CLARIFICATION OF MILK. 239 



milk clarification, provided the present explanation which accounts for 

 the increased number of bacteria after clarification is tenable. At present 

 our knowledge is too restricted to draw stable conclusions. 



Other Considerations. 



Centrifugal force has been repeatedly and commonly employed to eject 

 micro-organisms when in suspension, which is the case in hand. Its 

 values for this purpose are in a very general way understood. From the 

 largest micro-organism with limited surface as compared with its content, 

 to the minutest with its extensive surface as compared with its content, 

 there seems to exist a gradation in effectiveness. In other words, the 

 large organisms are easily ejected, while the minutest are with difficulty 

 cast out. In the case of some of the invisible viruses the capacity to 

 produce disease is not reduced materially by centrifugalization. In the 

 foregoing tables it is apparent that the larger micro-organisms, as the 

 spores of Oidium ladis and the cells of Sacch. cerevisice, respond readily 

 to centrifugal force, while such organisms as B. prodigiosus respond 

 poorly. Likewise, colonies seemingly act as large and small cells. Again, 

 it is well known that micro-organisms contain a variable amount of fat, 

 as B. tuberculosis. Fat is easily determined, too, in varying amounts in 

 mold and yeast cells when subjected to certain conditions of growth. 

 The presence of fat must influence the specific gravity of cells, which in 

 turn is closely related to results from centrifugalization. The age of 

 a microbial cell, or the stage of development, is also bound up with its 

 specific gravity, due probably to the degradative changes taking place. 

 This is easily seen in the development of a culture when the old cells 

 settle to the bottom. 



It is verj^ evident from physical laws that the material in which micro- 

 organisms are suspended has a very important and peculiar influence in 

 their sedimentation by mechanical force. Milk, with its higher specific 

 gravity and viscosity, acts as a deterrent in the removal of micro- 

 organisms by centrifugalization, as is clearly evidenced by the preceding 

 tables for specific organisms. In spite of deterrent influences referred to, 

 micro-organisms are removed from milk in as large quantities as 75 per 

 cent, and over. Inasmuch as the plate colony-counts probably represent 

 colonies removed from milk, the percentage may rise much higher. The 

 results presented in the preceding tables, in which the work of the clarifier 

 upon specific organisms is shown, have an illuminating bearing on the 

 action of the clarifier in its practical application to market milk. 



In considering micro-organisms in milk it is necessary to remember 

 the "ebb and flow" of species. All who are students of milk have 

 learned that in the course of fermentation-development certain types 

 of micro-organisms in milk gradually reach the crest of their growth 

 then gradually decline in numbers, as the rise and fall in numbers of the 

 many species which are present in fresh milk, and which practically dis- 



