Citric acid manufacturing: What fermentation modes are commonly used to obtain citric acid as a fermentation product?

Introduction / Context:
Citric acid is one of the world’s highest-volume organic acids. It is produced microbially, classically by Aspergillus niger, using processes designed to drive carbon flux through the tricarboxylic acid pathway and to secrete citrate at high yields.

Given Data / Assumptions:

• Organism: typically Aspergillus niger.
• Industrial goal: high titer, high yield, and efficient downstream recovery.
• Common fermentation configurations include submerged aerated and surface (tray) processes.

Concept / Approach:
Submerged aerated fermentation uses stirred-tank or bubble-column reactors with strong aeration and controlled pH, nutrients, and trace metals to favor citrate overflow. Surface (stationary tray) fermentation spreads the culture on shallow trays to foster high oxygen transfer at the air–solid interface. Historically, both methods have been used; modern plants favor submerged aerated for scalability and control, but surface methods remain instructive and sometimes economical.

Step-by-Step Solution:

Verification / Alternative check:
Process histories show transitions from surface to submerged methods as engineering controls improved; nevertheless, both are recognized production routes.

Why Other Options Are Wrong:

• Only submerged or only tray: each alone is incomplete.
• None of these: contradicts standard industrial practice.

Common Pitfalls:
Confusing “surface” with “solid-state” only; tray processes often involve moist solid substrates with high oxygen exposure.

Final Answer:
Both (a) and (b)