242 THE PRESERVATION OF FOOD PRODUCTS. 
Lastly, it should be noticed that ordinary sun-dried hay will 
sometimes, especially if stored in too moist a condition, undergo 
a similar heating in the mow. The hay may be considerably 
injured by such heating, so that it will lose some of its nutriment. 
Sometimes the heat is sufficient to cause an actual ignition of the 
hay. 
Certain phenomena sometimes seen in cotton are clearly closely 
akin to the fermentation just described, for cotton may undergo a 
spontaneous heating sufficient to render it in danger of combustion, 
and this must be due to processes similar to those just mentioned. 
The same thing is true of hops which occasionally develop a like 
spontaneous heating during the curing. 
PRESERVATION BY COLD. 
All the common species of bacteria grow more slowly as the 
temperature is lowered, and cease growing entirely when it reaches 
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 material 
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 particularly 
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 
