54 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



colonies. It ferments lactose more slowly and requires 

 a higher temperature than the Streptococcus lacticus. 

 Acetic acid is the principal product of the lactose fermen- 

 tation, although lactic and succinic acids and gas are 

 also formed. Gas bubbles are rather numerous in the 

 curd and the fluid expressed from the latter is not always 

 clear. The sour taste of the milk is frequently un- 

 pleasant. The Bacterium acidi lactici is regarded by some 

 bacteriologists as a distinct species with strains showing 

 differences resulting from environment and by others 

 as the type of a number of species or varieties. The 

 mastitis organisms, Bacillus Guillebeau (a and b) 9 and 

 some of the bacteria which produce slimy or viscid milk 

 are closely related forms. 



Several varieties of long, thin, rod-shaped organisms, 

 of which the Bacterium bulgaricus is a type, also form 

 acid from lactose, but they operate so slowly at the usual 

 temperatures that they are not a factor in the ordinary 

 souring of milk. They are chiefly of interest because of 

 their use in the preparation of the oriental milks (mazun, 

 kefir, yoghurt) . The organisms of the Bacterium bul- 

 garicus group are usually present in ensilage and those 

 found in milk are no doubt derived directly or indirectly 

 from this source. 



The temperature at which the milk is kept has an 

 important influence upon the character of the lactose 

 fermentation. In milk kept at 15 to 20 C. (59 to 68 

 F. ) , the organisms of the Streptococcus lacticus type will 

 grow much more rapidly than those of the Bacterium 

 acidi lactici type. The Streptococcus lacticus grows quite 

 well at 15 C. (59 F.) and continues to grow at 10 

 C. (50 F.) , while the Bacterium acidi lactici grows bet- 

 ter at higher temperatures and practically stops grow- 



