454 



MICROBIOLOGY OF MILK AND MILK PRODUCTS 



ity of the milk. If milk is strained through cheese cloth or wire 

 gauze much of the insoluble dirt can be removed. This has led to the 

 general belief that straining improves the sanitary and keeping quali- 

 ties of the milk. 



The effect of straining on removal of insoluble dirt is shown by 

 the following results of tests: 



DIRT REMOVED BY PASSING MILK THROUGH Two THICKNESSES OF FINE CLOTH 

 (Weight of insoluble dirt given in milligrams per liter of milk) 



Experiment 



Before straining 



After straining Per cent removed 



It may be noticed that even after straining the milk contained 

 appreciable quantities of insoluble dirt which had passed through 

 the strainer cloth. The difference in per cent of dirt removed in 

 different samples is due to the nature of the dirt itself. The coarser 

 the dirt the greater the proportion that will be removed by straining. 



It is not true, however, that the keeping quality is necessarily 

 improved by the simple process of straining. It depends largely upon 

 the condition of the milk and the nature of the strainer. Not infre- 

 quently passing milk through a strainer not only fails to improve its 

 keeping quality but actually injures it. This has been shown by a 

 number of investigators. The effect of straining upon the germ con- 

 tent may be seen in the following figures where the milk was passed 

 through a strainer composed of three thicknesses of fine cheese cloth 

 supported by wire gauze. 



EFFECT OF STRAINING UPON BACTERIAL CONTENT OF MILK 



Experiment 



After straining, 

 bacteria per c.c. 



