Question

Assertion A: Microbial growth is increased when agitation speed rises.
Reason R: Oxygen mass transfer improves but shear damage to cell also occurs.
In the light of the above statements, choose the most appropriate answer:

• A. Both A and R are correct and R is the correct explanation of A
• B. Both A and R are correct but R is NOT the correct explanation of A
• C. A is correct but R is not correct
• D. A is not correct but R is correct

Hint:

In bioprocessing, agitation is a balancing act. You want enough speed to "feed" the cells oxygen, but not so much that you "crush" them with shear forces.

Answer 1

The Correct Option is B

Concept: In bioreactors, agitation serves to mix nutrients and, most importantly, to enhance the Dissolved Oxygen (DO) available to the microorganisms.

Step 1: Evaluate Assertion A.
Increasing agitation speed generally increases the oxygen transfer rate ($k_La$), which usually supports higher microbial growth rates in aerobic cultures. Thus, A is generally considered correct in a standard context.

Step 2: Evaluate Reason R.
It is true that higher speeds improve mass transfer. However, it is also true that high speeds create mechanical shear stress that can rupture or damage fragile microbial cells. Thus, R is a correct statement of the dual-effect of agitation.

Step 3: Determine the relationship.
While both are true, R describes a conflict (benefit vs. damage) rather than a direct, singular reason why growth $increases$. The increase in growth is specifically due to the improved oxygen transfer part of R, but the presence of shear damage actually works against growth. Therefore, R is not a simple "explanation" of a growth increase, but rather a more complex statement of factors.