Citric acid fermentation — in the traditional liquid surface culture (surface fermentation) process, which statement correctly describes how Aspergillus niger grows and how the culture is set up? Select the most accurate description of organism placement and medium arrangement.

Introduction / Context:
Citrate production by Aspergillus niger can be performed by two classic bioprocess configurations: surface (also called liquid surface culture) and submerged fermentation. Understanding how the fungus grows in each configuration is crucial for exam questions and for scaling up industrial processes.

Given Data / Assumptions:

• Organism: Aspergillus niger.
• Process type: liquid surface culture (surface fermentation), not submerged.
• Focus: physical location of fungal growth in relation to the liquid medium.

Concept / Approach:

In liquid surface culture, shallow trays or pans of nutrient solution are inoculated, and the filamentous fungus grows as a mycelial film on the air–liquid interface. This setup maximizes oxygen access without sparging, unlike deep-tank aerated systems used for submerged fermentation, where hyphae form pellets or dispersed mycelia throughout the broth.

Step-by-Step Solution:

Verification / Alternative check:

Industrial microbiology texts consistently describe surface culture as shallow liquid with surface mycelium; submerged culture uses aerated, agitated tanks with fungal pellets or dispersed hyphae.

Why Other Options Are Wrong:

(a) Describes solid-state fermentation, not a liquid surface system. (c) Describes submerged fermentation. (d) and (e) combine incompatible modes, so they cannot both be correct for the same process.

Common Pitfalls:

Confusing “surface culture” with “solid-state” due to similar oxygenation logic; assuming all citric acid fermentations are submerged because that is common in modern plants.

Final Answer:

A. niger forms a floating mycelial mat on the surface of the liquid medium