MILK. 367 



which gradually separates into a solid white curd, and a thin, 

 transparent milk-serum or whey. 



The coagulation of the casein is caused by lactic acid, pro- 

 duced by the so-called lactic fermentation of lactose. The fer- 

 ments causing this fermentation are undoubtedly floating in the 

 air, as it is possible to prevent the decomposition of milk-sugar 

 for a considerable length of time by taking proper precautions 

 for destroying and excluding them. Simultaneously with the 

 coagulation of milk the alkaline reaction becomes acid and the 



O 



sweet taste gradually more and more sour. 



These changes in milk can to some extent be artificially 

 produced, hindered, and controlled. Thus, the casein may be 

 precipitated by the addition of rennet or acetic acid (or any 

 mineral acid) and heating. 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 phy- 

 sical treatment, such as cooling or icing, boiling and aeration; 

 3, by condensation or evaporation, with or without the addition 

 of a preservative agent. All these systems of preservation, 

 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 form- 

 ing a lactate of calcium or of sodium. 



Of antiseptics, salicylic acid has been used with good results 

 (2 grains to a pint). 



Of all preservatives, cold is the most efficient and least objec- 

 tionable, and milk when cooled to within a few degrees of the 

 freezing-point may be kept for 8 to 12 days sweet and without 

 change. 



The condensation of milk is effected either simply by evapo- 

 rating (generally in vacuum pans) a portion of the water, or by 

 first dissolving in it a certain quantity of sugar (generally cane- 

 sugar) and then evaporating to the consistence of a thick syrup, 

 which is placed in suitable air-tight jars. The sugar which is 

 added serves as an additional preventive of decomposition. 



The following gives the constituents of milk which may be 



