FOOD PRESEEVATION 175 



It has been previously learned that bacteria are not killed 

 off instantly by heat, but that the temperature determines 

 the rate of death or rate at which the microorganisms die 

 off. Other things being equal, the larger the initial number 

 of microorganisms to be destroyed the longer will be the 

 time necessary for their destruction. It is also apparent 

 that the kinds of organisms present may have some influ- 

 ence. The spore-producing bacteria are more difficult to 

 destroy than the nonsporulating types. A process com- 

 monly used preliminary to canning is termed blanching. 

 The food products, usually vegetables or fruits, are dipped 

 first into hot water, then into cold water. This process 

 shrinks the tissues somewhat, makes them somewhat firmer, 

 and tends to set the color, but also washes off many of the 

 bacteria which were present, thus reducing the initial infec- 

 tion and possibly thereby somewhat decreasing the time 

 necessary for sterilization. It is sometimes stated, further- 

 more, that bacteria are more easily destroyed when first 

 heated, then cooled (or chilled) and reheated. Several tests 

 on this point have failed to show any marked effect of this 

 1 ' cold shock." 



The second factor in determining time of sterilization is 

 the size of the food container or can. It is evident that the 

 center of any can is that portion which last reaches the 

 desired temperature. The larger the container the longer 

 will it take this central portion to assume the desired tem- 

 perature. 



The third factor is the ease with which heat may pene- 

 trate to the center of the canned material. Where the 

 particles of food are solid and are surrounded by water or 

 thin syrup, convection currents are at once set up in the 

 water and the interior heats relatively quickly by a process 

 analogous to that used in a hot water system in the heating 

 of a home. If the material lying between the food particles 

 is viscous or gelatinous, or there is not much, if any, free 



