8 GENERAL CONDITIONS AFFECTING BACTERIA IN MILK 



standard. The following is Konig's table setting forth the differ- 

 ences succinctly : — 



These have been briefly referred to already with the exception 

 of the butter-milk and whey. Of the former it may be said that it 

 is the fluid remaining after the fat has been removed from cream 

 by churning : its sourness is due to lactic acid ; its proteids are 

 present in finely flocculent form. Whey is the watery fluid which 

 exudes from clotted milk and possesses a small degree of nutri- 

 ment. 



The physiology of milk. — Under this heading we propose, for 

 convenience, to deal with two matters, leaving for treatment 

 elsewhere the question of the secretion of milk, and the circum- 

 stances affecting it. The two points to which reference will be 

 made here are the curdling and coagulation of milk and the effects 

 of heat upon it, apart from the question of sterilisation, to which 

 reference will be made at a later stage. 



The natural curdling oi milk is brought about by the precipitation 

 of caseinogen, owing to the action of lactic acid fermentation. The 

 caseinogen is broken from its union with lime salts, and not being 

 soluble is thrown down in bulk. At first the precipitation is partial 

 and gives an appearance of flakiness and flocculence. But eventu- 

 ally it is complete, the separation being even more definite than in 

 the clotting of blood. The accompanying Plate illustrates the 

 natural curdling of milk almost to perfection. This particular case 

 occurred in the laboratory, where incidentally a bottle of whole 

 milk had been left for some months at rest. At the top is seen 

 the cream {or fat) : in the middle is the clear serum {inilk plasma) : 

 and at the bottom is the curdled precipitate (caseinogen). 



Such curdling which invariably occurs in standing milk, though 

 milk is rarely allowed to remain for a sufficiently long period to 

 provide such a clear demonstration, must not be confused with the 



