CHAPTER II. 



CHANGES IN MILK CAUSED BY BACTERIA. 



Significance of bacterial contamination. The mere consid- 

 eration of the multiple sources of contamination of milk, em- 

 phasizes the truth of the statement that freshly drawn milk 

 contains a wide variety of bacterial forms. Certainly no food 

 used so universally by man is exposed during its preparation 

 to a greater variety of contaminating influences. The signifi- 

 cance of these bacteria in the subsequent changes in milk de- 

 pends upon their character, adaptability for existence in milk, 

 and upon the conditions under which milk is kept. The pre- 

 cautions exercised in restricting the growth of bacteria are of 

 greater importance in controlling the character of the milk 

 when consumed, than those exercised to prevent the initial 

 contamination. The results of changes of bacterial origin in 

 milk, concern not merely the commercial requirements that 

 it must be unaltered in taste and appearance when consumed, 

 but also have a direct bearing upon the hygiene of food. 



The constituents of milk and the form in which they exist, 

 together with the temperatures to which milk is exposed, offer 

 conditions favorable to the growth of quite a variety of bac- 

 terial groups. The carbohydrates represented by the milk 

 sugar are present in solution, a most favorable form for bac- 

 terial nutrition. The albuminous constituents, either in solu- 

 tion or in a finely divided state in suspension, are likewise 

 freely available to satisfy bacterial requirements. 



Fat alone, of the constituents of milk, resists the action of 

 the common bacteria found in milk. When consideration is 

 paid to the rapidity of the grovrth of bacteria, and to the fact 

 that decomposition of organic matter is the chief function of 

 bacteria as a class, the full significance of the vulnerability of 

 milk becomes apparent. 



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