Question

In the citrate metabolic pathway, reduction of diacetyl results in the formation of which compound?

• A. Oxaloacetate
• B. Pyruvate
• C. Acetoin
• D. \(\alpha\)-acetolactate

Hint:

The reduction chain of flavor compounds in starter cultures proceeds as:
Diacetyl (high aroma) \(\rightarrow\) Acetoin (weak aroma) \(\rightarrow\) 2,3-Butanediol (no aroma).
This reduction explains why buttery flavor can decrease if fermented products are stored too long at warm temperatures.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Citrate metabolism in flavor-producing lactic acid bacteria (such as Lactococcus lactis ssp. lactis biovar. diacetylactis and Leuconostoc species) is responsible for the characteristic buttery aroma in fermented dairy products.
The primary aroma compound produced is diacetyl.
However, this compound is biochemically unstable and can undergo further reduction by the bacterial cells.

Step 2: Detailed Explanation:
Let us trace the metabolic pathway of citrate:
Citrate is internalized and converted into oxaloacetate, which is then decarboxylated to pyruvate.
Excess pyruvate is converted to \(\alpha\)-acetolactate, which undergoes oxidative decarboxylation to form diacetyl.
Diacetyl is highly volatile and gives the desirable buttery aroma.
Active starter culture cells contain the enzyme diacetyl reductase, which catalyzes the reduction of the intense aroma compound diacetyl into acetoin (acetylmethylcarbinol).
Acetoin is much less volatile and has a much weaker aroma than diacetyl.
Acetoin can be further reduced by acetoin reductase to 2,3-butanediol, which has almost no aroma.
Therefore, the direct reduction of diacetyl yields acetoin.

Step 3: Final Answer:
The reduction of diacetyl in the citrate metabolic pathway results in the formation of Acetoin.