THE MILK CONTRACTOR 291 



thus utilizing the maximum heating power of the water and raising the 

 temperature of the milk gradually. The efficiency of machines of this 

 type depends on securing a slow enough flow of the milk and sufficient 

 heating surface to certainly heat all of the milk to the required tempera- 

 ture. It is important that these pasteurizers shall be so built that there 

 shall be no danger of intermingling of the milk and water through leakage, 

 and so that they can be easily taken apart for thorough cleaning. The 

 advantages of this type of pasteurizers are that they are built so strongly 

 that they can be sterilized by steam under pressure and that they are 

 constructed in sectional units whose capacity can be increased by the 

 addition of more units as the growth of business demands it. An objec- 

 tion to this apparatus is that the pipes are so slightly inclined from the 

 horizontal that at the end of the run it is difficult to empty all the milk 

 from the machine. 



Type 6. Various makes of tanks that are insulated and heated by 

 hot water jackets or in some patterns by revolving screws or coils of pipe, 

 are extensively used for pasteurizing milk on a small scale. The extreme 

 simplicity of the vat system highly recommends it. The life of dairy 

 machinery is short because of the rough usage it must necessarily re- 

 ceive. The simple tanks may outlast the more complicated heater and 

 holders and therefore cost less. They are also economical because the 

 tank may be used as a heater, holder and cooler. The great disadvantage 

 is that it takes a long time to raise the milk to the desired temperature and 

 likewise to cool it. Besides, unless the tank is kept covered the loss from 

 evaporation is considerable. The advantage of the tanks is that the 

 heating of the milk may be checked right at the desired temperature and 

 the milk held there till the tank is emptied. When the tanks are used for 

 cooling, cold water and afterward brine is pumped through the jackets 

 or screws. In some types of tanks the bearings or packing boxes of 

 the agitators come into contact with the milk, and contaminate it with 

 oil. While this is perhaps one of those things that are more unsightly 

 than dangerous it should be avoided if for no other reason than that there 

 is some slight danger of the oil imparting a bad flavor to the milk. 



In the spray vat system one tank is contained within another but in- 

 stead of being cooled by a water jacket filling the space between the inner 

 and outer tank it is cooled by a spray of cold water from a pipe that runs 

 along its upper edge, the spray being delivered in such a way that the jets 

 unite to form a film which runs down the outer surface of the inner tank 

 and away at the bottom. The milk is agitated by blades affixed to a 

 mechanism that travels on the top frame of the vat, thereby eliminating 

 all danger of oil contamination. 



Type 7. There is a tank for pasteurizing milk in cans. A double 

 row of cog wheels runs the full length of the tank bottom and on each 

 wheel is placed a 10-gal. can within each of which is a stationary paddle, 



