Question

Given below are two statements:
Statement (I): The rate of heat transfer to the thermal centre of a container depends on the size of container, but not the shape
Statement (II): The rate of heat transfer to the thermal centre of can is improved by agitating the container
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both Statement (I) and Statement (II) are true
• B. Both Statement (I) and Statement (II) are false
• C. Statement (I) is true but Statement (II) is false
• D. Statement (I) is false but Statement (II) is true

Hint:

To optimize retort processing in dairy canning:
- Use containers with high surface area-to-volume ratios (slender shapes) to minimize heat transfer distances.
- Use rotary/agitating retorts to induce convective heating, shortening sterilization times and preserving product color and flavor.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Thermal processing of packaged dairy foods (such as sterilizing canned evaporated milk) requires heat to transfer from the retort steam to the coldest point of the container, known as the thermal center.
The rate of this heat transfer determines the time required to sterilize the food safely without causing heat damage.

Step 2: Detailed Explanation:
Let us evaluate both statements individually:
- Statement (I): "The rate of heat transfer to the thermal centre of a container depends on the size of container, but not the shape."
This statement is false.
The rate of heat conduction or convection to the thermal center depends on both the size *and* the geometric shape of the container.
Shape determines the surface area-to-volume ratio of the container.
For example, a long, thin cylindrical can has a higher surface area-to-volume ratio than a short, squat can of the same volume.
This larger surface area allows heat to transfer much faster to the center, shortening the required sterilization time.
- Statement (II): "The rate of heat transfer to the thermal centre of can is improved by agitating the container."
This statement is true.
In stationary cans, heat transfers through the food primarily by slow conduction (especially in viscous dairy products).
Agitating or rotating the cans inside the retort forces the liquid inside to move, inducing forced convection.
This convective flow distributes heat rapidly throughout the container, accelerating heat transfer to the thermal center and significantly shortening the processing time.
Therefore, Statement (I) is false, and Statement (II) is true.

Step 3: Final Answer:
Statement (I) is false but Statement (II) is true, which corresponds to Option (D).