Nitrogen supplementation in citric acid fermentation: which of the following nitrogen sources are commonly added to the production medium for Aspergillus niger?

Introduction / Context:
Although citric acid is a carbon-heavy product formed under conditions that limit certain nutrients, a modest supply of assimilable nitrogen is necessary to initiate growth and support the biosynthetic machinery of Aspergillus niger. The choice and level of nitrogen source influence yield and by-product formation.

Given Data / Assumptions:

• Organism: A. niger requires nitrogen for biomass and enzyme production.
• Goal: high citric acid yield with minimal by-products.
• Common nitrogen sources: ammonium salts (e.g., (NH4)2SO4) and urea.

Concept / Approach:
Both ammonium salts and urea are classic inorganic/simple nitrogen sources for citric acid fermentations. Their concentrations are carefully controlled; excess nitrogen promotes biomass over product, whereas slight nitrogen limitation encourages overflow metabolism toward citric acid.

Step-by-Step Solution:

Verification / Alternative check:
Industrial media formulations frequently include (NH4)2SO4 and sometimes urea; both are standard.

Why Other Options Are Wrong:

• Single-source options are unnecessarily restrictive.
• Peptone is not typical for bulk citric acid due to cost and nitrogen complexity.
• Omitting nitrogen entirely impairs growth and production.

Common Pitfalls:
Over-supplying nitrogen; ignoring the impact of ammonium on pH and buffering.

Final Answer:
Both ammonium salts and urea