CONTROL OF BACTERIA IN FOOD SUPPLIES 343 



dioxide in the soil, and in this form they can be taken into the 

 roots. 



The Indirect Relation of the Fertility of Soil to the City Dweller. 

 — Those of us who live in cities are aware of the importance of 

 fresh vegetables brought in from the neighboring market gardens. 

 But we sometimes forget that our great staple crops, wheat and 

 other cereals, potatoes, fruits of all kinds, our cotton crop, and all 

 plants we make use of grow in proportion to the amount of raw 

 food materials they take in through the roots. When we remember 

 also that many industries within the cities, as mills, bakeries, and 

 the like, as well as the earnings of our railways and steamship lines, 

 are largely dependent on the abundance of the crops, we may 

 recognize the importance of what we have read in this chapter. 



Means of Control of Bacteria in Food Supplies. — We have 

 found that bacteria act in two ways upon our food supplies : Di- 

 rectly, they feed upon them, causing them to decay ; indirectly, 

 they renew our food supplies through this very process of decaj^ 

 and by the partnership that exists between the nitrogen-fixing 

 bacteria and the legumes. Man, since the time of the Napoleonic 

 wars, has won a fight against the spoiling of foods, by discovering 

 that heat will kill bacteria in food. Food supplies may be con- 

 served by cold storage, canning, and the use of preservatives. 



Cold Storage. — Man has learned to use cold to keep bacteria 

 from growing in foods. The icebox at home and cold storage on a 

 larger scale enable us to keep foods for a more or less long 

 period. If food is frozen, as in cold storage, it might keep without 

 growth of bacteria for years. But fruits and vegetables cannot 

 be frozen without spoiling their flavor. All frozen foods after thaw- 

 ing are particularly susceptible to the bacteria of decay. For 

 that reason products taken from cold storage must be used at once. 



Canning. — Canning is simply a method by which first the 

 bacteria in a substance are killed by heating and then the sub- 

 stance is put into vessels and covered so that no more bacteria can 

 gain entrance. In this method of canning often used at home the 

 fruit or vegetable is first boiled with salt or sugar, as either of these 

 substances aids in preventing the growth of bacteria. The time 

 of boiling will be long or short, depending upon the materials to 

 be canned. Some vegetables, as peas, beans, and corn, are very 



