What is 
mold? 
48 WORLD OF INVISIBLE LIFE 
outside of the cheese, and the other gives the 
soft inside its peculiar taste. The holes in 
Swiss cheese are formed by carbon dioxide gas 
produced by certain kinds of microbes in the 
cheese. 
Cheese-making is, as you can see, a very 
complicated process. It seems to depend on 
many different kinds of microbes acting upon 
the curds of the milk. It is difficult, for this 
reason, to be sure that the cheese will turn out 
in the way desired, for often a wrong kind of 
microbe gets into it and ruins the taste. The 
only way this can be avoided is by taking great 
care that everything used in the dairy and 
factory is very clean. 
Everybody has seen the white felt-like fuzzy 
growth which sometimes appears on bread that 
has become damp and stale. Usually, it turns 
greenish-colored after it has grown for a while. 
This is one of the most common molds. 
However, it is only one of many kinds of molds. 
All of them are plants, and so small that a single 
plant cannot be seen with the naked eye. 
