110 Dairy Bacteriology. 



1. Diminished "body" When milk, but more especially 

 cream, is heated to 140 F. or above, it becomes thinner in 

 consistency or "body, 1 ' a condition which is. due to a 

 change in the grouping of the fat globules. In normal 

 milk, the butter fat for the most part is massed in micro- 

 scopic clots as shown in Fig. 22. When exposed to a tem- 

 perature of 140 F. or above, these fat-globule clots break 

 down, and the globules become homogeneously distributed 

 as in Fig. 23. Under these conditions milk does not cream 

 readily. When cream itself is so heated, its consistency is 

 materially reduced, giving the impression that it contains 

 a less per cent of butter fat. These changes seriously af- 

 fect the general adoption of heat as a means of preserving 

 milk for ordinary market use, but fortunately this defect 

 can be overcome. 



2. Cooked taste. If milk is heated to 160 F., it acquires 

 a cooked taste that becomes more pronounced as the tem- 

 perature is further raised. Milk so heated develops on its 

 surface a pellicle or " skin. 1 ' The cause of this change in 

 taste is not well known. Usually it has been explained 

 as being produced by changes in the nitrogenous elements 

 in the milk, particularly in the albumen. Recently, Thoer- 

 ner ! has pointed out the coincidence that exists between 

 the appearance of a cooked taste and the loss of certain 

 gases that are expelled by heating. He finds that the 

 milk heated in closed vessels from which the gas cannot 

 escape has a much less pronounced cooked flavor than if 

 heated in an open vessel. The so-called " skin " on the 

 surface of heated milk is not formed when the milk is 

 heated in a tightly-closed receptacle. By some 8 it is as- 

 serted that this layer is composed of albumen, but there is 



.. Thoerner, Chem. Zeit., 18: &45. 

 8 Snyder, Chemistry of Dairying, p. 59. 



