322 
Journal of Agricultural Research 
Vol. II, No. 4 
indicate that the varieties of streptococci associated with or responsible 
for this disease are among those varieties which have a low thermal death 
point. 
determination of thermal death point 
In these experiments the following method of determining the thermal 
death point has been used. The streptococci were grown first in plain 
neutral extract broth for 18 hours and then inoculated by means of a 
small-bore pipette into litmus-milk tubes. Four drops constituted an 
inoculation in each milk tube. In making the inoculation care was taken 
not to have any of the culture touch, or any of the inoculated milk wash 
up on, the sides of the tube, either during the handling or during the 
subsequent heating. 
The inoculated milk tubes were heated in a large water bath and the 
temperature of the milk was recorded in a control milk tube by a ther¬ 
mometer placed in the milk. The temperature in the tubes was not 
allowed to vary over half a degree in either direction. In all the experi¬ 
ments the heating period was 30 minutes at a given temperature. After 
heating, the tubes of milk were quickly cooled to about io° C. (50° F.), 
incubated at 37 0 C. (98.6° F.), and the reactions recorded. Growth in the 
tube indicated that the organism was not destroyed at the particular 
temperature to which the milk had been subjected. In every case the 
tubes were run in duplicate, and in general both tubes had to show 
growth before the test was considered positive. The only exception to 
this was when only one tube showed growth after the highest heating tem¬ 
perature; in such cases one tube was considered a positive reaction, and 
the organism was recorded as surviving the process. 
This method of determining the thermal death point was used in order 
to render the conditions of heating similar to Pasteurization. 
1. the thermal, death point op the cultures as a whole 
The thermal death point of 139 cultures 1 of streptococci was studied. 
These cultures were isolated from cow feces, from the udder and mouth 
of the cow, and from milk and cream; therefore they represent a wide 
range of sources of the streptococci commonly found in milk. 
The cultures were heated in milk, as previously described, to tempera¬ 
tures ranging from 48.9° C. (120° F.) to 73.9 0 C. (165° F.). The results 
given in Table I show the number and percentage of cultures which with¬ 
stood the different temperatures. Of the total cultures, 138, or 99.28 
per cent, survived heating for 30 minutes at 54.5 0 C. (130° F.). At 
57.2 0 C. (135 0 F.) 118, or 84.89 per cent, of the cultures survived. At 
6o° C. (140° F.), the lowest Pasteurizing temperature used commer- 
1 The cultures of streptococci were supplied by Mr. L. A. Rogers, of the Dairy Division, Bureau of Ani¬ 
mal Industry. 
