180 AGRICULTURAL BACTERIOLOGY 



tures equaling or exceeding the boiling-point. The former 

 method is employed when it is not desired to keep the ma- 

 terial for long periods, as in the commercial handling of 

 milk. It is also used when the presence of bacterial spores 

 is of no importance, because of the fact that their germi- 

 nation is prevented by the reaction of the material. The 

 higher temperature is employed in the preparation of 

 canned foods where complete destruction of bacterial life 

 is necessary to preserve such material for relatively long 

 periods of time. 



Pasteurization of milk. The organisms concerned in the 

 spoiling of milk are non-spore-forming bacteria, and are 

 easily destroyed. Milk may also contain pathogenic or- 

 ganisms which it is necessary to destroy. Fortunately, 

 those disease germs that are likely to be spread through the 

 agency of milk are non-spore-forming, so that protection 

 of milk supplies may be secured through pasteurization. 



In the treatment of any food with heat, the physical 

 effect on the material must be considered. In many cases 

 the chemical and physical changes are such as to injure 

 the commercial value of the food. If milk is heated above 

 145 F. for any length of time, the process of creaming 

 is much retarded. The fat globules in milk are not uni- 

 formly distributed throughout the entire milk, but are 

 grouped in masses that present a relatively smaller surface 

 in proportion to their volume than do the separate globules. 

 The viscosity of the serum is such that it offers a certain 

 amount of resistance to the rising of the fat. The larger 

 the mass of fat the more readily does it rise to the surface. 



In the commercial handling of milk in bottles, it is de- 

 sirable that the creaming take place rapidly, since an in- 

 distinct or thin cream line is considered by the consumer as 

 indicating a milk low in fat content. The cooked taste 

 that is imparted to milk by too high a degree of heat is ob- 



