STR capabilities check: which statement about mechanical agitation and aeration in stirred-tank fermenters (STFs) is incorrect for typical bioprocess operation?

Introduction / Context:
Stirred-tank fermenters (STFs) are versatile reactors. With proper impeller selection (e.g., helical ribbon, anchor, pitched-blade) and baffle configuration, they can handle a broad viscosity range. This item asks you to identify the incorrect statement about their capabilities.

Given Data / Assumptions:

• Modern STFs support aeration, agitation, and cooling coils/jackets.
• Impellers can be tailored for gas dispersion or viscous mixing.
• Viscosity may increase during culture due to biomass or polymers.

Concept / Approach:
Agitation generates flow for mixing and suspension, while aeration and dispersion add interfacial area for oxygen transfer. Although gas–liquid mass transfer becomes harder at high viscosity, STFs remain usable by switching to low-shear, high-torque impellers and by operating at suitable speeds and air rates, sometimes with oxygen enrichment or micro-spargers.

Step-by-Step Solution:

Verification / Alternative check:
Industrial production of xanthan and fungal broths uses modified STFs with appropriate impellers to handle very viscous fluids.

Why Other Options Are Wrong:

• (a) True: agitation suspends cells/solids.
• (b) True: agitation ensures homogeneity.
• (d) True: dispersion boosts kLa.
• (e) True: mixing enhances heat transfer coefficients.

Common Pitfalls:
Confusing “challenging for aeration” with “impossible to use”; ignoring impeller customization.

Final Answer:
STFs cannot be used for high-viscosity media