RELATION OF BACTERIA TO DAIRY PRODUCTS 561 



Fourth, by heating the whole milk to 176 F. and skimming 

 it while hot, thus pasteurizing both the skim-milk and the 

 cream in one operation. 



Of the four methods suggested, the utilization of exhaust 

 steam of the creamery engine is undoubtedly the most economi- 

 cal. The skim-milk is somewhat diluted by the steam and a 

 small amount of cylinder oil from the engine also passes into 

 it ; but these additions are of little importance compared with 

 the economy of heating in this way, as neither the small quan- 

 tities of steam nor the trace of cylinder oil will affect the feed- 

 ing value of the skim-milk to any appreciable extent. It is 

 important, however, in using the exhaust steam for the pasteuri- 

 zation of the skim-milk to make sure that sufficient steam 

 is obtained to heat all the milk to the desired temperature 

 of 176 F. 



Forcing high-pressure steam into skim-milk is the easiest 

 and surest method of getting all the milk heated to the required 

 temperature. It is a more expensive way of heating than using 

 exhaust steam, but this is about the only objection that can be 

 made to it. 



The use of milk and cream pasteurizers which heat the skim- 

 milk as it passes over a metal surface, which is heated either 

 by steam or hot water, protects the skim-milk from dilution 

 with steam, but the machines designed for pasteurizing in this 

 way are somewhat expensive and they require more attention 

 while in operation than the first two methods of heating. 



Skimming the whole milk at a temperature of 176 F. is 

 also expensive in the use of fuel, and further objection is made 

 to it because of the difficulties of separating milk at this high 

 temperature. More sediment or bowl slime accumulates in 

 the separator when hot milk is skimmed than in the case of 

 milk having a temperature of 80 F. The clogging of the bowl 

 makes it necessary, therefore, to stop the machine and clean 

 the bowl more frequently than when colder milk is skimmed ; 

 2o 



