Correct answer: (3) Aspergillus

Why Aspergillus is correct

For commercial citric acid production, the main industrial microbe is Aspergillus niger, a filamentous fungus that can secrete very high levels of citric acid when grown on sugar-rich mediums under specific conditions (low pH, limited metals, high sugar). Industrial fermentation plants worldwide rely predominantly on Aspergillus niger for large‑scale citric acid manufacture, making “Aspergillus” the correct choice in this MCQ.

Explanation of each option

• (1) Neurospora

  • A genus of filamentous fungi used mainly as a genetic model (e.g., Neurospora crassa).

  • Not used as the standard commercial organism for citric acid production.

• (2) Penicillium

  • Includes species such as Penicillium chrysogenum used for penicillin production.

  • Some species can produce organic acids, but they are not the principal industrial workhorse for citric acid.

• (3) Aspergillus – Correct

  • Especially Aspergillus niger is optimized and widely used for commercial citric acid fermentation.

  • Advantages: high yield, ability to use cheap substrates (molasses, starch hydrolysates), and robustness in acidic conditions.

• (4) Saccharomyces

  • Yeast genus (e.g., Saccharomyces cerevisiae) primarily used for ethanol (alcohol) and baking.

  • Not the main industrial strain for citric acid manufacture.