Citric acid medium design: typical sugar (carbon source) concentration maintained during production is approximately:

Introduction / Context:
Citric acid accumulation by Aspergillus niger requires a high initial sugar concentration to drive flux through central carbon metabolism under conditions that limit biomass but favor product formation. The common exam ask is the typical range used industrially.

Given Data / Assumptions:

• Carbon sources include sucrose, glucose, or refined molasses.
• High sugar is tolerated at low pH by A. niger strains.
• Metal ion limitation and nitrogen/phosphate balance are adjusted for citrate overproduction.

Concept / Approach:

Operationally, many surface and submerged processes maintain sugar at roughly 150–200 g/L (15–20 % w/v) to sustain production without frequent feeding, while controlling viscosity and oxygen transfer. Concentrations above this can impair oxygen transfer and create osmotic stress; below this may reduce yields.

Step-by-Step Solution:

Verification / Alternative check:

Classic fermentation handbooks cite initial sugars often between 140 and 200 g/L for efficient citric acid accumulation, with pH control and trace metal management.

Why Other Options Are Wrong:

(a) 10–15 % may be used in some cases but is below the typical exam standard. (c)–(d) 20–30 % often causes mass transfer issues. (e) 5–8 % is too low for industrial citrate runs.

Common Pitfalls:

Confusing w/v with w/w; ignoring that fed strategies can alter “maintained” concentration while initial charge still targets ~15–20 %.

Final Answer:

15–20 % (w/v)