Question

Increasing viscosity in fermentation broth leads to ________.

• A. \( \text{Lower } k_La \)
• B. \( \text{Higher oxygen transfer} \)
• C. \( \text{Faster growth} \)
• D. \( \text{Better mixing} \)

Hint:

Fermentations involving fungi or bacteria that produce extracellular polysaccharides (like xanthan gum) suffer the most from oxygen limitation due to high viscosity.

Answer 1

The Correct Option is A

Concept: The volumetric mass transfer coefficient (\( k_La \)) determines how efficiently oxygen is transferred from gas bubbles into the liquid broth.
• Efficient oxygen transfer is vital for aerobic fermentations.
• Fluid properties like viscosity have a profound impact on the physics of bubble formation.

Step 1: Viscosity and Bubble Dynamics.
As a fermentation broth becomes more viscous (thicker), it becomes harder for bubbles to move and break apart. Higher viscosity leads to the formation of larger bubbles, which have a smaller surface area-to-volume ratio.

Step 2: Impact on Mass Transfer.
The liquid film resistance around the bubbles increases with viscosity, slowing down the diffusion of oxygen. Since \( k_La \) represents the overall efficiency of this transfer, a thicker broth results in a much lower \( k_La \) value. This usually limits the amount of biomass that can be supported in the bioreactor. Final Answer: Increasing the viscosity of the broth results in a Lower \( k_La \).