Downstream processing of citric acid: In classic lime–sulfuric acid recovery, citric acid is first precipitated from the fermentation broth as which salt?

Introduction / Context:
Citric acid produced by Aspergillus niger must be recovered and purified. A classical downstream sequence uses lime treatment and acidification to isolate and release citric acid. Recognizing the intermediate salt is a standard question in biochemical engineering and industrial microbiology.

Given Data / Assumptions:

• Broth contains citric acid along with cells and impurities.
• Calcium hydroxide (lime) is added to precipitate a sparingly soluble salt.
• Subsequent sulfuric acid treatment liberates citric acid and forms gypsum (calcium sulfate) as a byproduct.

Concept / Approach:
Adding lime converts citric acid to calcium citrate, which precipitates and can be filtered. Acidifying with sulfuric acid regenerates citric acid in solution and yields gypsum. This route is robust, economical, and widely documented historically.

Step-by-Step Solution:
Recall the “lime–sulfuric” sequence used in classical citric acid recovery.Identify the precipitated intermediate as calcium citrate.Choose “calcium citrate” as the correct option.

Verification / Alternative check:
Many process descriptions explicitly list calcium citrate precipitation followed by sulfuric acid treatment.

Why Other Options Are Wrong:
Magnesium or zinc salts are not the standard precipitated intermediates in the conventional industrial route; “none of these” is thus incorrect.

Common Pitfalls:
Confusing the precipitation step with newer solvent extraction or ion-exchange routes; the question specifically targets the classical process.

Final Answer:
calcium citrate