CHANGES PRODUCED IN MILK BY HEAT 235 



calcium that it is not lost, and if heated in a special vessel can 

 be obtained by the infant if this is desired. 



The Effect of Heating on the Coagulation by Rennet. The 

 effect of heating on the coagulation of caseinogen by rennet has 

 been studied by numerous investigators, and it is not proposed 

 to deal with many of the papers. 



There is no doubt that the time of coagulation with rennet is 

 prolonged by heating. This was mentioned by Soldner (1888) who 

 believed that the delay in the coagulation was due to deficiency 

 of calcium, the calcium having been thrown out of solution as a 

 result of heating. 



Ringer (1890) showed that coagulation of caseinogen can be 

 brought about if calcium chloride is added to milk, and that the 

 addition of lime salts hastens coagulation by rennet, while their 

 reduction delays clotting. 



This was confirmed by Conradi, who showed also that if the milk 

 is heated to a temperature of 70 C. before the addition of calcium 

 chloride, it will coagulate at a temperature of from 8-12 degrees 

 lower than if the calcium is added before heating. The coagula- 

 tion with rennet, he believed, was not affected by heating the 

 milk for half an hour at 70 C., and was very little delayed by a 

 temperature of 75 C. After this temperature the coagulation 

 time gradually increases, but coagulation will still occur until the 

 temperature has been raised to well above 100 C. (about up C.). 

 Milk which has been just boiled and then cooled coagulates just as 

 well as raw milk. 1 



Kreidl and Lenk investigated the question of rennet coagula- 

 tion in heated milk and dealt fully with the literature. These 

 authors state that when the milk and all the apparatus concerned 

 in the process are sterile, coagulation does not occur, but on the 

 least contamination by any unsterilised material, even a finger 

 on the top of the test-tube, coagulation occurs. 



Rupp, working on the changes produced in milk as a result 

 of heating, found that using low temperatures, i.e. from 55 C. up 

 to 65 C., the coagulation with rennet takes place more rapidly 

 than was the case with raw milk. Retardation commenced at 

 70 C. and at 75 C. The time of coagulation was approximately 

 doubled, and the coagulation was of a flocculent character. Similar 

 observations were made by Stassano and Talarico some years 

 previously. 



Sidler showed that human milk does not coagulate with rennet, 

 and generally speaking this is the opinion of later investigators, 

 although it appears that human milk does alter somewhat on the 

 addition of rennet but does not form into a solid clot. 



There is some evidence to show that the delay in clotting is 

 connected both with the falling out of calcium and with the acidity 



1 See Conradi 's paper for the early literature j 



