132 MILK 



The fat is not decomposed by heating to or even above the 

 boiling-point. The physical condition, however, is easily dis- 

 turbed. Commencing at about 70° C, or even lower, the clusters 

 of fat globules break up, and then the cream rises slowly and in- 

 completely. The amount of cream appearing at the surface of 

 heated milk appears to be smaller than in the same milk raw. 

 The higher the temperature, the more rapidly are the fat clusters 

 broken up and the slower is the tendency to rise. The length 

 of time at which milk is exposed to a certain temperature has a 

 similar influence. The longer milk has been exposed, the more 

 are the fat clusters broken up. Moreover, while the milk is hot 

 agitation increases the tendency to break up clusters. BoiUng 

 causes the fat globules to coalesce. 



In boiling mUk soluble calcium phosphates are changed to 

 insoluble salts and are precipitated. The loss of carbon dioxid 

 by heating aids in the precipitation of salts. These salts dissolve 

 again, in part at least, when boiled milk stands and absorbs 

 carbon dioxid from the air. 



The taste of milk is changed by boiUng, due chiefly to decom- 

 position of protein with evolution of hydrogen sulphid. When 

 casein in boiled milk is coagulated by acid the flakes are smaller 

 than when raw milk is coagulated. It is claimed, therefore, that 

 the casein of boiled milk is more easily digested than the casein 

 of raw milk. 



A great deal has been written about the changes which occur 

 in milk when it is pasteurized, but little experimental evidence on 

 this point is available. Recently Rupp has made a careful in- 

 vestigation of this important problem and has obtained results 

 which have aided much in overthrowing existing prejudices against 

 the use of pasteurized mUk. The author has shown that the 

 chemical changes occurring in milk pasteurized at 62.2° C. (145° F.) 

 for thirty minutes are so small as to range within experimental 

 error. The albumin does not coagulate at this temperature, 

 soluble phosphates of lime are not precipitated, and the acidity 

 is slightly less than in raw milk. Even at 68.3° C. (155° F.) 

 the quantity of phosphoric acid, lime, and magnesia is the same 

 in the serum of both raw and heated milk. At 65.6° C. (150° F.) 

 the albumin begins to coagulate, but the amount is only 5.75 per 

 cent., while at 68.3° C. (155° F.) the amount of coagulated albu- 

 min is 12.75 per cent., and at 71.1° C. (160° F.) the amount rises 

 to 30.55 per cent. Coagulation by rennet requires slightly less 

 time in milk heated up to 65° C. (149° F.) than in raw milk, but 

 at higher temperatures there is a retardation. 



Centrifugation of Milk.— Since the introduction of centrifugal 

 machines for cream separation and milk clarification it has become 



