18 



The American Public Health Association 



ciencies of the design and operation of 

 heating or pasteurizing processes. 



Bacterial Flora in Raw Milk. The 

 floras of raw milk are obviously sub- 

 ject to extensive variations as to ori- 

 gin, character and condition of the bac- 

 teria and of the effects of the growth 

 or other activities of the same on each 

 other and on the product itself. 

 1 The bacterial flora of a milk about 

 to be pasteurized is derived from many 

 sources, i. e., from the cow, i. e., milk 

 ducts and reservoirs, udder, hair, skin, 

 feces, saliva, etc., and from the subse- 

 quent growth of the same if any in the 

 milk ; from the human beings handling 

 and milking the cows, i. e., skin, urine, 

 feces, saliva, etc., and from the subse- 

 quent growth if any of the same in the 

 milk; from the surroundings of the 

 cow and human attendants, i. e., hay, 

 grains and other feeds, soil, water, air, 

 etc., and from the subsequent growth if 

 any of the same in the milk; from flies, 

 insects and other animal contamina- 

 tions and from the subsequent growth 

 of them in the product; from the farm 

 utensils with which the milk on pre- 

 vious occasions has come in contact, 

 the flora on the surface of which may 

 be the result of failures in proper 

 cleaning and are, therefore, of origins 

 already mentioned, with or without 

 subsequent growth due to bacterial in- 

 cubation on moist apparatus surfaces 

 or in cracks and crevices; or they may 

 be due to the direct contamination of 

 the milk handling apparatus from any 

 or all of these sources of milk pollu- 

 tion just mentioned and from the incu- 

 bation of these added micro-organisms 

 on the moist and greasy surfaces of the 

 utensils, cans, etc. 



Factors Causing Changes in Bac- 

 terial Flora. The combined floras 

 from these various sources may have 

 been greatly modified by the time the 

 product reaches the pasteurization sys- 

 tem by action of bacterial antagonisms 

 or associated growths, the character of 



which may have been subject to change 

 by the likely alterations in tempera- 

 tures, by agitations and other physical 

 states to which milk in transit is con- 

 tinually exposed. These may likewise 

 be influenced by the character of the 

 metallic surfaces with which the milk 

 has been in contact, such as brass, cop- 

 per, and inappropriate metallic combi- 

 nations. 



Chemical Treatment of Milk. There 

 occur frequently outbreaks of effort to 

 keep down the bacterial counts in raw 

 milk by various chemical treatment of 

 milk and of milk handling apparatus. 

 Those in which the germicide or pres- 

 ervatives have been added directly to 

 the product itself have been but short 

 lived. They are not often indulged in 

 at present. Those in which the germi- 

 cides are utilized in the sterilization or 

 prevention of bacterial incubation in 

 certain forms of milk handling appa- 

 ratus, such as rubber milking machine 

 parts, are not objectionable when the 

 product used is harmless and special care 

 is taken to remove the germ killing or 

 restraining solutions by thorough rins- 

 ing. Where this rinsing is not done 

 the solution may gain direct access to 

 the milk subsequently handled in the 

 apparatus. As the use of such solu- 

 tions is extended to the washing or 

 sterilization of pails, cans, etc., fairly 

 substantial amounts of even more than 

 one solution each having its own sep- 

 arate chemical characteristics, may 

 finally find their way into the raw 

 product. Here we must consider the 

 probable selective germicidal effect 

 upon different types of bacteria and 

 other micro-organisms which, if some 

 subsequent incubation occurs, is very 

 likely to upset that important and more 

 or less normal "balance" between the 

 numbers of acid producing, inert, and 

 alkali developing groups of bacteria. 

 Any procedures liable to suppress or 

 eleminate the natural preponderance of 

 the non-spore forming acid types 



