274 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



In order to understand these developments, the chemical 

 constituents of milk must be first considered. According to 

 Warington, cow's milk has the following composition : — 



It is evident, therefore, that the possible chemical changes 

 that may take place in mUk are concerned with the decom- 

 positions of albumin which are brought about by proteolytic 

 bacteria and enzymes, with the splitting of fats, and with the 

 various fermentations which lactose or mUk sugar is capable of 

 undergoing. If these various changes are to be controlled, it 

 will be seen that in dairy practice the utmost cleanliness is of the 

 first importance, lest normal proteolysis should become putre- 

 faction, or the breaking up of fats and of sugar should give 

 rise to abnormal developments of butyric acid, and consequent 

 rancidity, as distinguished from merely pleasant souring. 



These considerations may first be applied to the manu- 

 facture of butter. Butter, as is well known, is obtained by the 

 churning of cream, a process by which the fat globules, present 

 as an emulsion in nulk, are collected together to form the mass 

 known as butter. The fat of milk consists largely of glycerides 

 of palmitic and oleic acids, together with smaller quantities of 

 the glycerides of other fatty acids, notably butyric. 



The oldest method of separating the cream from milk is to 

 allow the latter to stand in wide shallow dishes, when the fat 

 particles, being specifically lighter than the rest of the milk, rise 

 to the surface, and can be skimmed off. Such a process 

 obviously offers conditions for contamination of the cream by 

 air infection, especially if any carelessness is permitted. The 

 danger of contamination is reduced if the cream is allowed to 

 rise in deep closed vessels. But the modern process in which 



