Question

Which of the following amino acids can act as a source for acetaldehyde production by lactococci ?

• A. Valine
• B. Leucine
• C. Lysine
• D. Threonine

Hint:

The cleavage of threonine by the enzyme threonine aldolase is the standard biochemistry pathway responsible for generating the green-apple flavor (acetaldehyde) in fermented milk products.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Lactic acid bacteria generate a wide array of volatile flavor compounds during milk fermentation.
Among these, acetaldehyde is the critical compound responsible for the fresh, green-apple flavor characteristic of yogurt and other cultured dairy products.
Although several metabolic pathways can yield acetaldehyde, the primary amino acid precursor utilized by lactococci and other starter bacteria is threonine.
This conversion is driven by a specific cleavage reaction catalyzed by the enzyme threonine aldolase.

Step 2: Detailed Explanation:
Acetaldehyde is the key aroma component in fermented milks, and its balance with other compounds like diacetyl determines the final flavor quality.
During the metabolic breakdown of milk proteins, free amino acids are released by the proteolytic system of the starter culture.
Threonine, a polar amino acid, is directly converted into acetaldehyde and the amino acid glycine.
This reaction is catalyzed by the enzyme threonine aldolase (glycine hydroxymethyltransferase, EC 5).
The chemical reaction can be represented as:
\[ \text{Threonine} \xrightarrow{\text{Threonine Aldolase}} \text{Acetaldehyde} + \text{Glycine} \]
This enzymatic pathway is highly active in both mesophilic lactococci and thermophilic yogurt cultures.
Other amino acids like valine, leucine, and lysine undergo different metabolic degradation pathways that lead to other aldehydes or branched-chain fatty acids, but do not directly yield acetaldehyde.
Therefore, threonine is the designated amino acid precursor.

Step 3: Final Answer
Threonine is the primary amino acid source for the production of acetaldehyde by lactococci.