12 SCIENCE AND PRACTICE OF DAIRYING. 



the amphoteric reaction of milk is caused by the presence in it of neutral 

 and acid phosphates and carbonates of the alkalies. By warming the milk 

 the alkaline reaction becomes more pronounced. Warming, however, has 

 no influence on the acid reaction. To phenol-phthalein milk only shows an 

 alkaline reaction after it has been neutralized with a certain amount of 

 alkali. As a rule 100 c.c. of fresh milk require about 7 c.c. of a J normal 

 soda solution for the alkaline reaction. In order to determine the acidity 

 in fresh milk caused by the acid phosphate, Soxhlet and Henkel treat 

 50 c.c. of milk with 2 c.c. of an alcoholic 2 per cent phenol-phthalein 

 solution, and titrate with a J normal soda solution. The number of c.c. 

 required serve as an indication of the acidity. By the addition of diluted 

 acids milk can be immediately coagulated, and, in a somewhat longer time, 

 by means of a strong rennet solution. Dilute lactic acid and rennet 

 change the milk into a coagulated adhesive mass. Acetic and diluted 

 mineral acids, under similar conditions, produce flocculent coagulation. 

 By warming milk at 50 C., or at higher temperatures, it undergoes 

 changes which specially affect its proteids, as well as its taste and colour. 

 Under such conditions the addition of diluted acids does not produce a 

 lumpy coagulation, but a finely flocculent and pulpy one. The milk is 

 also rendered much more sensitive to the action of rennet, which, under 

 certain conditions, exerts its full coagulating influence. Milk coagulated at 

 a temperature of 130 to 140 C. assumes the peculiar flavour of cooked milk, 

 and becomes slightly yellowish or yellowish brown in colour. The higher 

 milk is heated between the limits of 50 C. and 140 C., the more quickly do 

 the above described changes take place, and the shorter is the time within 

 which increased temperature produces the various changes. It is obvious 

 that heating milk to 100 C. can only be accomplished in a closed vessel. 



The properties of the proteids of milk are dependent, in the first place, 

 on the nature of the chemical combinations of the mineral constituents of 

 milk, and especially of the lime salts. If, as is actually the case, the 

 constitution of the mineral salts of milk is changed under the influence of 

 high temperatures, and if a portion of the soluble lime salts is converted 

 and precipitated into an insoluble condition, it naturally follows that the 

 condition of the proteids also undergoes change. 



The peculiar smell and flavour of milk strongly heated is very pro- 

 bably connected with the small quantities of sulphuretted hydrogen which 

 have been proved to be present in boiled milk. (Fresh milk, treated with 

 tincture of guaiacum, assumes a blue colour, while boiled milk does not 

 show this reaction.) 



The change of colour which takes place on heating milk for some time 

 at temperatures over 80 C., and which increases the higher the tempera- 

 ture and the longer the duration of the exposure to such temperatures, 



