Studies on Starter Metabolism. IV. Effect of Various Substrates on the Formation of Acetoin by a Mixed Strain Starter Culture

Abstract

The mechanism and the source of C for acetoin formation by a mixed culture consisting of predominantly Streptococcus cremoris and Leuconostoc species was studied using both a reconstituted milk and a dialyzed milk medium. Of the various metabolites added to the dialyzed medium, only citric, oxalacetic, pyruvic, and [alpha]-keto butyric acids stimulated ketol formation; 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 a-koto glutaric acid had no effect. Using the dialyzed milk medium, it was shown tnat 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 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 decarboxy-lase activity was demonstrated in the ground cells of a Leuconostoc isolate obtained from the mixed culture.