Citric acid via the koji (solid-state) process: which statement accurately describes this production method using Aspergillus niger?

Introduction / Context:
Citric acid can be produced by Aspergillus niger using different cultivation modes. The “koji” process is a classic solid-state/surface method where the mold grows on moist solids rather than submerged liquid, historically including substrates like sweet potato or cassava.

Given Data / Assumptions:

• Organism: Aspergillus niger.
• Process variant: koji (solid-state) versus submerged fermentation.
• Substrate examples: sweet potato, wheat bran, cassava.

Concept / Approach:
Koji refers to a solid-state culture where filamentous fungi develop on the surface and within a moist solid substrate with controlled humidity and aeration. This contrasts with submerged/deep-tank processes where mycelium is suspended in liquid and oxygen transfer is via sparging/agitation.

Step-by-Step Solution:

Verification / Alternative check:
Historical industrial routes and textbooks consistently classify sweet potato–based A. niger cultivation as a koji/solid-state process for citric acid.

Why Other Options Are Wrong:

• Floating surface growth in liquid is surface/submerged, not solid-state koji.
• Deep-tank growth describes submerged fermentation, not koji.
• Koji relies on fungi; bacteria are not “only” used.

Common Pitfalls:
Mixing up surface fermentation in shallow liquid with true solid-state koji; assuming all citric acid today is submerged (solid-state remains relevant historically and for specific contexts).

Final Answer:
A. niger is grown on a moist solid substrate (e.g., sweet potato) in a surface/solid-state system