POPULAR BULLETIN REPRINTS. 



A NEW METHOD OF DETERMINING MILK QUALITY.* 



F. H. HALL. 



Microscopic To the unaided eye, normal fresh cows' milk is a 

 bodies in faintly yellowish, white liquid, apparently uniform 

 milk. throughout and simple in composition. Probably most 



of its consumers realize, however, that milk is more 

 complex in make-up than it appears at first view, since they see fat 

 rise to the surface as cream or find the casein coagulating after 

 longer keeping. Yet few milk drinkers appreciate the great com- 

 plexity in composition of this common article of diet, or know how 

 many and how delicate means and methods must be used by scientists 

 to identify the varied components of milk and to trace their intimate 

 relationships as these affect the handling of milk and the making 

 of other dairy products. 



Fundamental study of milk is very essential, for apparently slight 

 variations in the relationships of its constituents may greatly affect 

 its value or even change it from a most wholesome food to a menace 

 to health or to an actual poison. 



Many of these studies may be left to the chemist, for he must 

 determine the ultimate composition of all the milk constituents; 

 and some of them he, only, can find, since they are in solution and 

 therefore beyond the range of vision, even if aided by the most power- 

 ful microscope. 



But milk contains, ordinarily, three classes of bodies, or under 

 some conditions four, which can be brought into view by the micro- 

 scope; and two of these classes of microscopic objects — the fat 

 globules and the bacteria — have most intimate relations to the 

 value and wholesomeness of milk; and the third class — cells and 

 cell fragments — may be indicative of sanitary quality. Casein 

 belongs in the " possible " fourth class referred to, for the minute 

 particles of this colloid, or jelly-like substance, are only just beyond 

 the power of the compound microscope. They are revealed by "ultra- 

 microscopic " methods. 



'Reprint of Popular Edition of Bulletins Nos. 373 and 380; for Bulletins 

 see pp. 79 and 117. 



[893] 



