784 BACTERIA : THEIR WORK 



In addition to the production of butyric acid from the action 

 of these organisms upon carbohydrates and lactates, it is pro- 

 bable that this acid is sometimes produced in the fermentative 

 decomposition of proteids by various species of bacteria. 



Butter which has developed the peculiar aroma and flavour 

 termed 'rancidity' contains butyric acid as well as other com- 

 pounds possessing a disagreeable odour. 



5. Acetic Fermentations. — The surface of beer and wines 

 containing not more than 14 per cent, of alcohol when ex- 

 posed to the air for a few days becomes covered with a thin 

 tough whitish skin or filmy scum and the liquids turn sour. 

 The film on examination is found to be a bacterial zooglcea 

 (p. 770), the individual organisms forming it being united 

 together by the swollen gelatinous external portions of their 

 cell-membranes. The sourness is due to the activity of 

 the bacteria which produce acetic acid from the alcohol 

 originally present in the liquids, the oxygen necessary for 

 the chemical change involved in the process being derived 

 from the air. The fermentation does not always cease with 

 the production of acetic acid, for as soon as all the alcohol 

 has disappeared the organisms attack the acetic acid, which 

 has been formed and oxidise it to carbon dioxide and 

 water. 



Acetic fermentation is the basis of the manufacture of vinegar 

 from fermented malt-liquors or fermented grape-juice, and the 

 gelatinous zooglcea is sometimes spoken of as the 'mother of 

 vinegar,' or the ' vinegar-plant' The bacteria constituting 

 these ' vinegar-plants ' are not always the same, several distinct 

 species being recognised. Most of them carry on their work 

 vigorously at 27° C. : those best known are Bacterium aceti, 

 B. Pasteurianum, B. Kutzingianum of Hansen, Bacterium aceti 

 and Bacterium xylinum of Brown, and B. oxydans and B. 

 acetosum of Henneberg. The three former species are ex- 



