94 ELEMENTS OF GENERAL SCIENCE 



106. Relation of bacteria to decay. Since it is chiefly bac- 

 teria, yeasts, and molds which cause the change known as 

 decay in plant and animal substances, one way to prevent 

 decay is to prevent the growth of these organisms. The most 

 successful methods of preventing harm by bacteria, yeasts, or 

 molds is to kill those present by the use of heat or chemicals 

 and to prevent the entrance of others. 



The latter method is sometimes used to preserve milk, 

 formaldehyde being added in small quantities for this pur- 

 pose. Formaldehyde is effective in preserving the milk, but 

 it is very objectionable in a food material. An interesting 

 and proper use is in preserving specimens in museums. Other 

 chemicals are used in canned goods for the same purpose, but 

 in general it may be said that any chemical which will destroy 

 bacteria is not likely to be a suitable article of food. Most 

 states have laws which prohibit the use of injurious pre- 

 servatives in food. What are the laws in your state? The 

 preservation of food by means of salt, sugar, or vinegar is a 

 common and effective method. This is practiced in the can- 

 ning of fruits and other food products. Yeasts and molds 

 are present on the skins of all fruits and sooner or later 

 cause the fruit to spoil unless they are destroyed. In can- 

 ning, the fruit is first boiled to kill yeast and mold; it is 

 then sealed up air-tight while still hot, in order to exclude 

 all dependent plants ; and if both operations have been suc- 

 cessful (that is, if there are no living things in the can and 

 no opportunity exists for them to get in) the fruit will keep 

 indefinitely. Yeast and bacteria cannot grow if a good deal 

 of sugar is added to the fruit. It is a matter of common 

 experience that jellies and jams may be kept for a long 

 time without being tightly sealed. This is not a satisfactory 

 protection against molds, however, unless the food is kept 

 in a cool, dry place. 



The process by means of which all life of every kind is 

 destroyed by heat, chemicals, or other means is known as 



