342 MILK 



can be found in most milks, and these groups must form the sub- 

 ject of our study. 



According to their ability to grow best at different temperatures 

 bacteria in milk are sometimes classified as follows: 1, Cryoflora, 

 those multiplying at 5 to 20 C.; 2, Mesoflora, those multiplying 

 at 20 to 37 C.; 3, Thermoflora, those growing above 37 C. 

 This classification is, in reality, of little value, because the limits 

 of temperature at which bacteria grow overlap considerably, and 

 different strains of the same type may have different optimum 

 temperatures. The majority of bacteria in milk belong to the 

 mesoflora, although the other two groups have not yet been 

 sufficiently studied. Especially is this true of the cryoflora. 

 These may constitute a very important group, since possibly the 

 bacterial flora of ice-cream belongs to this group. Furthermore, 

 it is claimed by some authors that milk kept near the freezing- 

 point will soon teem with bacteria that are able to grow at this 

 temperature, and that these are not 'desirable. In view of the 

 importance of keeping milk cold for preservation, the cryoflora 

 should receive attention. The above-mentioned Bacillus bul- 

 garicus group contains some members that multiply at tempera- 

 tures considerably above 37 C. The function of this group of 

 bacteria has not been well understood. They are active in the 

 formation of some fermented milks and probably in the ripening of 

 some cheeses. 



Aside from the large number of saprophytic types that com- 

 monly occur in milk, pathogenic organisms may be present under 

 unusual conditions. Epidemics due to milk-supplies have been 

 noted, although it must be admitted that disease is actually dis- 

 seminated through milk in surprisingly small proportion when we 

 consider the enormous amount of milk and milk products that are 

 consumed. Disease germs gain access to milk chiefly from two 

 sources, namely: 1, from the cow, as, for example, bovine tuber- 

 -culosis, foot-and-mouth disease, and others; and 2, from the 

 milkers and others concerned in the handling of milk. Among 

 infections of the second group are typhoid fever, diphtheria, 

 scarlet fever, sore throat, and others. The important subject of 

 transmission of disease through milk will be dealt with in a 

 separate chapter. 



For the present we are concerned with the saprophytic organ- 

 isms in milk, and these will be discussed for convenience under 

 the following grouping: 



1. Lactic acid bacteria. 



2. Spore-bearing bacteria. 



3. Bacteria causing abnormal conditions in milk. 



4. Molds, yeasts, and torulae. 



