Question

Hehner test is used for detection of which one of the following in milk?

• A. Sugar
• B. Glucose
• C. Skim milk powder
• D. Formalin

Hint:

Milk Adulteration Detection Tests:
FORMALIN Preservative $\rightarrow$ HEHNER TEST (Violet/Purple ring).
Cane Sugar $\rightarrow$ Resorcinol Test (Red color).
Starch $\rightarrow$ Iodine Test (Blue color).
Urea $\rightarrow$ DMAB Test (Yellow color).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Chemical detection of chemical adulterants in milk: the Hehner test detects formaldehyde (formalin, 40% aqueous HCHO) as an illegal preservative by reaction with milk tryptophan and ferric chloride in concentrated sulfuric acid, producing a violet/purple ring.
Key Formula or Approach:
\[ \text{Formalin (HCHO)} + \text{Milk Tryptophan} + \text{FeCl}_3 \xrightarrow{\text{Conc. } \text{H}_2\text{SO}_4 \text{ Interface}} \mathbf{Violet / Purple \text{ Ring}} \]

Step 2: Detailed Explanation:
In milk adulteration testing and chemical preservatives detection (FSSAI Manual IS: 1479):
- Formalin (40% aqueous Formaldehyde solution, HCHO): Illegally added as a toxic preservative to increase the shelf-life of raw milk in summer.
- Hehner Test (Hehner's Test) (D): The official regulatory chemical colorimetric test for detecting Formalin in milk:
1. To $5\text{ mL}$ of milk in a test tube, carefully layer concentrated Sulfuric Acid ($\text{H}_2\text{SO}_4$) containing trace ferric chloride ($\text{FeCl}_3$) along the inner wall.
2. In the presence of formaldehyde, tryptophan residues in milk casein condense with formaldehyde in the presence of $\text{Fe}^{3+}$ oxidant at the acid-milk interface, producing a distinct Violet or Purple ring. (Pure unadulterated milk produces a yellow/brown ring).

Step 3: Final Answer:
Therefore, Hehner test is used for detection of Formalin, corresponding to option (D).