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Studies on Starter Metabolism. IV. Effect of Various Substrates on the Formation of Acetoin by a Mixed Strain Starter Culture^{1 ,2,}

Department of Dairy Husbandry, University of Minnesota, St. Paul

ABSTRACT

The mechanism and the source of carbon for the formation of acetoin by a mixed culture consisting predominantly of Streptococcus cremoris and Leuconostoc species have been studied, using both a reconstituted milk and a dialyzed milk medium.

Of the various metabolites added to the dialyzed medium, only citric, oxalaoetic, pyruvic, and alpha-keto butyric acids stimulated the formation of ketol; the alpha-keto butyric acid gave rise to a ketol tentatively identified as 3-hydroxy-2-pentanone. Acetaldehyde, isobutyraldehyde, and crotonic acid inhibited acetoin formation. The tricarboxylic acid cycle intermediates, malic, succinic, cis-aconitic, and alpha-keto glutarie acid, had no effect.

Using the dialyzed milk medium, it was shown that the addition of glucose alone did not result in the formation of acetoin, regardless of the initial pH of the medium. When citric acid alone was added, only a trace of acetoin was formed at pH 7.0, but a considerable amount was formed at pH 5.0. When glucose and citric acid were added, a considerable amount of acetoin was formed, regardless of the initial pH.

Studies with various metabolic inhibitors indicated that, in general, the acid formation was more readily inhibited than was acetoin formation. At an inhibitor concentration such that a partial inhibition of the formation of acetoin was manifested, the addition of more citric acid overcame the inhibition.

Alpha-acetolactic acid decarboxylase activity was demonstrated in the ground cells of a Leuconostoc isolate obtained from the mixed culture.

¹ Paper No. 3978, 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, Minnesota.