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Bacteria and Their Relations to Life 393 



In order to avoid the danger that Kes in the use of milk con- 

 taminated with disease germs, milk that is shipped into the 

 large cities is usually pasteurized before being sold. This treat- 

 ment kills most of the bacteria, destroying all the kinds that 

 produce disease in human beings. By " pasteurization '* is 

 meant the heating of the liquid to 150° or 160° 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 produced 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 preventing 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 

 available after months and years of storage. 



Soil bacteria and humus. In the process by which the bac- 

 teria of decay destroy animal and vegetable bodies, the humus 

 represents the products of partial decomposition, particularly 

 of cellulose. Carbohydrates, fats, proteins, and related com- 

 pounds are all subjected to bacterial action. Some are oxidized, 

 and some are split into less complex substances. Among the 

 many products of decay are hydrogen, marsh gas, organic acids 

 (e.g., acetic, butyric), ammonia, hydrogen sulfide, carbon dioxide, 

 and water. Usually the production of the final products CO2, 



