BACTERIOLOGY OP WATER AND MILK 187 



The elimination of danger from infection. The danger 

 from bacteria in milk may be eliminated by (1) having the 

 milk produced under absolutely healthy conditions cleanli- 

 ness, healthy animals, and healthy attendants, and (2) steriliz- 

 ing or pasteurizing the milk before it is used. 



1. Sterilization. This is accomplished by boiling, al- 

 though chemicals are sometimes used. Heating, however, is 

 the more preferable. There are certain serious objections- 

 raised to the use of boiled milk, such for example as (a) the 

 casein is rendered more indigestible; (b) the taste and odor 

 are disagreeably altered; (c) a large part of the C0 2 is driven 

 off, resulting in a change of phosphates and a precipitation of 

 casein and magnesium carbonates; (d) many of the fat glob- 

 ules coalesce; and (e) the lact-albumin is coagulated, giving 

 rise to the albuminous film or skin which forms on the surface 

 of boiled milk. 



The objection to the use of chemicals is their possible 

 deleterious effect upon the body. The chemicals most often 

 used are: soda, borax, boracic acid, salicylic acid, quick lime, 

 and formalin. 



2. Pasteurization. This consists in heating the milk at a 

 temperature high enough and for a time sufficient to destroy 

 the objectional bacteria. In some cases it sterilizes the milk 

 but in others it does not. The process consists of two parts, 

 (1) heating the milk usually from 60 to 65 C. for 20 min- 

 utes, and (2) the cooling of the milk as quickly as possible 

 after heating it to a tenjperature of about 15 C. and keeping 

 it cool. 



Foreign matter in milk. In addition to dirt from ex- 

 traneous sources that may be in milk it must be remembered 

 that milk as it comes from the cow usually contains leucocytes 

 or pus corpuscles in considerable numbers. Market milk occa- 

 sionally contains blood itself coming from some injured or 

 diseased udder. The leucocytes in large numbers are objec- 

 tionable, as they seem to interfere with the digestion of the 



