Question

The cause for higher insolubility index of dried milk can be attributed to :

• A. Milk having high antibiotic residues
• B. Milk has been stored in chilled condition for 2 days prior to concentration and drying
• C. Milk had disturbed salt balance
• D. Late lactation milk was pooled with mid lactation milk intended for drying

Hint:

To prevent a high insolubility index, dairy processors often add stabilizing phosphate or citrate salts to the milk concentrate before drying.
This restores the salt balance and protects the proteins from heat denaturation.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The insolubility index is a measure of the amount of undissolved sediment left after reconstituting milk powder.
A high insolubility index indicates that the milk proteins have undergone denaturation and aggregation during processing, reducing the solubility of the powder.
Detailed Explanation:
The heat stability of milk proteins, particularly casein micelles, is highly dependent on the mineral balance (salt balance) of the milk.
This balance is primarily determined by the ratio of calcium and magnesium ions to phosphate and citrate ions:
\[ \text{Salt Balance Ratio} = \frac{[\text{Ca}^{2+}] + [\text{Mg}^{2+}]}{[\text{PO}_4^{3-}] + [\text{Citrate}^{3-}]} \] If this salt balance is disturbed (e.g., due to an excess of calcium or magnesium ions):
- The casein micelles lose their stable electrical charge and hydration shell.
- During the preheating, concentration, and spray-drying processes, these unstable proteins easily denature and aggregate.
- These aggregated proteins cannot rehydrate when the powder is reconstituted, resulting in a high insolubility index.
Other factors, such as antibiotic residues, cold storage, or pooling late-lactation milk, can affect other properties of milk but do not directly cause the protein destabilization that leads to a high insolubility index.

Step 2: Final Answer:
The cause of a higher insolubility index is a disturbed salt balance in the milk.