236 THE PRESERVATION OF FOOD PRODUCTS. 
juices, jellies, jams, in short, anything preserved by sugar, are 
liable to yeast action. Therefore, in their preservation, it is 
to be borne in mind that we are dealing chiefly with yeasts which 
are much more easily killed than bacteria. Sterilization of these 
products is much easier than sterilization of proteid foods. 
By Molds and Higher Fungi. Although these are less impor- 
tant agents in the spoiling of foods than bacteria, they are important 
in several directions. Many kinds of food bread, cheese, etc. 
will support a mold growth if kept rather moist. Molds grow 
chiefly on the surface, but when they become luxuriant they cause 
the material to become "musty" and to develop unusual as well 
as unpleasant flavors. Almost any food might in time be completely 
spoiled by molds, but usually the bacteria and yeasts act more 
rapidly than the molds, so that mold action is secondary. In the 
decay of wood and timber it is the higher fungi that play the chief 
part (bracket fungi and other tree fungi). They force their mycelia 
into the trunk of the solid tree, softening it and beginning the 
process of decay. The common molds are the primary cause of 
the decay of fruit, for they force their mycelia through breaks 
in the skin of the fruit and then through the whole fruit. While 
yeasts and bacteria may sometimes be concerned in the rotting of 
fruit the molds are almost universally the cause of this phenomenon. 
PRESERVATION OF FOODS. 
The extremely varied nature of farm products has made it 
necessary to find many different methods of preservation, since 
what is well adapted for one may be useless for another. The 
method of preserving wheat, for example, is not adapted for pre- 
serving milk or fruit. There are several fundamental methods in 
use, each of which has numerous modifications. 
Protection from Microorganisms. If it were possible to pre- 
vent bacteria, yeasts, and molds from gaining access to food materials, 
the food could be preserved indefinitely. But these organisms or 
their spores are so abundant everywhere that this is impossible, 
except by hermetical sealing. Some foods, however, are thus pro- 
tected. Fruits have a certain amount of protection against the 
