Industrial citric acid fermentation: Which organism is most widely used commercially to obtain citric acid by fungal fermentation?

Introduction / Context:
Citric acid is a high-volume organic acid used in foods, beverages, and pharmaceuticals. Understanding the standard production organism is foundational for discussions about substrate choice, metal ion control, and process modes (surface vs submerged) in industrial biotechnology.

Given Data / Assumptions:

• The question asks for the traditional fungal producer organism.
• Options list several molds and a yeast.
• We assume typical large-scale, submerged aerobic fermentation.

Concept / Approach:
The overwhelming majority of commercial citric acid is produced by Aspergillus niger. Its physiology, tolerance to acidic conditions, and ability to hyper-produce citric acid under controlled metal ion limitation make it ideal for industrial processes. Other organisms can produce organic acids, but A. niger is the established workhorse for citric acid.

Step-by-Step Solution:

Verification / Alternative check:
Industrial biotech references and process patents consistently center on A. niger for citric acid, validating this choice.

Why Other Options Are Wrong:

• A. oryzae: enzyme and koji fermentation organism, not the main citric acid producer.
• A. flavus: known for aflatoxin risks—unsuitable for food-grade citric acid.
• S. cerevisiae: ethanol fermentation yeast.
• Rhizopus oryzae: associated with fumaric/lactic acid production pathways.

Common Pitfalls:
Assuming any Aspergillus species is interchangeable; overlooking contamination control and trace metal effects crucial for citric acid yields.

Final Answer:
Aspergillus niger