XV. 



MILK AND MILK PBODUCTS 



319 



the mercury from the filtrate by addition of caustic soda and sul- 

 phuretted hydrogen, and evaporating the clear liquid until the milk 

 sugar crystallises out on cooling. 



The reaction of milk towards indicators depends, of course, greatly 

 upon the indicator. 



Fresh milk is usually described as amphoteric when 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 contain- 

 ing 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 cor- 

 responds to less than 0*2 per cent lactic acid. As the acidity in- 

 creases, a sour taste becomes perceptible (at about 0*40 per cent) and 

 when the amount reaches 0'7 per cent, coagulation or curdling is pro- 

 duced. However long it may be kept, milk rarely develops an acidity 

 exceeding 2 per cent lactic acid. 



The Ash of Milk is white and contains the inorganic constituents, 

 together with some products resulting from the oxidation of the sul- 

 phur, 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 per 

 cent and its composition varies slightly. 



Schrodt and Hansen 1 give the following numbers as the extremes 

 of seven analyses of the mixed milk of ten cows taken at various times 

 of the year : 



Potash . 



Soda 



Lime 



Magnesia 



Ferric oxide 



Sulphur trioxide 



Phosphorus pentoxide 



Chlorine 



Per cent. Per cent. 

 22-55 to 26-94 

 10-26 11-97 

 19-71 

 1-78 

 traces 

 3-75 

 22-41 



13-15 



23-57 

 3-15 

 0-21 

 4-38 

 26-51 

 16-15 



Of the ash, about one-third is usually soluble in water and 

 consists mainly of alkaline chlorides and carbonates. Much of the 

 basic material of the ash exists in the milk in association with the 

 casein and in union with citric acid. 



Citric acid is present in cows' milk to the extent of about 1 per 

 cent 2 and crystals of calcium citrate are sometimes found in con- 

 densed milk. The presence of the citric acid probably accounts for a 

 portion of the calcium phosphate of milk being in solution. 



The gases contained in milk are chiefly carbon dioxide, oxygen 



r. Chem. Soc., 1884, Abstracts, 1397. 

 2 Henkel and Soxhlet, Jour. Chem. Soc., 1889, Abstracts, 178. 



