Citric acid from sucrose: what initial medium pH is commonly set when sucrose is the substrate for Aspergillus niger production runs?

Introduction / Context:
Citric acid accumulation depends on carbon source, trace metals, and pH strategy. With sucrose as substrate, classical practice sets the initial pH higher than the final pH observed during production, which later drops as acid accumulates.

Given Data / Assumptions:

• Organism: Aspergillus niger.
• Substrate: sucrose (refined sugar).
• Question focuses on initial set pH of the medium.

Concept / Approach:

Traditional guidance: when using sucrose or glucose solutions, the initial pH is typically adjusted around 5–6 to optimize spore germination and early growth. During fermentation, pH progressively decreases (often to near 2) as citric acid accumulates; low pH also minimizes contamination and by-products. Molasses media may be set somewhat lower due to impurities and buffering differences, but refined sucrose media commonly start near pH 5.

Step-by-Step Solution:

Verification / Alternative check:

Process descriptions in fermentation manuals cite initial pH 5–6 for sucrose media; the broth later acidifies substantially to near pH 2–3 during peak citrate formation.

Why Other Options Are Wrong:

(a) and (b) are more typical of later stages or specific molasses protocols; (d) 6 can be used but 5 is the more commonly cited single value; (e) 2 is a typical late fermentation pH, not an initial set-point.

Common Pitfalls:

Confusing initial set pH with the final pH after acid accumulation; assuming the same pH target for all carbon sources regardless of buffering and impurity profiles.

Final Answer:

5