Question

During ripening of cheeses, citrate metabolism leads to formation of

• A. Butyric acid
• B. Propionic acid
• C. Diacetyl
• D. Lactic Acid

Hint:

Citrate fermentation is the biochemical pathway utilized by aroma-producing lactic acid bacteria to generate diacetyl, acetoin, and carbon dioxide (which forms tiny holes or eyes in some cheeses).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Citrate is a minor but highly important organic acid present in milk.
During the cheese ripening process, non-starter and starter lactic acid bacteria metabolize citrate through a specialized pathway.
This pathway leads to the production of volatile compounds that are essential for the characteristic aroma of many cheese varieties, such as Gouda and Cheddar.
The key aromatic compound generated through this metabolism is diacetyl, which provides a distinct buttery flavor.

Step 2: Detailed Explanation:
Citrate is transported into the bacterial cell by the enzyme citrate permease.
Inside the cell, citrate is cleaved by citrate lyase into oxaloacetate and acetate.
Oxaloacetate is subsequently decarboxylated to pyruvate by oxaloacetate decarboxylated.
The excess pyruvate generated from this pathway cannot be entirely directed to the main lactic acid pathway.
Instead, it is converted into active acetaldehyde, which condenses with another molecule of pyruvate to yield $\alpha$-acetolactate.
Under aerobic or oxidized conditions, $\alpha$-acetolactate undergoes non-enzymatic oxidative decarboxylation to form diacetyl:
\[ \alpha\text{-Acetolactate} + O_2 \xrightarrow{\text{Oxidation}} \text{Diacetyl} + CO_2 \]
Diacetyl can be further reduced by the bacteria to acetoin and 2,3-butanediol, which have much higher flavor thresholds.
Other organic acids like butyric and propionic acids are produced via lipolysis or specific bacterial fermentations (such as propionibacteria in Swiss cheese), not directly from citrate.
Therefore, diacetyl is the major aroma compound resulting from citrate fermentation.

Step 3: Final Answer
Citrate metabolism during cheese ripening leads to the formation of the flavor compound diacetyl.