When molasses is used as the carbohydrate substrate for citric acid production by Aspergillus niger, the fermentation medium is typically adjusted to which initial pH range for optimal production?

Introduction / Context:
pH control is a key lever in citric acid fermentations. Low pH not only supports product stability but also suppresses contaminants and shifts Aspergillus niger metabolism toward citric acid over biomass or by-products such as oxalic acid and gluconic acid.

Given Data / Assumptions:

• Substrate: molasses, containing minerals and impurities.
• Organism: A. niger optimized for acid production.
• Operational target: maximize citric acid yield and minimize contamination.

Concept / Approach:
Industrially, initial pH is often adjusted around 2–3.5 depending on the process variant and feedstock. For molasses-based processes, pH around 3 is widely cited as a practical starting point that balances productivity and robustness against contamination and metal ion effects.

Step-by-Step Solution:

Verification / Alternative check:
Process manuals and case histories for molasses-based citric acid plants commonly report initial pH ~3 with subsequent natural acidification.

Why Other Options Are Wrong:

• pH 4–6 increases contamination and alters product profile.
• Neutral to alkaline pH is unsuitable for A. niger citric acid production.

Common Pitfalls:
Allowing metal contamination to interfere at higher pH; inadequate buffering leading to pH drift.

Final Answer:
pH 3