458 MAN CONTROLS HIS ENVIRONMENT FOR HEALTH 



valve 



gauge, 



Why anl when is a pressure cooker used? 



Sterilization. Bacteria grow very slowly, if at all, in the tem- 

 perature of an ice box, very rapidly from 70° to 98°, and much less 

 rapidly (or are killed) at a higher temperature. Those bacteria 

 which form spores resist a great deal of heat and may even be 



boiled for some time without 

 injury. The practical lessons 

 drawn from these facts are 

 many. We boil our drinking 

 water if we are uncertain of its 

 purity ; we cook foods that we 

 believe might harbor bacteria, 

 and thus keep them from spoil- 

 ing. The industry of canning 

 is built upon this method of 

 sterilization. 



Canning. Canning is simply 

 a method by which first the 

 bacteria in a substance are killed 

 by heating and then the substance is put into vessels and covered 

 so that no more bacteria can gain entrance. The use of canned 

 goods has completely changed the life of the sailor and the soldier, 

 who in former times used to suffer from various diseases caused 

 by lack of a proper balance of food. 



Practical Exercise 1. What is the " cold pack " method of canning ? What 

 scientific principles are used in canning ? 



Cold storage. Man has learned to use cold to keep bacteria 

 from growing in foods. The refrigerator at home and cold storage 

 on a larger scale enable us to keep foods for a more or less long 

 period. If food is frozen, as in cold storage, it might keep without 

 growth of bacteria for years. But frozen foods after thawing are 

 particularly susceptible to the bacteria of decay. For that reason 

 products taken from cold storage must be used as soon as possible. 



Demonstration 2. To determine the effect of pasteurization on the 

 keeping quality of milk. 



Place half of the milk in a sterilized jar, cover, and leave in a warm 

 place for 24 to 48 hours. 



Place the remainder of the milk in another jar, cover, and put it 

 in the double boiler or pasteurizing apparatus. Keep the hot water 



