532 MICROBIOLOGY OF FOODS 



which hold about 450 g., and this quantity is equivalent to about i J^ 1. 

 of normal milk. The addition of cane sugar acts as a preservative, 

 and although the finished product may contain some living organisms, 

 it is said to keep indefinitely if unopened, and will even keep for a 

 number of days after opening. Occasional losses do occur by spoilage 

 of the finished product, either from the growth of occasional types 

 of bacteria tolerant of the high percentage of cane sugar, or from 

 yeasts. 



PROCESSING AND STERILIZATION 



ECONOMIC CONSIDERATIONS. For certain classes of food products, 

 pasteurization is widely applicable, and is of great value from an 

 economic standpoint. Preservation by pasteurization is at best, how- 

 ever, temporary. Bacterial spores are certain to be present on many 

 kinds of foods, and these, unharmed by pasteurizing temperatures, 

 develop vegetative cells, and spoilage occurs. 



For permanent preservation therefore, a higher temperature and 

 longer periods of time must be adopted. The final heating of canned 

 foods for permanent preservation has formerly been termed steriliza- 

 tion. In the light of recent researches, this terminology must be 

 modified. 



As stated in a previous paragraph, it has been shown by several 

 investigators that canned food may keep for a long period of time al- 

 though not in a sterile condition, possibly containing viable spores of 

 bacteria. These spores are unable to grow in the food due to the ab- 

 sence of oxygen. The principle employed in the canning of food 

 to-day is the same as that of Appert over 100 years ago. Although he 

 knew nothing of microorganisms or their relation to the spoilage of food, 

 Appert's experiments taught him that not only must the food to be 

 conserved be heated thoroughly, but it must also be so sealed as not to 

 allow air to enter the container. 



With the development of knowledge of microbiology it was con-' 

 sidered that the success of the process did not depend so much upon 

 keeping out the air, but upon keeping out organisms which might be 

 carried in the air. This is true provided the canned food is sterilized. 

 As recently proven, however, commercially canned foods may keep 

 perfectly although not actually sterile, provided a vacuum is established 

 in the can thereby preventing the viable spores of bacteria from 

 developing. 



