EXPERIMENT STATION BULLETINS. 303 



the combination from the pure cultures used as checks and running paral- 

 lel under exactly the same conditions. 



It is not necessary in this article to discuss in general the symbiotic in- 

 fections and fermentations. Some of these have been referred to in the 

 previous contributions. It is desired to emphasize but one aspect of the 

 problem under consideration in this article — the real significance of these 

 microbial relations to actual dairy practices. 



Selecting micro-organisms for demonstrating lactic relationship, as has 

 been our practice in the past, is truly suggestive and seemingly conclusive, 

 yet milk as it exists under commercial conditions is so fraught with micro- 

 bial complications that it would scarcely be advisable to conclude from 

 mere possibility. Accordingly it is hoped that the suggestions and tenta- 

 tive conclusions of the past may take a more permanent nature as final con- 

 clusions in the present. Each sample of milk will be considered by itself 

 for the purpose of keeping the unity of the sample. A comparative study 

 at the end will furnish sufficient data to illustrate the harmonies and discords 

 of the various samples and the peculiarities existing within each will be 

 discussed as they arise to our attention. 



METHODS. 



Each sample of milk was secured in Erlenmeyer flasks, the acidity* de- 

 termined as soon as reaching the laboratory, and plates made in ordinary 

 and whey agar. The flasks were then placed at a uniform temperature of 

 21° C. From time to time, as indicated under each sample, the acidity 

 was determined and plates made for ascertaining the bacterial content. 

 Any changes in the milk apparent to the e5^e were noted. All determinations 

 of the above character ceased with the loppering of the milk. 



As the plates developed the total count was made and an effort put forth 

 to obtain some notion of the various kinds of micro-organisms present in 

 each set of plates and their relative numbers. The study of the rise and 

 fall of the different kinds of micro-organisms present in the given sample 

 has its importance in the final conclusions to be gathered from the various 

 combinations of the sample. 



It would be a great advantage if the ordinary lactic fermentation of milk 

 could be followed from beginning to end along with its various modifications 

 and each deviation or change in the course noted with satisfactory accuracy. 

 While approximation is suggestive and doubtless holds the investigator 

 to a rigid course, yet a failure, as is really the case, in ascertaining exactly 

 the number of micro-organisms of a given species existing in milk at a definite 

 period of its fermentation and the reason for increase or decrease, as the 

 case may be with each species, must leave many problems wide open for 

 future determination. 



In investigations which involve so many possibilities, it has not been the 

 intention of the writers or is it feasible to attempt exhaustive work, but 

 rather to hew to a line which may be a fair average of the many divergent 

 lines which might be undertaken with profit in connection with our theme. 



It would make our findings quite different had we chosen another tem- 

 perature than 21° C at which the cultures were maintained; yet, had another 

 temperature been chosen to compare with 21° C, the required work would 

 have been doubled. Again, any change in temperature would be likely 



♦Acidity was always determined by deci-normal solution of sodium hydroxide with phenol-phthalein 

 as an indicator. 



