178 AGRICULTUEAL AND INDUSTRIAL BACTERIOLOGY 



Food Preservation by Pasteurization. Pasteurization is 

 that process of food preservation in which the food is heated 

 to a temperature sufficient to destroy certain types of 

 undesirable bacteria, but not necessarily to destroy all living 

 microorganisms present. The process was first used with 

 wines and beers in which it was desirable to destroy certain 

 nonspore-producing bacteria. Heating such materials to 

 high temperatures would injuriously affect their flavor. By 

 careful tests the time and temperature sufficient to destroy 

 the undesirable organisms may be determined without in- 

 juring the flavor. The process has in recent times come to 

 be used most extensively with milk. In the modern milk 

 pasteurizing plants, however, the effort is made to heat the 

 milk to that temperature and for a sufficient length of time 

 to kill all disease-producing germs. Other kinds of organ- 

 isms may not be destroyed. Milk, or more particularly 

 cream, may also be pasteurized to destroy microorganisms 

 which may give undesirable flavors or aroma to the cream 

 products, particularly butter. It is evident that pasteuriza- 

 tion of milk is primarily a sanitary measure rather than a 

 means for increasing its keeping qualities. 



Preservation by Low Temperatures. The rate at which 

 microorganisms grow and at which enzymes act decreases 

 with decrease in temperature. Articles of food which may 

 be frozen without damage may be preserved almost in- 

 definitely, although even in frozen foods, such as poultry 

 and fish, there is a slow deterioration, probably due to the 

 action of enzymes. Other foods may be kept at low tem- 

 peratures above the freezing point. Such, for example, are 

 eggs and various other foods. It should be noted that at 

 low temperatures the types of organisms which may grow 

 may differ from those which develop at higher temperatures 

 and the changes brought about will not be the same. 



Preservation by Chemicals. Some foods naturally con- 

 tain substances which are preservative, for example, many 



