THE PHYSICAL AND CHEMICAL EXAMINATION OF 



MILK 



In making routine examinations it is usually sufficient to 

 examine milk for the following factors: Specific gravity, fat, total 

 solids, and soUds-not-fat — also called plasma solids. For more 

 detailed analyses the quantity of ash, total protein, casein, albu- 

 min, and milk-sugar should be determined. It may also be 

 desirable to estimate the acidity, sediment, viscosity, the speed of 

 rennet coagulation^ alcohol coagulation, and to determine if the 

 milk has been heated. Sometimes it is necessary to distinguish 

 between human and cow's milk. Furthermore, coloring-matter 

 and gelatin are sometimes added to milk, the former to impart a 

 yellowish color and the latter to increase the viscosity. Pre- 

 servatives, too, are still sometimes added to milk, so their presence 

 should be tested for. For special investigations detailed analyses 

 of the ash may be made. 



A correct sample of the milk to be examined must be obtained. 

 As the fat rises within a relatively short time when the milk is 

 quiescent, thorough mixing is necessary prior to the removal of 

 samples. The longer the milk stands before sampling, the more 

 difficult becomes the even distribution of the cream. This is 

 especially true when the milk has been kept at the proper low 

 temperature. The fat then solidifies and adheres to the walls of 

 the vessel. Under these conditions the milk should be warmed 

 to 40° to 45° C. to liquefy the fat. 



Mixing is best accomplished by pouring the milk repeatedly 

 from one vessel to another. When samples from large cans are 

 to be taken the milk should be stirred with a large clean spoon or 

 dipper. An instrument which is designed to stir the milk and 

 take a sample at the same time is very useful. This consists of 

 a stiff iron rod with a sUghtly concave disk attached to the lower 

 end (Fig. 28). The milk is mixed by moving the stirrer up and 

 down. Under no conditions should the milk be shaken violently, 

 as the viscosity of milk prevents air-bubbles from rising, and there- 

 fore a sample taken after violent shaking would not represent 

 the desired volume. Furthermore, violent agitation of milk has 

 a tendency to churn it, with the result that even distribution of 

 the fat becomes difficult. If milk is first diluted with water and 

 then shaken, the result is not as detrimental as when undiluted 

 milk is agitated. Therefore it is possible to obtain well mixed 

 milk by the addition of a definite amount of water. 



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