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The Source of the Volatile Acidity Produced in Milk by the Citric Acid Fermenting Streptococci

Iowa State College, Ames, Iowa

ABSTRACT

Lactic acid increased the volatile acidity produced in milk cultures of the citric acid fermenting streptococci, but similar increases were secured with tartaric, phosphoric and sulphuric acid and, accordingly, it appears that lactic acid is not a source of the volatile acidity produced by these organisms. The influence of the various acids presumably involves the establishment of a hydrogen ion concentration that is very favorable for the production of volatile acid in milk by these organisms and may include the freeing of citric acid from its salts. The effect of acids in increasing the volatile acidity produced in milk varied greatly with different cultures and was greatest with cultures that produced comparatively small amounts of titrable acidity and least with cultures that produced comparatively large amounts of titrable acidity. Added citric acid gave such large increases in the volatile acid production of the organisms in milk cultures that it is evidently a source of volatile acid.

The importance of citric acid as a source of volatile acid with the citric acid fermenting streptococci was also shown by striking increases in the volatile acid production when citric acid was added to a modified beef infusion bouillon or to milk which had been fermented to remove the citric acid and then distilled to drive off the volatile acids; with the bouillon glycollic, lactic, beta-hydroxypropionic, succinic, malic and tartaric acids failed to give an increase in volatile acid while with the fermented milk lactic acid failed to give an increase.