Question

In lactococcal fermentation, acetaldehyde is formed due to

• A. Lipid metabolism
• B. Carbohydrate metabolism
• C. Arginine hydrolysis
• D. Threonine metabolism

Hint:

Acetaldehyde Synthesis in Starters: Precursor = L-THREONINE. Enzyme = THREONINE ALDOLASE $\rightarrow$ Cleaves Threonine into Glycine + Acetaldehyde.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Bacterial aldehyde biogenesis: in Lactococcus lactis and Lactobacillus bulgaricus, acetaldehyde is synthesized via threonine aldolase cleavage of amino acid L-threonine into glycine and acetaldehyde.
Key Formula or Approach:
\[ \text{L-Threonine} \xrightarrow{\mathbf{Threonine \text{ } Aldolase \text{ (Glycine Cleavage System)}}} \text{Glycine} + \mathbf{Acetaldehyde} \]

Step 2: Detailed Explanation:
In metabolic biochemistry of dairy starter cultures:
- In Lactococcus lactis (and Lactobacillus delbrueckii subsp. bulgaricus):
- Although minor amounts of acetaldehyde can arise from pyruvate decarboxylation in carbohydrate glycolysis,
- Acetaldehyde is predominantly formed via Threonine Metabolism (D) catalyzed by the intracellular enzyme Threonine Aldolase (EC 4.1.2.5).
- Threonine aldolase directly hydrolyzes the amino acid L-Threonine into Glycine and Acetaldehyde, supplying the essential flavor aldehyde that defines the aroma profile of yoghurt and cultured milks.

Step 3: Final Answer:
Therefore, acetaldehyde is formed due to Threonine metabolism, matching option (D).