1264 The Cornell Reading-Courses 



usually, however, cur efforts are concentrated on getting rid of the trouble- 

 some micro-organisms. In bread-making yeast is desirable, but the 

 yeast that finds its way from the air into a can of fruit and causes it to 

 ferment, or work, is a pest. 



HOW TO PREVENT FOODS FROM SPOILING 



If food is to be kept for any length of time, then, it is necessary to retard 

 or prevent natural ripening or developing processes and to protect the 

 food from invasion by invisible as well as visible enemies. If we know 

 what conditions favor life processes, either in the food or in the micro- 

 organisms that may be on it, we can prevent to a certain extent the occur- 

 rence of those conditions. 



All micro-organisms need the conditions of warmth, moisture, food, and 

 oxygen to make them grow. Warmth, moisture, light, and oxygen also 

 favor the maturing, ripening, and decay changes occurring in eggs, meat, 

 fresh fruits, and vegetables. From an understanding of the few simple 

 facts, it has been possible to develop large and successful systems of food 

 preservation, some of which are mainly commercial but most of which 

 may find a practical household illustration. 



The following convenient grouping of systems of food preservation may 

 be made, according to the conditions that they furnish to oppose the 

 development of micro-organisms and the na.tural changes occurring in 

 such foods as eggs, fruits, and vegetables: 



1. By means of low temperatures. 



2. By means of high temperatures. 



3. By means of preservatives. 



4. By means of the removal of moisture. 



Preservation of Jood by means of high temperatures 

 Although warmth directly stimulates growth in micro-organisms and 

 causes fruits and vegetables to ripen more rapidly than when they are 

 kept in a cool or cold place, great heat is not favorable to life processes. 

 A temperature as high as that of boiling water is very destructive to the 

 vitality of both micro-organisms and the food on which they may occur. 

 That destructive effect of heat is the corner stone of the method of food 

 preservation which we know as canning. Successful canning depends on 

 two things : first, the complete destruction by heat of all life in or on the 

 food, and on all parts of the can that are to come in contact with the food ; 

 second, subsequent treatment of the can by sealing or by other methods 

 to prevent further entrance of micro-organisms. The process of heating 

 a food as just described is known as sterilization. 



When canned foods spoil, the misfortune may be laid at the door of 

 one thing — the presence of some tiny invisible form of life, or micro- 



