240 MASS. EXPERIMENT STATION BULLETIN 187. 



appear as conditions change. This is also discernible in the ascendency 

 and decline of the lactic group followed by other types wliich appear and 

 disappear, leading finally to complete decomposition of the milk. This 

 "special growth-curve" which appears when conditions are favorable is 

 a factor in clarification, for by this mechanical act the conditions for 

 microbial development are apparently somewhat altered, and accordingly 

 there is resulting a more or less kaleidoscopic change. It follows, there- 

 fore, that an additional factor to those already controlling the stages of 

 alteration or fermentation in milk has been introduced, naturally yielding 

 somewhat different changes in the course of milk fermentation. 



The removal of large numbers of bacteria by clarification, as has been 

 established, must exert some influence upon the changes which take place 

 in the clarified milk. Especially will this be true if the types which yield 

 more readily to centrifugalization are cast out in large numbers and the 

 types which seem to respond but poorly remain behind. The balance 

 of growth equilibrium is disturbed. When conditions of growth are so 

 complex as in milk, it can at once be surmised that owing to the great 

 variation in the germ-content of milk, both in numbers and kinds, the 

 results must be widely different. It seems that there ought to be evidence 

 which will correlate this great change in germ-content with alterations in 

 clarified milk as different from unclarified. It will not be possible to 

 furnish all of our data at the present writing. Only such evidence as has 

 led us into a more intimate study of these changes will be given. 



When unclarified and clarified milk of the same original sample is 

 permitted to stand for some time at low temperature (15° C), so that the 

 fermentation changes appearing do not rush by unnoticed, \dsible altera- 

 tions are evident. The precipitated casein resulting from such a fer- 

 mentation may be collected then on a sterilized filter paper, and, after 

 covering carefully, allowed to stand at ordinary temperatures for some 

 time. The difference in the fermentation changes of the unclarified and 

 clarified milk casein is usually strikingly manifest. This demonstrates 

 that in the unclarified milk and casein there exist organisms which pre- 

 ponderate over those in the clarified milk and casein. Hence the clarifier 

 has ejected certain types of organisms in sufficient numbers to control 

 the character of the fermentation in the clarified milk and casein. Whether 

 these changes can be explained by the elimination of Oidium lactis and 

 other molds and yeasts (see page 234) cannot be definitely stated at 

 present. 



These observations have induced us into undertaking to demonstrate 

 the factors involved in these differences. To this task our energies have 

 been directed, and some of the data are at present available, but it is 

 felt that the answer should be given as a single answer and as completely 

 as possible. 



