30 



DAIRY BACTERIOLOGY 



practice we have to deal almost exclusively with cocci and rod- 

 shaped organisms ; special attention will therefore be paid to 

 these groups in this work. 



THE LACTIC ACID FERMENTATION 



This is the most important fermentation in dairy practice. In 

 the true lactic acid fermentation, lactic acid is practically the sole 

 product arising from the fermented sugar. The chief by-products 

 are carbonic and acetic acids. The production of carbonic acid 



FIG. 33. Thermobacierium bulgaricum. Grown in sterile milk. Stained with 

 methylene blue. The grains are round and dark blue, x 1,000. 



in recognisable amounts is a sign that the lactic acid bacteria are 

 in full vigour ; acetic acid, on the other hand, only appears in 

 appreciable quantities in stale cultures, or when the bacteria are 

 given a plentiful supply of air, either by aerating the liquid during 

 fermentation or by allowing it to expose a large surface to the air. 

 Pseudo lactic acid fermentations are characterised by the formation 

 of considerable quantities of volatile acids, gases and other by- 

 products, such as succinic acid and alcohol. In view of the fact 

 that in dairy practice only the true lactic acid fermentation can 

 be turned to economic advantage, while the pseudo lactic acid 

 fermentations generally cause trouble, the terms " true " and 



