beta\)-lactose that existed in the liquid milk prior to drying, maintaining the equilibrium ratio of 1.0 : 1.68.
Over time, if the powder absorbs moisture from the air, this amorphous glass can crystallize into \(\alpha\)-lactose monohydrate, causing the powder to lump or "cake."

Step 3: Final Answer:
The equilibrium ratio of \(\alpha\)- to \(\beta\)-lactose in milk and freshly made powder is 1.0 : 1.68.