PRESERVATION BY THE USE OF CHEMICALS 261 
freezing. The nearer to freezing a fermentable substance is kept, 
the greater the delay of the bacterial growth. In the large cold- 
storage houses the food which is to be preserved may be cooled 
to a temperature below freezing, and is, consequently, actually 
frozen. At this temperature the bacteria never act and the ma- 
terial may be kept indefinitely, although it is claimed that some 
molds can grow at temperatures below freezing. It must be 
remembered, however, that the low temperatures do not kill the 
bacteria, but simply delay their action, and as soon as such food 
products are warmed, the bacteria immediately begin to grow. 
Indeed, food spoils very rapidly when taken from cold storage. 
Where such low temperatures are not feasible, a moderate de- 
gree of cold may check bacterial growth and delay putrefaction. 
An ice chest usually maintains a temperature below 50°, and this is 
very efficient in helping preserve food. A cool cellar answers the 
same purpose, as well as the common plan of placing milk and 
other foods in cold water to delay the spoiling. Fruits are par- 
ticularly benefited by these cool dry temperatures, and a cellar 
which has a fairly uniform and low temperature is of great value 
on the farm in helping to preserve fruits and vegetables that would 
otherwise soon spoil. The value of ice as an aid in keeping all 
sorts of perishable material is too fully understood to require 
further notice. 
PRESERVATION BY THE USE OF CHEMICALS 
Many chemical substances are destructive to bacteria, and 
foods may frequently be protected from bacterial action by the 
addition of small quantities of some material, harmless in itself, 
yet having a checking action upon bacteria. Such agents are 
called preservatives. If they are to be used in the preservation of 
food products, it is of course necessary that they should not be 
deleterious to health and also that they should not impart dis- 
agreeable flavors to the food. The number of substances that can 
be used without hesitation is not very great. 
