224. The decomposition of citric acid by Betacoccus cremoris

The fermentation products of Betacoccus cremoris in neutral milk are acetic acid and carbonic acid. In acidified milk or in mixed cultures of this bacterium and lactic acid streptococci the products are acetic acid, C02, diacetyl, acetylmethyl carbinol and 2–3 butylene glycol. The latter three substances may be called “C4 compounds”. Diacetyl is only formed when an oxidation with atmospheric oxygen can take place. Carbinol is found in aerobic and in anaerobic cultures. It may be reduced to butylene glycol. This reduction is more complete at lower acidity, but also depends on the strain of Betacoccus used.

All these substances are formed from the citric acid of the milk. However, small amounts of acetic acid may be produced from sugar.

From 1 mol. of citric acid are formed: 2 mol. of carbon dioxide, 1–1·5 mol. of acetic acid, 0·5–0 mol. of C4 compounds.

A reciprocal relation exists between the quantities of acetic acid and C4 compounds. When the amount of acetic acid is high the C4 compounds content is low. The higher the acidity of the medium in which the betacocci are cultivated the higher is the amount of C4 compounds. This means that the fermentation in neutral media is the extreme case of the fermentation process. Then the production of C4 compounds is minimal (0) and that of acetic acid is maximal (1·5).