Butter flavor biochemistry — from acetoin to the key aroma compound Milk streptococci can produce acetoin, which spontaneously oxidizes to the classic butter aroma compound. Which compound is responsible for the desirable buttery note?

Introduction / Context:
The characteristic buttery aroma in cultured butter, sour cream, and some cheeses arises from specific metabolic pathways of lactic cultures. Understanding which compound delivers that note guides starter selection and process control for flavor consistency.

Given Data / Assumptions:

• Organisms: “milk streptococci” (for example, Lactococcus lactis subsp. lactis/cremoris).
• Intermediate: acetoin.
• Reaction: spontaneous oxidation to a key flavor molecule.

Concept / Approach:
Citratemetabolism by certain lactic cultures yields acetoin and, upon oxidation, diacetyl. Diacetyl imparts the buttery aroma prized in cultured dairy. Its formation depends on citrate availability, redox state, and strain selection, while excessive reduction to acetoin can dull flavor.

Step-by-Step Solution:
Recognize the citrate-to-acetoin pathway in mesophilic starters.Recall that acetoin oxidizes to diacetyl, the buttery note compound.Choose diacetyl as the correct product.

Verification / Alternative check:
Starter culture datasheets specify diacetyl production capacity to tailor flavor profiles.

Why Other Options Are Wrong:
Acetone is a solvent odor, not butter; acetyl-CoA is a metabolic intermediate, not a flavor; butyric acid gives rancid notes; propionic acid is nutty/sweet in Swiss-type cheeses.

Common Pitfalls:
Confusing acetoin (less aromatic) with diacetyl (strong buttery impact); ignoring oxygen availability that affects redox balance.

Final Answer:
Diacetyl.