54 Dairy Bacteriology. 



With reference to the number of kinds present, the more dirt 

 and foreign matter the milk contains, the larger the num- 

 ber of varieties found in the same. While milk may con- 

 tain forms that are injurious to man, still the great majority 

 of them have no apparent effect on human health. In 

 their effect on milk, the case is much different. Depend- 

 ing upon their action in milk, they may be grouped into 

 three classes: 



1. Bacteria that exert no appreciable effect in milk. 



2. Bacteria that are beneficial by reason of the products 

 which they form. 



3. Bacteria that are injurious on account of the effect 

 which they produce in milk. 



A surprisingly large number of bacteria that are found 

 in milk belong to the first class. Undoubtedly they af- 

 fect the chemical characteristics of the milk somewhat, 

 but not to the extent that it becomes physically percep- 

 tible. Eckles l reports in a creamery supply from 20 to 55 

 per cent of entire flora as included in this class. 



Those species that are concerned in the production of 

 proper flavor and aroma in butter, and which are also 

 cdncerned in the development of acid and possibly asso- 

 ciated with formation of cheese flavor represent the sec- 

 ond type. Many of these organisms are lactic acid-pro- 

 ducing, but in addition to these, some of the casein 

 ferments are also associated with aroma production in but- 

 ter. Under normal conditions b} r far the larger proportion 

 of bacteria present in milk belong to the lactic acid type. 

 There are always present, though, digesting species that 

 are able to grow if the lactic acid forms are killed, as in 

 pasteurizing. 



i Eckles, Bull. 59, Iowa Expt. Stat., Aug., 1901. 



