136 MILK HYGIENE 



metal tank, through which cold water or, better, ice 

 water is passed and over the surface of which milk 

 slowly flows. 



For pasteurizing milk in bottles or in cans an appa- 

 ratus of different construction is used. Some consist 

 simply of a water bath; a large shallow metal tank is 

 partly filled with water and the bottles are so placed in 

 this that they are submerged as close as possible to the 

 neck, then the tank is closed by a cover and the water is 

 heated with steam. 



Other pasteurizers are built according to the follow- 

 ing principle: there is a perforated drawer in a large 

 metal tank upon which the filled bottles are placed close 

 together (Fig. 9) ; after the tank has been tightly closed 

 by means of a cover or door, steam is admitted until the 

 desired temperature is reached. Of course these ma- 

 chines are also adapted to sterilizing at a higher tem- 

 perature (100 to 110 C., 212 to 230 F.). After 

 from one-half to one hour's heating, the bottles are 

 allowed to cool a little before the tank is opened, then 

 they are immediately sealed. In some machines, all of 

 the bottles are sealed before the tank is opened, by 

 means of a specially arranged mechanism. The further 

 cooling of the bottles is attended with certain difficulties, 

 as they crack easily. This may be accomplished by 

 passing the bottles through several tanks of water of 

 progressively lower temperature, or by slowly lowering 

 the temperature of the water surrounding them. 



While the fact of the previous heating of milk to not 

 less than 80 C. (176 F.) may be established with cer- 

 tainty by the Storch or guaiac tests, it is not possible to 

 determine whether or not the milk has been subjected to 

 a less degree of heat ; moreover, since a brief heating at 

 80 C. (176 F.) is cheaper and far more convenient than 

 a longer heating at, for instance, 65 C. (150 F.), there 



