Bacteria and Fungi 257 



that produce disease in human beings. By " pasteurization " 

 is meant the heating of the liquid to 150 degrees or 160 degrees 

 F. for from 10 to 30 minutes. This does not kill the spores, 

 but they are to a large extent prevented from developing by 

 the subsequent cooling that the milk receives. 



The preservation of foods. The greatest losses that occur 

 in the utilization of crops are connected with the distribution 

 of the products to the consumer. Much of the food pro- 

 duced never reaches the consumer, because bacteria and 

 molds render it unfit for use before it can be distributed 

 through the markets. There are four methods of prevent- 

 ing this loss : (i) cold-storage warehouses and refrigerator 

 cars are used to keep foods below the temperature at which 

 bacteria grow appreciably; (2) fruits, vegetables, or other 

 foods are packed in cans, and the cans are then sterilized by 

 heat and are sealed so that they are bacteria-tight ; (3) food 

 products are dried to make it impossible for bacteria to grow 

 in them ; and (4) foods like meat and fish are treated with 

 salt or with some other chemical that will prevent the growth 

 of bacteria. Refrigeration enables us to preserve foods for 

 weeks and months. Canning and drying make foods avail- 

 able after months and years. 



Soil bacteria and humus. In the process by which the 

 bacteria of decay destroy animal and vegetable bodies, the 

 brown organic matter represents the products of partial 

 decomposition. This matter is called humus, and it is one 

 of the important soil factors in crop production. Humus is 

 of importance in light soils because it helps to hold the water 

 and so to keep the soils moist. In heavy clay it mellows 

 and loosens the soil and promotes aeration and drainage. 

 Humus may also contribute something directly to the nu- 



