In downstream processing of citric acid fermentations, calcium hydroxide [Ca(OH)2] is added to the fermentation broth (slurry). What is the purpose of this step?

Introduction / Context:
Commercial citric acid production with Aspergillus niger typically employs a precipitation and reconversion route. After fermentation, citric acid is recovered by forming an insoluble salt (calcium citrate), which is then treated with mineral acid to release citric acid and remove calcium as gypsum.

Given Data / Assumptions:

• Broth contains citric acid and impurities.
• Ca(OH)2 addition is controlled by pH and stoichiometry.
• Subsequent step uses sulfuric acid to regenerate free citric acid.

Concept / Approach:
Ca(OH)2 reacts with citric acid to form sparingly soluble calcium citrate. The solid can be filtered, washed, and then “acid cracked” with H2SO4 to produce free citric acid while precipitating CaSO4, simplifying purification.

Step-by-Step Solution:

Verification / Alternative check:
Standard flowsheets for citric acid recovery show the calcium citrate–gypsum route as a classic industrial method.

Why Other Options Are Wrong:

• Calcium carbonate/phosphate precipitation is unrelated to citric acid capture.
• Direct gypsum formation from broth without the intermediate is not the practiced sequence.
• Alkalinity alone does not sterilize nor recover product.

Common Pitfalls:
Confusing the two precipitation steps (calcium citrate first, gypsum later); overliming leading to product loss.

Final Answer:
To precipitate calcium citrate, allowing later acid cracking to free citric acid