EFFECT OF TEMPERATURE ON BACTERIAL GROWTH. 25 



increase in numbers, and those growing at that temperature 

 not lactic acid bacteria. At 20°C. (eS^F.) milk sours in forty 

 hours. At 37°C. (99°F.) curdling occurs in a few hours (ll). 

 The length of time required for transportation and delivery of 

 milk is frequently such that refrigeration must be used to insure 

 its reaching the consumer in proper condition. 



Methods of cooling. Some of the methods on the farm are 

 crude and inefficient. The process of passing milk through 

 the air to remove undesirable odors — aeration — will accomplish 

 some reduction of temperature. Cans of milk may be stood 

 in a tank of water, and by stirring the milk occasionally the 

 temperature may be reduced. In a hot dry climate cans may 

 be covered with burlap and dampened. The evaporation from 

 the burlap will lower temperature. In the same way, wet 

 blankets over cans on a wagon are of use. 



There are various mechanical devices for utilizin-g cold water. 

 These fail in hot weather, when most needed. The use of ice 

 with proper apparatus is productive of better results, and is 

 essential in hot weather. The use of ice constitutes the stand- 

 ard, approved method of cooling milk on the small dairy farm, 

 in climates where natural ice may be obtained. 



Mechanical refrigeration, by means of anhydrous ammonia or 

 carbon dioxide, furnishes the most efficient means for preserv- 

 ing milk in large quantities (38). It is employed in city 

 distributing depots, and in the best large dairies. It may, and 

 should, be very widely used in connection with dairies in warm 

 climates. 



Milk during transportation. The refrigerator car, with 

 abundant ice supply, solves the problem of shipping milk long 

 distances in tanks. It is not so effective for local traffic, when 

 the car doors are frequently opened. 



Bottles of milk in crates may be kept in good condition for 

 quite a time without a refrigerator car, by packing cracked ice 

 about them. Bottles of certified milk, packed in closed wooden 

 boxes, perhaps metal lined, and thoroughly surrounded with 

 cracked ice, may likewise be shipped. Milk in cans covered 

 with non-conducting jackets of hair may be kept cool longer 

 than otherwise (4, 14, 43, 44). 



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