Question

Which of the following statement is true with respect to Lobry de Bruyn-Alberda van Ekenstein rearrangement of lactose in milk during heating?

• A. Glucose moiety of lactose is changed to fructose
• B. Galactose moiety of lactose is changed to fructose.
• C. Glucose moiety of lactose is oxidized to gluconic acid.
• D. Galactose moiety of lactose is galactitol.

Hint:

Lactose is Galactose-Glucose. During the Lobry de Bruyn rearrangement, only the free reducing end (glucose) isomerizes to fructose, forming lactulose (Galactose-Fructose).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The Lobry de Bruyn-Alberda van Ekenstein rearrangement is a classic base- or heat-catalyzed isomerization of aldose sugars into their ketose counterparts or epimers.
Lactose is a disaccharide consisting of a D-galactose moiety linked via a $\beta$-(1,4)-glycosidic bond to a D-glucose moiety.
When milk is heated, lactose undergoes this chemical rearrangement, altering the structure of one of its monosaccharide units.

Step 2: Detailed Explanation:
During the heating of milk, the reducing end of the lactose molecule is affected.
Because the glycosidic linkage is between C1 of galactose and C4 of glucose, the glucose moiety contains the free reducing carbonyl group (an aldose).
Under the influence of heat, the glucose moiety undergoes enolization, forming an enediol intermediate.
This intermediate then isomerizes into a fructose moiety (a ketose) at the reducing end.
This conversion changes lactose (galactosyl-glucose) into lactulose (galactosyl-fructose):
\[ \text{Lactose (Galactose-Glucose)} \xrightarrow[\text{Rearrangement}]{\text{Heat}} \text{Lactulose (Galactose-Fructose)} \]
The galactose moiety is locked in the glycosidic bond and remains completely unchanged during this rearrangement.
Therefore, the statement that the glucose moiety of lactose is changed to fructose is chemically correct.

Step 3: Final Answer
The correct statement is that the glucose moiety of lactose is converted to fructose during heating.