Starter Metabolism. V. The Mechanism of Acetoin Formation as Determined with C14-Labeled Substrates

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Abstract
The carbon source and the mechanism of formation of acetoin produced by a mixed starter culture consisting predominantly of Streptococcus cremoris and Leuconostoc species was studied with C14-tagged substrates: uniformly labeled glucose and citric acid, 1-5 labeled citric acid, and pyruvates labeled in the 3 different positions. The tracer substrates were added to a dialyzed milk medium in which the culture was grown. The C14 from carbon atoms 2, 3, 4, or 6 were found to be incorporated into acetoin, but not carbon atoms 1 and 5. The addition of uniformly labeled glucose to the medium containing either unlabeled citrate or pyruvate resulted in tagged acetoin. The addition of uniformly labeled citric acid to the medium containing un-tagged glucose produced labeled acetoin. When the tagged pyruvates were used, the carboxyl carbon appeared in the CO2 whereas the methyl and alpha-keto carbons were transformed to the acetoin.