FERMENTED MILK 491 



stantly. After the pasteurization is completed, cold water is 

 run into the jacket and the milk cooled to about 24°-27° C. 

 (75.2°-80.6° F.). A bottle of the mother starter is added and 

 the can is covered and allowed to stand overnight. This 

 gives a large and active pure culture of lactic acid bacteria to 

 start the acid formation in the cream. Better results are 

 obtained if the cream is first pasteurized. 



When lactic acid bacteria grow in milk, the lactose is con- 

 verted into lactic acid with slight traces of certain other organic 

 acids . This acid breaks up the combination of calcium phosphate 

 and casein which holds the casein in solution, and the casein 

 is precipitated as a firm, jellylike mass. When this occurs in 

 cream, the fat globules are entangled in the precipitated casein. 

 In the process of churning the casein is broken into fine particles, 

 and the fat globules are collected into large granules that float 

 on the top of the buttermilk. Buttermilk, then, is the water 

 of the milk holding the sugar, acids, ash, and other soluble 

 constituents in solution and the finely divided particles of 

 precipitated casein in suspension. The amount of fat in the 

 buttermilk depends on the completeness with which the fat is 

 removed in the churning. Even with the best methods a little 

 of the fat in the form of very small globules remains in the 

 buttermilk. On standing, the suspended casein settles slowly 

 to the bottom. 



The composition of an average buttermilk is about as fol- 

 lows : x s 



Per Cent 



Fat 0.5 



Casein 2.4 



Albumin 6 



Lactose 5.3 



Ash J_ 



Total solids 95 



1 Vermont Agricultural Experiment Station. Annual Report, 1891, 

 p. 119. 



