Question

Which property of lactose is utilized for estimation of lactose in Lane-Eynon method?

• A. Oxidising property
• B. Mutarotation
• C. Hydrolysis
• D. Reducing property

Hint:

Sugar Estimation Methods:
Lane-Eynon Method $\rightarrow$ REDUCING PROPERTY (Reduces Fehling's $\text{Cu}^{2+} \rightarrow \text{Cu}_2\text{O}$, Methylene blue indicator).
Polarimetry $\rightarrow$ Optical rotation.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Titrimetric determination of sugars: the Lane-Eynon method quantitatively titrates boiling Fehling's alkaline copper sulfate solution using methylene blue indicator based on the stoichiometric reducing property of the free hemiacetal aldehyde group of lactose.
Key Formula or Approach:
\[ \text{Lactose (Reducing Sugar)} + 2\text{Cu}^{2+} + 4\text{OH}^- \xrightarrow{\Delta, \text{ Fehling's Solution}} \mathbf{\text{Cu}_2\text{O} \downarrow \text{ (Red Precipitate)}} + \text{Lactobionic Acid} \]

Step 2: Detailed Explanation:
In quantitative dairy carbohydrate analytical chemistry:
- The Lane-Eynon Volumetric Method (IS: 1479 AOAC Official Method) is the standard reference chemical method for determining lactose in milk and dairy products:
- It relies strictly on the Reducing Property (D) of lactose.
- In hot alkaline solution, the open-chain aldehyde group on the C-1 carbon of the glucose moiety of lactose reduces alkaline cupric tartrate complex (Fehling's Solution A and B) to an insoluble red precipitate of cuprous oxide ($\text{Cu}_2\text{O}$).
- The titration endpoint is indicated by the irreversible reduction and decolorization of Methylene Blue indicator to colorless leucomethylene blue upon complete reduction of all cupric ions.

Step 3: Final Answer:
Therefore, the property utilized in Lane-Eynon method is Reducing property, corresponding to option (D).