104 MICRO-ORGANISMS AND FERMENTATION. 



many foreign micro-organisms, attempts are made to suppress 

 the dangerous kinds (mycoderma, mould, etc.), by the addition 

 of about 2 per cent, of acetic acid. 



Finally, an acetic acid bacterium may be mentioned, 

 found by J. C. Holm on cocoa beans, which he has named Bact. 

 aceticum rosaceum. It forms short, rounded, motionless rods 

 1'6 fjL in length, single or in pairs. On wort or beer it forms 

 a very weak, pale-coloured film, whilst the colonies on wort- 

 gelatine and agar are distinctly pink in colour. 



Acetic acid bacteria play an important part in the fer- 

 mentation of beer, spirits, and wine. They do much harm, 

 especially in wine, and if they once attain a strong develop- 

 ment, the wine is irretrievably spoilt. 



In low-fermentation breweries they usually do less mischief, 

 as their growth requires a high temperature and an abundant 

 supply of air. Thus, they are readily suppressed in a well- 

 arranged lager beer cellar. Hansen's experiments have shown 

 that Bact. aceti and Bact. Pasteurianum are able to exist during 

 the whole time of storage, whether the infection takes place at 

 the beginning or end of the principal fermentation. In his 

 experiments the contamination, however, did not manifest 

 itself during the whole course of the fermentation either by 

 the taste or by the smell of the beer. When the beer was 

 bottled, and exposed to a higher temperature, the bacteria 

 developed further ; yet, when the bottles were well corked, 

 the beer did not turn sour. Just the same result was arrived 

 at when the finished beer was infected. If, on the contrary, 

 the bottles were badly corked, the growth turned the beer 

 sour. 



In high-fermentation breweries, on the other hand, where 

 fermentation is carried on at higher temperatures, these 

 bacteria are capable of doing much mischief, even before 

 the beer leaves the brewery. 



It is of practical interest to note that the species described 

 by Hansen exert no influence on the colour or brightness of the 

 beer, whilst most other bacteria cause turbidity. 



In distilleries, and more especially in air-yeast factories, 

 acetic bacteria may occur in large quantities, as shown by 

 numerous experiments made by the author. They are most 



