300 CITY MILK SUPPLY 



air. Probably the greatest loss occurs when pasteurizing is done in open 

 vats or the cooling is done over an open cooler. To prevent loss from 

 the latter source many firms use an internal cooler of the type that has 

 one set of tubes enclosed within another. Usually such coolers are re- 

 generative, the incoming cold milk being used to cool the outgoing 

 pasteurized milk and being itself heated in the process. It is impossible 

 to state the loss that occurs from evaporation because it varies so much 

 in different plants. In some it amounts to as much as 4 per cent, of the 

 milk pasteurized but this is excessive ; with careful management it should 

 be kept under 1 per cent. 



Besides the loss of milk that evaporation causes, there is that which 

 arises from spilling the milk in passing it through the pasteurizing machin- 

 ery and from that which is left behind in the machine. This, too, varies 

 greatly in amount, being little in some plants and large in others accord- 

 ing to the care and types of machines used. 



The tendency of the treated milk to froth or foam causes considerable 

 trouble because it makes the milk difficult to handle, because it tends 

 to produce excessive evaporation, and because the foam is likely to be 

 several degrees cooler than the body of the milk in the holder and so to 

 cause imperfect pasteurization. The trouble has been overcome to some 

 extent by filling the holders from the bottom. 



Milk should never be pumped after it is pasteurized because it is 

 likely to be recontaminated and because, if the pumping is done while 

 the milk is hot, the volume of the cream that will rise on it is greatly 

 reduced. 



At certain seasons of the year there is a tendency for the cream of 

 pasteurized milk to form a tough leathery layer on top of the milk. This 

 is believed to be due to needlessly rough handling in the course of pasteur- 

 ization and to occur at those seasons when atmospheric temperatures 

 approach those of churning, about 62F. 



Importance of Temperature Control. The vital part of pasteuriza- 

 tion is the use of the proper degree of heat; hence temperature control is 

 of the utmost importance. Skill and care are required to successfully 

 operate a pasteurizer. In the first place in starting a machine the tempera- 

 ture rises gradually and the apparatus as a whole is not promptly heated 

 to the required temperature so that the first milk to pass is apt to be under- 

 pasteurized. Therefore, it is customary to run it through the machine a 

 second time; when this is done the milk should not be allowed to pass over 

 and contaminate the cooler before being returned to the heater. In some 

 plants hot water is run through the apparatus to heat up and sterilize 

 the apparatus. This procedure is excellent but care must be taken to 

 remove all of the hot water before actual pasteurization is begun. Again, 

 in starting it takes some machines time to settle down to the maintenance 

 of even temperatures and until they have done so the milk they are han- 



