Industrial microbiology check: which microorganism is classically employed at scale for commercial production of citric acid?

Introduction / Context:
Citric acid is a high-volume organic acid used in foods, beverages, pharmaceuticals, and cleaning products. The organism used for its production must tolerate low pH, high sugar, and produce citric acid at high yield.

Given Data / Assumptions:

• The question asks for the canonical organism.
• Process is aerobic and favors acid accumulation.
• Product inhibition and metal ion balance are relevant but secondary here.

Concept / Approach:

Aspergillus niger is the historical and current workhorse for citric acid thanks to its robustness, high flux through the tricarboxylic acid pathway under specific nutrient limitations, and tolerance to acidic conditions (broth pH often near 2–3 during production). Yeasts and lactic streptococci listed do not match the classical industrial process for citrate.

Step-by-Step Solution:

Verification / Alternative check:

Industrial bioprocess references and FAO/WHO reports cite A. niger as the organism of choice for citric acid, with optimized strains and fed strategies.

Why Other Options Are Wrong:

(a) and (d) are lactic fermenters used for dairy fermentations. (b) Saccharomyces cerevisiae is used for ethanol, not citrate. (e) Candida utilis is used for SCP (single-cell protein), not bulk citric acid.

Common Pitfalls:

Confusing any acid-producing microbe with the specific production organism; assuming yeasts dominate all food-related fermentations.

Final Answer:

Aspergillus niger