92 MANUAL OF MILK PRODUCTS 



likewise dependent on these conditions. Temperature also has 

 a marked effect, the consistency being increased or diminished 

 as it is lowered, or raised. 



Effect of physical agents 

 Heat. 



It has previously been stated that variations in temperature 

 have a marked effect on the consistency of milk and cream ; 

 that within moderate limits the effect is the same as with most 

 viscous substances, it being made thinner when warmed, and 

 the original consistency returning when cooled to the initial 

 temperature. 



Pasteurized milk and cream show an apparent exception to 

 this rule, inasmuch as the original consistency does not return 

 after the milk or cream is thoroughly chilled. This condition 

 is a serious objection to the use of these products on the part 

 of the consumer, as he invariably refers this relative thinness 

 to a lack of butter-fat. So serious has this complaint become 

 that it has greatly militated against the general introduction 

 of these products. As this is the only objection, and as these 

 products have every advantage from a sanitary and economic 

 standpoint in comparison with ordinary milk and cream, this 

 has been the chief incentive from the practical standpoint for 

 undertaking this study. 



Before attempting to find a remedy for this difficulty, it 

 became necessary to study the general question of the conditions 

 affecting the consistency of natural milk and cream when sub- 

 jected to high temperatures such as are used in the pasteurizing 

 and sterilizing processes. 



Here the microscope gave us an important clue to the physi- 

 cal constitution of milk and cream that had been submitted to 

 the action of heat. A microscopic examination of pasteurized 

 milk or diluted pasteurized cream presents a very different pic- 

 ture from that of normal milk or cream. In the case of the 



