IMPORTANCE OF PLANTS TO MAN 247 



or to prevent their further growth. This may be done 

 in several ways. The methods most commonly employed 

 for killing them are boiling or using chemicals called 

 preservatives. The former method is the better 

 because chemicals which will destroy bacteria are 

 likely to be poisonous to man, and therefore should 

 not be used in his food. Preservatives frequently used 

 are formaldehyde, borax, boracic acid, and salicylic 

 acid. In canning fruits and other foods heat is used. 

 The fruit is first boiled to kill all bacteria in it. The 

 can is sterilized by putting it in boiling water, and 

 then the fruit is sealed in* it air-tight while still hot. 

 If this process has been carefully done, there will then 

 be no bacteria in the can and none can enter. 



Other methods for preserving foods are keeping 

 them cold (refrigeration), drying, smoking, 'or using 

 salt, sugar, or vinegar. 



Useful bacteria. The injury done by bacteria is so 

 very apparent that we sometimes overlook the fact 

 that many bacteria are of benefit to man. Mention 

 has already been made of their value in causing decay, 

 and in enriching the soil by adding nitrogen compounds 

 to it (see page 122). Bacteria are also useful in the 

 production of several foods. Vinegar, for example, is 

 formed as a result of their action upon the fruit juice, 

 cider. The flavor of butter is partly due to bacteria, 

 and that of cheese is almost entirely the result of the 

 kinds of bacteria used in "ripening" it. 



