THE RELATION OF MICROORGANISMS TO MILK. 



315 



organisms, which constituted over 99 per cent of the total organisms 

 present at the time of curdling. While this may be considered as showing 

 the normal effect of the original contamination upon the milk, it is well 

 to bear in mind the fact that there are many apparent exceptions due to 

 some particular type of organism predominating and interfering with the 

 normal development of the lactic types. 



STRAINING. The straining of milk is one of the most common opera- 

 tions in connection with its handling and is considered by most dairymen 

 as one of the most essential from the standpoint of the quality 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 qualities 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 infrequently passing milk 

 through a strainer not only fails to improve its keeping quality but ac- 

 tually injures it. This has been shown by a number of investigators. 

 The effect of straining upon the germ content may be seen in the following 



