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Starter Metabolism. V. The Mechanism of Acetoin Formation as Determined with C¹⁴-Labeled Substrates^{1, 2,}

Departments of Dairy Industries and Bacteriology, University of Minnesota St. Paul and Minneapolis

ABSTRACT

C¹⁴-labeled substrates, including uniformly labeled glucose and citric acid, 1-5-labeled citric acid, and pyruvates labeled in three different positions were used to determine the source of carbon and mechanism of formation of acetoin by a mixed starter culture consisting predominantly of Streptococcus cremoris and Leuconostoc species.

The starter culture was found to transform the C¹⁴ from Carbon Atoms 2, 3, 4, or 6 of citric acid into the acetoin molecule; whereas, Carbon Atoms 1 and 5 were not.

The addition of uniformly labeled glucose to a dialyzed milk medium containing either unlabeled citrate or pyruvate resulted in labeled acetoin, as did the addition of uniformly labeled citric acid to the dialyzed milk medium containing unlabeled glucose.

The methyl and alpha-keto carbons, but not the carboxyl carbon, of pyruvic acid were transformed into acetoin. Carboxyl carbon appeared in the CO₂.

A possible pathway for the formation of acetoin is proposed.

¹ Paper No. 4515, Scientific Journal Series, Minnesota Agricultural Experiment Station.

² Taken from data submitted to the graduate faculty of the University of Minnesota by the senior author in partial fulfillment of the requirements for the Ph.D. degree.

³ Present address: Economics Laboratory, Inc., St. Paul 1, Minnesota.