MILK 17 



up a definite series of changes in milk but even these are to be accounted 

 for rather in the fact that they are commonly found in the surroundings 

 with which milk comes into touch than that milk is their natural habitat. 



Classification of Bacteria Found in Milk. While the bacteria in 

 milk are a heterogenous lot it is possible to separate them into groups and 

 it is helpful to do so for it simplifies the problem they present by enabling 

 one to clearly distinguish the useful and important organisms from those 

 that are hurtful and those not of material interest. There are several 

 classifications, but that in Table 10, a slight modification of that of 

 Hastings, is believed to be as serviceable as any. 



Importance of the Various Classes of Bacteria. With regard to the 

 classes as a whole it may be said that in the first, the organisms of groups 

 1 and 2 are most important. They are the ones that effect the ordinary 

 souring of milk; the value of the product is in no small degree determined 

 by which of them participates most fully in the process. The bacteria 

 of the first group give milk a pleasant mildly acid flavor, whereas those 

 of the second give it a sharp tang and other tastes of a less definite 

 character. Moreover, the microbes of the second group are the par- 

 ticular enemies of the cheese maker, because they are the cause of 

 gassy curds. When slovenly conditions surround the production of milk, 

 manure and dust in which these microbes abound, will get into the milk 

 in large quantities, and heavily seed it with the organisms. They are 

 best controlled by scrupulous attention to cleanliness and keeping 

 the milk well cooled. As a rule organisms of group 2 outnumber those 

 of group 1 at the outset, but are soon overgrown by the latter. In fine, 

 the effect of these organisms is to keep milk from putrefying; were it 

 not for them, bacteria of the second class would find ample opportunity 

 for development, and milk would be more perishable than it now is, and 

 its consumption would be attended with a certain sort of danger from 

 which it is now largely free. Bacteria of the third group have rapidly 

 acquired commercial importance since their discovery. Lang has shown 

 that they are particularly adapted for culturing buttermilk that is 

 obtained from churning pasteurized cream. 



The bacteria of class 2 are wholly objectionable; not only do they 

 produce bad flavors in milk but they rot it and produce decomposition 

 products that arouse suspicion because they are evolved from proteins. 

 Moreover it is all but certain that some of these germs are the cause of 

 serious intestinal disturbances in children. Their presence in milk is 

 believed to indicate uncleanly methods of production. The pasteuriza- 

 tion of milk at high temperature, by killing off all vegetative forms and 

 the lactic acid bacteria in particular, may give an opportunity for these 

 putrefactive forms to develop, which is one reason that has brought the 

 "holder" process of pasteurizing into favor. 



The microbes of class 3 betray their presence to the consumer; con- 



