Question

Which of the following reagents is useful to prepare soluble caseinate from insoluble casein

• A. Acetic acid
• B. Benzene
• C. Ethyl alcohol
• D. Sodium hydroxide

Hint:

Casein behaves as an amphoteric molecule. Below its isoelectric point (pH 4.6), it acts as a cation, whereas above pH 4.6, it acts as an anion. Treating it with strong bases like NaOH or KOH converts it into highly soluble anionic caseinate salts.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Casein is the primary group of phosphoproteins found in mammalian milk, constituting approximately 80% of the total protein in bovine milk.
The solubility of casein is highly dependent on the pH of the surrounding medium.
At its isoelectric point (pH 4.6), casein carries a net neutral charge, which minimizes electrostatic repulsion and causes it to precipitate out of solution as an insoluble curd.

Step 2: Detailed Explanation:
In its precipitated or acid-precipitated state, casein is practically insoluble in water.
To convert this insoluble acid-casein into a highly functional, soluble form for food and industrial applications, it must be neutralized using an alkaline reagent.
- When an alkali such as sodium hydroxide (NaOH) is added to insoluble casein, it neutralizes the acidic carboxyl groups of the protein.
- This reaction converts the neutral, insoluble protein molecules into soluble salt forms known as sodium caseinates:
\[ \text{Insoluble Casein} + \text{NaOH} \rightarrow \text{Soluble Sodium Caseinate} + \text{H}_2\text{O} \]
- The resulting sodium caseinate is highly soluble in water due to the increased net negative charge on the protein molecules, which promotes electrostatic repulsion and hydration of the protein chains.
- Let us examine the other options:
- Acetic acid is a weak acid that would lower the pH, maintaining or promoting casein precipitation rather than solubilizing it.
- Benzene is a non-polar organic solvent that cannot dissolve hydrophilic proteins like casein.
- Ethyl alcohol is a polar solvent used to dehydrate and precipitate proteins rather than solubilizing them.
Thus, sodium hydroxide is the correct reagent for preparing soluble caseinate from insoluble casein.

Step 3: Final Answer:
The reagent useful for preparing soluble caseinate is sodium hydroxide.