314 MILK AND MILK PRODUCTS. XIV. 



abundant in dairies, &c., though probably all are not of the 

 same kind. The chemical change involved is apparently of a 

 very simple character 



C 12 H. 2 ,O n + H 2 Q == 4C a H 4 (OH)COOH, 



but in many cases other products are formed and much more 

 complicated reactions must occur. 



Milk sugar is prepared from whey or from milk by removing 

 the nitrogenous matter and fat by means of mercuric nitrate, 

 precipitating the mercury from the nitrate by addition of 

 caustic soda and sulphuretted hydrogen, and evaporating the 

 clear liquid until the milk sugar crystallizes cut on cooling. 



The reaction of milk towards indicators depends, of course, 

 greatly upon the indicator. 



Fresh milk is usually described as ampJioteric \vhen tested 

 with delicate litmus paper, i.e., it turns red litmus blue and 

 blue litmus red. Towards phenol-phthalein milk is distinctly 

 acid. It should always be borne in mind that the nature of 

 the reaction of a product containing organic acids and acid 

 phosphates, as milk does, depends mainly upon the character 

 of the indicator used. On keeping, milk almost invariably 

 becomes more and more acid owing to the activity of the lactic 

 bacilli, the development of acidity being more rapid in warm 

 than in cold weather. The average acidity of milk as sold 

 probably corresponds to about 0'2% lactic acid. As the 

 acidity increases a sour taste becomes perceptible (at about 

 O40%) and when the amount reaches 0*7% coagulation or 

 curdling is produced. However long it may be kept, milk 

 rarely develops an acidity exceeding 2% lactic acid. 



The Ash of Milk is white and contains the inorganic con- 

 stituents, together with some products resulting from the 

 oxidation of the sulphur, phosphorus, and carbon existing in 

 the proteids and other organic compounds. In the milk the 

 ash constituents doubtless exist in very different states of 

 combination to those in which they are left in the ash. The 

 amount of ash in milk is usually about 0'7 % and its compo- 

 sition varies slightly. 



Schrodt and Hansen* give the following numbers as the 



* Jour. Chem. Soe. 1884 abst. 1397. 



