478 



FARMER'S CYCLOPEDIA OF LIVE STOCK 



Another type of aerator depends upon 

 forcing the air through the milk. The 

 animal odors are thus rapidly removed, 

 but the milk is not thereby cooled and 

 does not keep appreciably longer than 

 unaerated milk. Marshall, in Michigan, 

 found that in aerating milk a consider- 

 able per cent of the carbon dioxide was 

 removed and this was considered an un- 

 favorable result of aeration. Carbon 

 dioxide is not harmful when taken into 

 the stomach, and its presence in the 

 milk has a tendency to che^k the growth 

 of bacteria and prevent the souring of 

 the milk. In some of Marshall's experi- 

 ments, the aerated milk soured more 

 quickly than that which was not aerated. 

 In these tests, however, the milk was in 

 an unusually clean condition, and the 

 results perhaps do not apply to milk 

 which contains a relatively high num- 

 ber of bacteria and considerable filth. 



Doane suggests that the chief use of 

 the aerator should be in helping the city 

 dealer to remove bad odors which milk 

 may contain, while the farmer should 

 strive to produce milk which does not 

 need the aerator in order to eliminate 

 bad smells. 



Pasteurization— Of the various meth- 

 ods which have been adopted for elim- 

 inating bacteria from milk, or greatly 

 reducing their number, pasteurization 

 occupies an important place. It may be 

 assumed as certain that even under 

 the best possible conditions a consid- 

 erable number of bacteria will gain en- 

 trance to the milk. The possibility 

 cannot be excluded that among these 

 bacteria there are some which belong 

 to harmful species. The simplest and 

 surest means of destroying these bac- 

 teria is found in pasteurization. This 

 may be accomplished by exposing the 

 milk for 10 to 15 minutes at a tempera- 

 ture of 158° F. This method has a dis- 

 tinct advantage, since the milk does not 

 thereby acquire a cooked taste, and its 

 keeping qualities are greatly improved. 

 It is desirable, however, immediately af- 

 ter pasteurization that the milk be re- 

 duced to a temperature of 50° F. and 

 kept at that point. The recommenda- 

 tions made by different dairy experts re- 

 garding the length of exposure, and the 

 temperature of pasteurization, vary con- 

 siderably. In Michigan, pasteurization 

 was successfully accomplished by heat- 

 ing the milk to 155° F. and keeping it at 

 that temperature for 20 minutes. Later 



experiments in Wisconsin showed that 

 when milk was maintained at 140° F. 

 for 15 or 20 minutes, about 99 per cent 

 of bacteria were killed and the milk 

 kept sweet for six days. On this point, 

 however, opinions vary somewhat, and 

 the results obtained depend upon the 

 skill applied to the process of pasteuriza- 

 tion. Thus Harrington estimates that 

 about 75 per cent of ordinary bacteria in 

 milk are destroyed by keeping the milk 

 for one hour at 140° F., for 15 minutes 

 at 150° F., or for 10 minutes at 158° F. 

 If milk is carried above 158° F. it may 

 acquire a cooked taste. For practical 

 purposes, therefore, a temperature of 

 about 155° F. for 20 or 30 minutes 

 should be considered satisfactory for pas 

 teurization. 



Many physicians claim that the use 

 even of such low temperatures unfavor- 

 ably influences the digestibility and nu- 

 tritive value of milk for infant feeding, 

 and that, for this reason, preference 

 should be given to raw milk for infant 

 feeding obtained under strict sanitary 

 conditions. 



Kober and others, however, call atten- 

 tion to the difficulty of obtaining raw 

 milk in such a way as to guarantee its 

 absolute wholesomeness for children, 

 and, therefore, recommend pasteurization 

 as the lesser evil of the two. In fact, 

 many physicians believe that the dangers 

 from the use of pasteurized milk have 

 been greatly exaggerated. 



Pasteurization is commonly accom- 

 plished by placing the milk in vessels 

 surrounded by hot water. A considerable 

 number of satisfactory pasteurizers have 

 been placed on the market and may be 

 obtained from dealers in dairy products. 

 Detailed directions for their use are fur- 

 nished with the apparatus. 



Bacteria in milk as related to tern 

 perature — The chief purpose for cooling 

 milk immediately after it is drawn is to 

 check the growth of bacteria and thereby 

 lengthen the time during which the milk 

 will remain sweet. It has been found by 

 the extensive investigations of Conn and 

 others that variations in temperature 

 have a striking influence upon the rate 

 of multiplication of bacteria in milk. At 

 a temperature of 50° F. bacteria mny 

 multiply only fivefold in 24 hours, while 

 in the same period at 70° F. they may 

 multiply 750 times. It is obvious, there- 

 fore, that temperature has a great in- 

 fluence upon the keeping property of 

 milk. Milk maintained at 95° F. will 



