222 OUTLINES OF BACTERIOLOGY 



placed is maintained at a temperature best suited for the growth of 

 these bacteria, viz. 18-22 C., and the casks are kept closed, except 

 for a small vent at the top, for the admission of air. The closure of 

 the casks keeps off extraneous organisms that would otherwise drop in 

 from the atmosphere and develop opposition fermentations ; but the 

 vent is also necessary because the acetic-acid bacteria are very strongly 

 aerobic. At the end of every week some more wine is added to each 

 cask, until finally each contains about forty gallons. By this time the 

 acetic-acid bacteria have done their work, and the wine has been 

 changed into vinegar. When once started a certain portion of the 

 vinegar is periodically withdrawn, and its place taken by wine. A 

 cask can be used in this way for six to eight years without interrup- 

 tion, but at the end of that time it has to be cleaned out to get rid of 

 the impurities which have accumulated at the bottom of the cask. 



(b) The other method is called the "quick vinegar method." The 

 raw material in this case is spirit. Spirit diluted with vinegar, techni- 

 cally termed "goods," is run over shavings of beechwood or birchwood 

 contained in vats which are closed except for a few vents. The acetic- 

 acid bacteria are present in the vinegar constituent of the " goods." 

 They settle on the shavings and ferment the alcohol, as it slowly 

 trickles over the shavings, into acetic acid. 



The number of known acetic-acid bacteria is comparatively numerous, 

 but very few have been accurately investigated. The best known are 

 Bacterium aceti, Bacterium Pastorianum, Bacterium Kutzingianum. 



Bacterium Aceti (Kutzing) Hansen is frequently present in the dust 

 of the air, and occasionally in water. It grows well in " double " beer 

 and in wort. In the former, when cultivated at 34 C., it forms 

 after 24 hours a smooth gelatinous film. The individuals of the 

 film are usually hour-glass shaped, and arranged in chains (Fig. 123). 

 At 40 C. long thin threads are formed. On wort gelatine grey, waxy, 

 round colonies with sharp contours are formed. The colonies consist 

 of small free rod-bacteria. The maximum temperature of growth in 

 " double " beer is about 42 C., the minimum 4-5 C. When a film is 

 formed the mucilage binding the individuals together is not stained by 

 a solution of iodine. 



Bacterium Pastorianum Hansen. This species forms in "double" 

 beer at 34 C. a dry wrinkled film after 24 hours. In general 

 appearance the individuals are somewhat larger and thicker than those 

 of Bacterium aceti when growing in this medium (Fig. 124). The 

 threads which are formed when this organism is cultivated at 40 C. 

 are similarly thicker than those of Bacterium aceti at the same 



