342 THE FOODS WE EAT 



under the same conditions of temperature. Examine the tubes at 

 intervals of 24 hours for three or four days. Note the taste and 

 odor of each. 



Raw Milk and Pasteurized Milk. Careful pasteuriza- 

 tion will not harm milk, but if the temperature is raised too 

 high, the Vitamin C in the milk will be destroyed and thus 

 the milk will not be of as much value as a food. A special 

 grade of raw milk called certified milk is now sold in most 

 communities. The price is higher because of the special 

 care taken in producing and handling it. 



A Cold Temperature Unfavorable to Bacteria. If you 

 will think back to the experiments on bacteria, you will 

 recall that cold is unfavorable for the growth of bacteria. 

 However, some bacteria will live in ice for a long time. 

 Intense cold prevents the growth of bacteria, but it does 

 not always kill them. We have come to make use of this 

 knowledge in our homes by using refrigerators. A well- 

 made electric refrigerator or even a good ice box will keep 

 the temperature below 45 F., which is sufficient to prevent 

 bacterial growth and will keep foods from spoiling before 

 they can be eaten. 



Construction of the Refrigerator. The household 

 refrigerator is a large box with thick, heat-insulated walls, 

 and with doors or covers to the several inside compart- 

 ments. There is always one chamber for the ice or the 

 ice-making machine, and another with shelves for food. 

 These compartments are connected by tubes or openings, 

 so that there is a free circulation of air throughout the 

 entire refrigerator. The drainage pipe leads to a pan, or 

 to a waste pipe protected by a trap that prevents warm 

 air from coming in. 



How We Use the Refrigerator. When air comes in 

 contact with ice, it gives up its heat, becomes colder and 

 heavier, and sinks to the bottom of the ice chamber. 

 An outlet below the ice allows this cold air to pass out 



