700 PHYSIOLOGICAL CHEMISTRY. 



Physical and chemical changes in milk on standing. 



After leaving the body milk undergoes physical and chemical 

 changes. The principal physical change is the separation of milk 

 into two layers : the upper, cream, contains practically all the fat, 

 and its proportionate quantity of other constituents; the lower, 

 skimmed milk, is almost fat-free. By removing varying quantities 

 of skimmed milk by siphon or otherwise, the proportion of fat in the 

 remainder of the milk is increased. 



Of chemical change, occurs, particularly on standing in a warm 

 place, conversion of lactose into lactic acid through lactic acid fer- 

 mentation. The reaction of milk then becomes acid, the casein coag- 

 ulates and separates as a solid white curd carrying with it fat. The 

 remaining thin, transparent liquid, whey, contains all the inorganic 

 salts, that portion of lactose which has not been decomposed, as also 

 the lactic acid formed. 



There has recently been an extensive use of milk which has been 

 fermented by the Bacillus Bulgaricus, a powerful lactic acid pro- 

 ducer. This use has been based upon the belief of Metchnikoff that 

 acid-producing bacteria are antagonistic to the putrefactive bacteria 

 which are normally present in the intestine, hence the milk is advised 

 in cases of intestinal disorder. 



Milk also undergoes another peculiar fermentation, by which it is 

 converted into a thick, ropy, gelatinous mixture. 



The decomposition of the milk-sugar and with it the "curdling" 

 may be prevented 1, by chemical treatment with alkaline salts or 

 antiseptics ; 2, by physical treatment, such as cooling or icing, boil- 

 ing and aeration ; 3, by condensation or evaporation, with or without 

 the addition of a preservative agent. All these systems of preserva- 

 tion, however, are subject to serious disadvantages because they either 

 interfere with the natural constitution and properties of the milk, or 

 because they serve their purpose for too limited a time. 



The addition of alkalies such as lime-water, sodium carbonate or 

 bicarbonate, does not prevent the lactic fermentation, but prevents 

 the action of the liberated acid on the casein by forming a lactate of 

 calcium or sodium. 



Milk preservatives. The chemical changes in milk are best pre- 

 vented by cleanliness and preservation at a low temperature. Various 

 antiseptics, such as salicylic acid, boric acid, formaldehyde, benzoic 

 acid, etc., are added to milk with the view of preventing decomposi- 

 tion. While the small quantities used appear to be harmless, yet 



