778 EXPERIMENT STATION RECORD. [Vol.35 



Effect of age and temperature on bacterial growth in milk. 



Kind of organism. 



Age, in days, of milk kept at various tem 

 peratures, which gave maximum 

 counts on nu' ;ient agar. 



Incuba- 

 tor. 

 35° C. 



Room, 

 15-28° C. 



"Water 

 tank, 



13° C. 



Cold stor- 

 age, 

 -1°C 



Bacterium lactis acidi 



Sarcina lutea 



Bacillus coli 



B . cyanogenes 



B. proteus vulgaris 



B . XTogenes 



B. fluorescens liguefaciens 



B . puiidum 



Microspira tyrogena 



B.subtilis 



M. citricu.i 



Oidium lactis 



B. prodigiosis 



All the organisms studied grew to some extent at the temperature of —1°. 

 Two types of micro-organisms were found very sensitive to this temperature. 

 B. lactis acidi, representing one type, increased from 30 per cubic centimeter at 

 the beginning to 2,870 per cubic centimeter on the third day, after which the 

 members gradually decreased to 50 per cubic centimeter on the forty-.second 

 day. The other class, represented by M. tyrogena, grew very slowly at first but 

 increased during the latter part of the observation period, reaching nearly 

 10,000 per cubic centimeter on the forty-second day. It is stated that this 

 growth relationship has an important bearing upon the storage of milk at low 

 temperatures for long periods. The milk might be appreciably changed in 

 chemical composition yet remain sweet. Certain organisms commonly asso- 

 ciated with filth, e. g., B. fluorescens liquefaciens, were more successful in grow- 

 ing at lower temperatures than the lactic acid bacteria. 



" The temperatures of previous incubation appeared to have an influence upon 

 the members of organisms developing upon gelatin and agar plates, as well as 

 the optimum temperature for the different organisms. The members of organ- 

 isms developing on agar plates were more or less closely correlated with the 

 amount of acid formed and the curdling of the milk. The number of organisms 

 developing on gelatin plates were more or less closely correlated with the forma- 

 tion of enzyms capable of reducing methylene blue." 



Studies on the clarification of milk, B. W. Hammek (Iowa Sta. Research 

 Bui. 28 (1916), pp. 19-32).— The results obtained in these studies show that 

 plates poured from clarified milk commonly, although by no means constantly, 

 revealed larger numbers of colonies of bacteria than plates poured from unclari- 

 fied milk. Since clarifier slime contains large numbers of bacteria and con- 

 tamination was practically excluded the increases in the number of colonies 

 developing on plates were only apparent increases, due to the breaking up of 

 clumps of organisms by the centrifuging. There was no definite relationship 

 between the effect of clarification, on the one hand, and such factors as the 

 original count, temperature of the milk, or the percentage of fat, on the other. 

 It is concluded that whether there will be an increase or a decrease in the 

 apparent number during clarification probably depends on the types of organ- 

 Isms and on the presence of clumps. 



Fifty-one comparisons of the bacterial content of clarified and unclarified 

 milk were made on samples showing less than 100,000 organisms per cubic 

 centimeter. In 3 cases the bacterial content was not influenced by clarification, 



