BUTYRIC ACID FERMENTATION 107 



The economic function of tlie lactic ferments concerns, of course, the 

 manufacture of butter and cheese. 



_ Van Laer has described a saccharo-bacillus which produces lactic 

 acid amongst other products, and brings about a characteristic 

 disease in beer, named tourne. The liquid gradually loses its bright- 

 ness and assumes a bad odour and disagreeable taste. The bacillus 

 is a facultative anaerobe. A number of workers have separated 

 organisms having a lactic acid effect, which diverge considerably from 

 the ordinary type of lactic acid bacillus. 



4. Butyric Acid Fermentation 



Cause, Bacillus butyricus and other forms ; medium, milk, butter, sugar and 

 starch solutions, glycerine; result, butyric acid. 



_ When sugars are broken down by Bacillus acidi lactici the lactic 

 acid resulting may, under the influence of the butyric ferment 

 become converted into butyric acid, carbonic acid, and hydrogen. 

 Neither butyric acid nor lactic acid is as commonly used as alcohol 

 or vinegar. Both, like vinegar, can be manufactured chemically, 

 but this is rarely practised. Butyric acid is a common ingredient 

 in stale milk and butter, and its production by bacteria was 

 historically one of the first bacterial fermentations understood. 

 Moreover, in its investigation Pasteur first brought to light the 

 fact that certain organisms acted only in the absence of oxygen. 

 In studying a drop of butyric fermenting fiuid, it was observed 

 that the organisms at the edge of the drop were motionless and 

 apparently dead, whilst in the central portion of the drop the 

 bacilli were executing those active movements which are character- 

 istic of their vitality. To Pasteur's mind this at once suggested 

 wha;t he was able later to demonstrate, namely, that these bacilli 

 were paralysed by contact with oxygen. When he passed a stream 

 of air through a flask containing a liquid in butyric fermentation, 

 he observed the process slacken and eventually cease. So were 

 discovered the anaerobic micro-organisms. The aerobic ferments 

 give rise to oxidation of certain products of decomposition; the 

 anaerobic organisms, on the other hand, only commence to grow 

 when the aerobic have used up all the available oxygen. Thus in 

 such fermentations certain bodies (carbohydrates, fatty acids, etc.) 

 undergo decomposition, and by oxidation become carbonic ,acid gas, 

 and the remainder is left as a " reduced " product of the ^hole 

 process. Hence sometimes this is termed fermentation by reduction. 

 The chemical formula of this butyric reaction may be expressed 

 thus : — 



CjHjgOg (by simple decomposition) = 2 CgH^Og, < ' 



Glucose. Lactic apid. ^ 



