Question

Which of the following techniques is not a procedure for estimation of protein in milk?

• A. Picric acid method
• B. Formol titration
• C. Kjeldahl method
• D. Dye binding method

Hint:

The Kjeldahl method is the gold standard reference, while Formol titration and Dye-binding are common rapid testing methods for milk protein estimation.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Protein estimation in dairy products is performed using methods that target specific chemical groups, such as peptide bonds, nitrogen content, or charged amino acid residues.

Step 2: Detailed Explanation:
Let us review the listed methods for protein analysis:
- Formol titration: This is a rapid volumetric method where formaldehyde is added to milk.
Formaldehyde reacts with the amino groups of proteins, converting them to neutral methylene-amino derivatives and freeing the carboxyl groups.
These carboxyl groups are then titrated with standard alkali, allowing for protein estimation. Thus, this is a standard method.
- Kjeldahl method: This is the international reference method for protein estimation.
It involves digesting milk with sulfuric acid to convert organic nitrogen into ammonium sulfate, distilling the ammonia, and titrating it.
The total nitrogen is then multiplied by a conversion factor (38 for milk) to determine protein content. Thus, this is a standard method.
- Dye-binding method: This method utilizes anionic dyes (such as Amido Black or Acid Orange 12) that bind quantitatively to the positively charged basic amino acid residues (lysine, arginine, histidine) of milk proteins at low pH.
The insoluble protein-dye complex is centrifuged out, and the remaining unbound dye is measured colorimetrically. Thus, this is a standard method.
- Picric acid method: Picric acid is a protein precipitant and is used in other assays (such as the Jaffe reaction for creatinine), but it is not a standard analytical procedure for estimating protein content in milk.
Therefore, the picric acid method is not used to estimate milk protein.

Step 3: Final Answer
The correct option is (A).