No. 7. DEPARTMENT OF AGRICULTURE. 267 



temperature will not destroy bacteria, with the exception of the lac- 

 tic acid group, unless they are exijosed to it for 15 minutes. Such 

 an exposure in the continuous pasteurizer is impossible, for the 

 milk must be drawn off as soon as it reaches the desired temperature. 



Continuous pasteurization at 185 degrees F. will prevent the milk 

 from souring, because it destroys the lactic acid bacteria. The lac- 

 tic bacteria may spoil the milk for market purposes by rendering it 

 sour, yet they are, on the whole, not injurious. The injurious or- 

 ganisms may remain undestroyed by such a partial pasteurization. 

 Hence, the intermittent pasteurizer must be relied upon for the suc- 

 cessful pasteurization. The continuous process is used chiefly for 

 pasteurizing milk and cream for buttermaking, and for pasteurizing 

 skim milk at creameries. 



The continuous process destroys the lactic acid group and the 

 pathogenic germs, but does not destroy the spore-bearing kind. For 

 this reason it is necessary to add a starter to pasteurized milk or 

 cream in order to obtain the normal sour cream for buttermaking. 

 If the starter were not added, the cream would putrefy or decay be- 

 fore it would sour. By first destroying all the chance lactic acid pro- 

 ducing bacteria and then reinoculatiog it with a pure culture, of 

 what might be called, pure bred or pedigreed bacteria, the butter- 

 maker is more able to control the flavors of his product. 



In pasteurizing milk by the intermittent process, either of two 

 methods may be employed, as follows: 



1. Pasteurization in bulk in an enclosed tank, 



2. Pasteurization in bottles. 



Pasteurization in Bulk: When large amounts of milk are to be 

 treated, the first mentioned method is to be preferred. 



There are on the market several forms of pasteurizers or one may 

 be made to order. 



If neither of these is convenient^, milk may be pasteurized in cans, 

 in double vats or in any vessels which may be heated gradually and 

 uniformity without danger of scorching the milk. When heated in 

 vessels of large size, it is necessary that the milk be constantly 

 stirred, in order to insure a uniform temperature and to prevent the 

 albumen from adhering to the sides of the vessel. The source of 

 heat should be hot water rather than steam, as there is much less 

 liability of the milk becoming overheated or being cooked on the 

 sides of the vessel. 



If desirable, milk may be bottled and then pasteurized afterward. 

 This method of pasteurization is the better of the two when not morf 

 than 200 to 250 quarts are to be pasteurized, since the milk is not 

 exposed to the air after pasteurization and the liability of re-con- 

 tamination is avoided. 



