Question

Among the following stages of milk powder manufacture, which stage involves highest degree of denaturation of whey proteins

• A. Pre-heating
• B. Concentration
• C. Spray-drying
• D. Atomization

Hint:

To control the properties of milk powder:
- High-heat powders (high denaturation) use intense pre-heating (e.g., 90°C for 30 minutes).
- Low-heat powders (low denaturation) use mild pre-heating (e.g., 72°C for 15 seconds).
In both cases, the pre-heating stage is where the denaturation is controlled.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Whey proteins (such as \(\beta\)-lactoglobulin and \(\alpha\)-lactalbumin) are globular proteins that are highly sensitive to heat.
They undergo irreversible thermal denaturation and unfolding at temperatures above 70°C.
The degree of denaturation depends on both the temperature and the duration of the heat treatment during processing.

Step 2: Detailed Explanation:
Let us evaluate the thermal conditions of each stage in milk powder manufacture:
-(A) Pre-heating (Forewarming):
In this initial stage, raw liquid milk is heated to high temperatures (typically ranging from 75°C to 120°C for times ranging from several seconds to minutes).
This severe heat treatment is necessary to destroy pathogens, inactivate enzymes (like lipases), and control the physical properties (such as viscosity and heat stability) of the final powder.
Because the whey proteins are in a dilute liquid state and exposed to high temperatures for a relatively long time, this stage causes the highest degree of whey protein denaturation (often exceeding 70% to 90% in high-heat powders).
-(B) Concentration (Evaporation):
Concentration is performed in falling-film evaporators under a vacuum.
Operating under a vacuum lowers the boiling point of the milk to between 50°C and 60°C.
These mild temperatures prevent thermal damage and cause negligible further denaturation of the proteins.
-(C) Spray-drying:
During spray drying, the concentrated milk is sprayed into hot air (inlet temperature 180°C to 200°C).
However, due to rapid evaporative cooling, the actual temperature of the product particles remains low (at the wet-bulb temperature, around 45°C to 50°C) as long as moisture is evaporating.
By the time the product reaches a higher temperature, it is dry, and proteins are much more stable against thermal denaturation in a dry state.
-(D) Atomization:
Atomization is a mechanical process that uses a nozzle or rotary disc to break the milk into fine droplets.
It does not involve any heat treatment and does not cause protein denaturation.
Therefore, the pre-heating stage is responsible for the highest degree of whey protein denaturation.

Step 3: Final Answer:
The pre-heating stage involves the highest degree of denaturation of whey proteins, which corresponds to Option (A).