Oxygen transfer in aerated fermentation: which change is most likely to increase the rate of oxygen transfer (typically expressed via k_La * (C* − C)) in a given system?

Introduction:
Oxygen transfer rate (OTR) determines aerobic fermentation performance. OTR is often modeled as OTR = k_La * (C* − C), where k_La is the volumetric mass-transfer coefficient, C* is the saturation concentration of oxygen, and C is the bulk dissolved oxygen concentration. Understanding which operating changes raise OTR is central to scale-up and control.

Given Data / Assumptions:

• Agitation influences interfacial area and turbulence, raising k_La.
• Antifoams generally suppress bubble dispersion and coalescence control, often decreasing k_La.
• Temperature increases reduce oxygen solubility (C*) even if k_La may rise modestly.

Concept / Approach:
Among standard levers, increasing stirrer speed reliably increases k_La by creating smaller bubbles, higher turbulence, and improved dispersion. Antifoams typically reduce interfacial area and can lower k_La. Temperature changes have competing effects (lower C* vs slightly higher k_La), and the net impact commonly reduces OTR because the solubility drop dominates in many aqueous systems.

Step-by-Step Solution:
Relate OTR to k_La and (C* − C).Note that agitation strongly increases k_La; select the factor with the clearest positive effect.Exclude antifoam addition due to typical k_La decrease.Exclude temperature rise because C* decreases and can offset any k_La gain.

Verification / Alternative check:
k_La correlations show a positive dependence on power input per volume (P/V), which rises with stirrer speed, supporting the choice.

Why Other Options Are Wrong:

• Addition of antifoams: Often lowers k_La by reducing bubble dispersion.
• Increase in temperature: Decreases C*; net OTR often declines in water-like media.
• Both (a) and (b): Combines two changes that are not reliably beneficial.
• Decrease gas flow: Reduces interfacial area and typically lowers OTR.

Common Pitfalls:
Assuming any variable that increases turbulence helps OTR without considering oxygen solubility; ignoring antifoam effects on bubble behavior.

Final Answer:
Increase in stirrer speed