Oxygen transfer in salty/sugary media:\nHow do higher concentrations of dissolved salts and sugars typically affect oxygen transfer rate (OTR) in bioreactors, and why?

Introduction / Context:
Oxygen transfer rate, OTR, is central to aerobic fermentations and is often written as OTR = KLa * (C* - C), where KLa is the overall mass transfer coefficient and C* is the saturation concentration. Media rich in salts or sugars (high osmolarity broths, high Brix feeds) frequently show reduced oxygen transfer performance. Understanding whether this is due to solubility, hydrodynamics, or both is key to troubleshooting scale-up.

Given Data / Assumptions:

• Addition of salts/sugars increases ionic strength and often increases viscosity.
• Gas type, sparger, and temperature are otherwise unchanged for comparison.
• We examine qualitative trends rather than exact numerical correlations.

Concept / Approach:
Two mechanisms lower OTR in such media. First, oxygen solubility (C*) decreases as dissolved solids increase (salting-out), shrinking the driving force (C* - C). Second, higher viscosity and altered surface properties reduce bubble breakup and circulation, lowering KLa by reducing interfacial area (a) and the liquid-side coefficient (KL). Therefore, both a drop in C* and a drop in KLa act in the same direction to depress OTR.

Step-by-Step Solution:

Verification / Alternative check:
Empirical correlations show KLa inversely related to viscosity; oxygen solubility tables demonstrate reduced C* in saline or high-sugar solutions at fixed temperature and pressure.

Why Other Options Are Wrong:

• (a) alone or (b) alone: each is true but incomplete; both occur together.
• (c): Claims KLa increases while OTR falls—contradicts typical viscosity effects.
• (d): Dismisses well-documented phenomena.

Common Pitfalls:
Assuming aeration rate alone fixes OTR; in viscous, salty media, impeller selection and oxygen enrichment may also be required.

Final Answer:
Both (a) and (b)