Fermentations of Milk. 91 



atures favor their growth, most rapid development tak- 

 ing place at 100 to 103 F. 



Spontaneous fermentation of milk. The normal 

 souring of milk is due to a mixture of these two groups 

 o bacteria. The relative proportions existing between 

 the two in any sample of milk is dependent on a number 

 of factors, most important of which is the degree of 

 cleanliness exercised in the production of the milk. 

 "Where careless conditions obtain under which dust and 

 manure particles find their way into milk, it becomes 

 more abundantly seeded with gas-generating bacteria,, 

 and consequently, the type of fermentation is undesir- 

 able. If, however, the milk is drawn into clean utensils 

 and care is taken to exclude dirt, the pure lactic acid 

 types are able to control the character of the changes 

 produced, and a clean, pleasant tasting liquid results. 

 It will be seen that things are well arranged by nature ; 

 one of the most important food products undergoes a 

 type of decomposition that is not offensive and when 

 produced under clean conditions, the sour milk is as 

 healthful a food as is the fresh product. Thus there is 

 every reason for cleanliness in the production of milk,, 

 for cleanliness' sake and because clean milk means bet- 

 ter products, and greater returns to everyone, producer 

 and dealer. 



There are other kinds of acid-forming bacteria in milk 

 but they are of small importance compared with those 

 just discussed. Some of the bacteria derived from the 

 inside of the udder of the cow form acid, but these forms 

 grow very slowly in milk at ordinary temperatures, and 

 have no influence on the keeping quality. 



