GENERAL CHEMISTRY OF MILK 131 



The Influence of Heat on Milk.— Profound physical and chem- 

 ical changes in milk are produced by heating it, and these changes 

 become more intense the greater the heat applied and the longer 

 milk is heated. This subject has been studied quite extensively, 

 especially during recent years, because boiled and pasteurized 

 milk have become popular. 



All milk constituents are affected by heat — casein is altered, 

 albumin is coagulated, fat globules coalesce, and some salts are 

 precipitated. Furthermore, gases escape; the milk acquires a 

 peculiar taste, usually called a "cooked taste," and the color 

 deepens. In contact with the air a film forms which does not re- 

 dissolve when the milk is cooled. 



The film which forms on boiling milk is not soluble in lime- 

 water and consists largely of casein which has been transformed 

 into the insoluble caseid. It also contains part of the other milk 

 solids, including some fat. At lower temperature, between 40° 

 and 60° C, a membrane is formed also, but this is soluble in lime- 

 water and can be coagulated from such a solution with rennet. 

 The membrane does not form if the milk is diluted with at least 

 twice its volume of water; if the surface is covered with a layer 

 of oU; or if the air is excluded by other means. 



Milk rich in albumin coagulates at the boiling-point. Colos- 

 trum contains an abnormally large percentage of albumin and 

 coagulates when boiled, the coagulum enclosing some of the 

 casein. When albumin is added to milk it also coagulates when 

 boiled — 8 c.c. of egg-albumen or one-tenth this quantity of desic- 

 cated egg-albumen will produce a jelly in boiling milk. 



When milk is heated to 80° C. for fifteen minutes the casein 

 is changed so that the time for rennet coagulation is prolonged. 

 At 100° C. the amount of protein which is precipitated by acetic 

 acid is reduced and the reduction is still greater at higher tempera- 

 ture. Raudnitz gives the following figures : 



THE AMOUNT OF PROTEIN PRECIPITATED BY ACETIC ACID AT DIFFERENT TEMPERA- 

 TURES 



Per cent. Per cent, in 



Temperature. precipitated. solution. 



Before heating 93.59 6.41 



Fifteen minutes at 100° C 93.47 6.53 



Fifteen minutes at 140° 76.37 23.63 



Onetiourat 140°C 74.39 25.61 



Milk-sugar begins to decompose at 120° C, and the change 

 becomes more profound as the temperature rises. The sugar is 

 caramelized and acids are formed. 



Boiling removes a large part of nitrogen and oxygen gas and 

 nearly all carbon dioxid. The loss of carbon dioxid renders boiled 

 milk more alkaline than raw milk. 



