Abstract
The dissimilations brought about by the propionic acid bacteria have received relatively little study. Phosphorylation appears to be a normal process with the intermediate formation of a hexose-monophosphate. 1 Methylglyoxal has been reported by Pett and Wynne 2 and Wood and Werkman have recently identified pyruvic acid 3 and a non-reducing carbohydrate 4 as intermediate products in the fermentation of glucose by Propionibacterium arabinosum. Virtanen 5 proposed the occurrence of lactic acid but was unable to detect its presence. Foote, Fred and Peterson 6 have reported the occurrence of lactic acid in the propionic acid fermentation. The intermediate mechanism leading to propionic acid is unknown. The fermentation of glycerol offers an opportunity of studying this phase. In this remarkable fermentation, glycerol is converted under anaerobic conditions practically quantitatively into propionic acid. No gas is produced (van Niel 10 ).
To detect intermediate compounds the following medium was fermented: Yeast extract (Difco) 0.5%, glycerol 2.0%, and calcium carbonate 2.0%. Calcium sulfite (1.0%) was included in the medium as a fixing reagent. P. arabinosum was the species employed. The fermentation took place under anaerobic conditions at 30°C. After 5 or 6 days'incubation, the fermentations were tested for aldehydes and ketones by addition of 2, 4-dinitrophenylhydrazine. An abundant precipitation was obtained. Distillation of the medium according to Peterson and Fred 7 gave a quantitative recovery of the unknown carbonyl compound. Derivatives were prepared from concentrated distillates and purified by crystallization. Table I gives the melting points obtained.
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