112 MODERN METHODS OF TESTING MILK 



EVAPORATION OF WATER 



In the case of all methods, the vessel containing the 

 weighed sample of butter is subjected to heat to evap- 

 orate the water. The means of applying heat vary; 

 in some cases, the vessel is heated directly over an 

 alcohol or gas flame, or there may be interposed a 

 thin sheet of asbestos or iron plate ; in other methods, 

 the dish is placed in some form of drying-oven, 

 heated by steam under pressure. 



In the process of evaporation, certain precautions 

 must be observed, especially in reference to (i) 

 length of time of heating, and (2) loss by sputtering. 



Length of time of heating. This depends mainly 

 upon the size of the sample, the degree of temperature, 

 and the diameter of the dish in which the sample is 

 heated. 



The objection to heating at high temperature longer 

 than is necessary to evaporate the water completely is 

 that other material is in danger of being driven off 

 from the butter, the consequence being that the re- 

 sults are too high. How can we tell when the water is 

 completely removed? This is, for the most part, a 

 matter of experience. Generally, the appearance and 

 behavior of the dried residue are depended upon to 

 indicate when evaporation is complete. When butter 

 is heated in an open dish, a foamy, snow-white cover- 

 ing collects over the surface as soon as the butter be- 

 comes quite hot; this white covering changes to a 

 dirty-brown color, and the crackling noise due to the 

 escape of water-vapor ceases, after the heating has 

 been continued some time, and then a slightly pungent 

 odor is usually noticeable. These conditions indicate 



