52 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



tinue to grow and to ferment lactose until the acidity 

 reaches 0.8 per cent., but the growth of other bacteria 

 stops when the acidity exceeds 0.2 per cent. The acid- 

 forming bacteria, therefore, perform the very important 

 service of inhibiting the growth of the more harmful 

 bacteria. If their development is interfered with, the 

 milk will undergo putrefaction instead of souring. 



Sour milk is not harmful to healthy adults; on the 

 contrary, in certain forms (curds and whey, buttermilk, 

 kefir, etc.) it is a regular article of diet, and milk contain- 

 ing certain acid-forming bacteria is beneficial in some 

 diseased conditions. But, nevertheless, milk in which the 

 fermentation is not sufficiently advanced to cause coagu- 

 lation or even to produce an appreciable sour taste may 

 cause vomiting and indigestion in small children and in 

 persons affected with catarrh of the stomach. 



The usual cause of the spontaneous curdling of milk 

 is the Bacterium lactis acidi (Fig. 5) , also called Strepto- 

 coccus lacticus, which occurs as a coccus or as a short oval 

 or pointed bacterium, arranged in pairs, frequently in 

 short and sometimes in long chains, and forms on solid 

 media very small, white, circular or lenticular colonies, 

 many of them being situated below the surface. In addi- 

 tion to the difference in form, variations occur in the 

 ability to ferment lactose and in other biological char- 

 acters. These are regarded by some bacteriologists as 

 changes due to environment, while others consider them 

 a sufficient basis for recognizing the existence of different 

 varieties or species. In general, however, organisms of 

 the Streptococcus lacticus type ferment lactose more 

 rapidly than the other species of acid-forming bacteria, 

 forming principally lactic acid, with little or no gas. The 

 milk has a clean, sour taste, while the fluid expressed from 



