Citric acid fermentation — in the submerged (deep-tank) process, which statement best describes the growth habit of Aspergillus niger?

Introduction / Context:
Modern citric acid production predominantly uses submerged, aerated, stirred tanks. Recognizing how filamentous fungi grow under these conditions distinguishes this method from surface or solid-state processes.

Given Data / Assumptions:

• Organism: Aspergillus niger (filamentous fungus).
• System: submerged, deep-tank bioreactor with aeration/agitation.
• Focus: spatial distribution of biomass.

Concept / Approach:

In submerged systems, A. niger forms pellets or dispersed hyphal clumps throughout the broth. Oxygen transfer, shear, and nutrient gradients influence pellet size and morphology, directly affecting productivity and viscosity. This is distinct from surface culture, where mycelium forms a film at the air–liquid interface, and from solid-state systems using moist solid substrates.

Step-by-Step Solution:

Verification / Alternative check:

Bioprocess engineering texts describe pellet control (by spore inoculum size, shear, metals) as a key lever in submerged citrate production.

Why Other Options Are Wrong:

(a) Solid-state setup; (b) surface film matches surface culture; (d) mixes incompatible modes; (e) is nonsensical.

Common Pitfalls:

Assuming that surface mat formation occurs in submerged systems; ignoring the impact of morphology on oxygen transfer and viscosity.

Final Answer:

Fungal mycelium grows throughout the aerated broth in a deep tank (pellets or dispersed mycelia)