Why vinegar (acetic acid) fermentations are often run in fed-batch mode: identify the key process control objective achieved by the feeding strategy.

Introduction / Context:
Vinegar production relies on aerobic oxidation of ethanol to acetic acid by acetic acid bacteria. Process mode influences productivity and cell health. Fed-batch is favored to control substrate levels and heat release while allowing high oxygen transfer.

Given Data / Assumptions:

• Ethanol is both substrate and potential inhibitor.
• Acetic acid and heat can also inhibit, but ethanol spikes are particularly harmful.
• Aerobic conditions are essential; oxygen demand is high.

Concept / Approach:

Feeding ethanol gradually prevents substrate inhibition and excessive exothermic heat from rapid oxidation. This stabilizes the culture, avoids ethanol toxicity, and helps keep dissolved oxygen within range. While acetic acid must also be managed, the principal control variable in fed-batch is ethanol concentration to protect cells and improve yield.

Step-by-Step Solution:

Verification / Alternative check:

Process descriptions of Frings or packed-generator acetators emphasize ethanol feed control (usually 0.5–1.5% v/v range) to avoid inhibition and maintain safe temperature.

Why Other Options Are Wrong:

(a) While acetic acid affects cells, fed-batch primarily targets ethanol; final titers rise by removing acid or buffering, not just by feed control. (c) The statement is false; high ethanol inhibits acetic acid bacteria. (d) Cannot be true because (c) is incorrect. (e) Aeration remains essential.

Common Pitfalls:

Assuming “fed-batch” always targets product inhibition rather than substrate inhibition; overlooking temperature rise from rapid ethanol oxidation.

Final Answer:

To maintain low ethanol concentrations that would otherwise inhibit or cause overheating