Frost-—Bacteriological Control of Public Milk Supplies. 1353 
good purpose in the control of pasteurization, and merits further 
study. 
This work seems to confirm the good opinion which Savage 
expresses of it. 
5. Spores or Heat-Resistant Fo)rms. The various grades of 
milk probably contain different proportions of spores. One 
would Suppose that the best grades would contain very few 
spores—that the more careless the operation of milking and 
handling, the greater the number of spores it would contain. 
These would come largely from dust and manure. Providing 
all milk was immediately cooled and kept cool, the care in milking 
could be quite accurately judged by the proportion of spores 
present. 
If milk stands for some time, and especially at temperatures 
at which bacteria, can grow rapidly, spores originally present 
would germinate and others would probably be formed so that no 
general law could be formulated, and their estimation would be 
of little value. Whatever their genesis, however, spores of bac¬ 
teria in milk would not be found in any quantity in the very good 
milks properly kept. While mjlks with a high initial contamina¬ 
tion, or milks which are not properly kept, would contain very 
many spores. 
Furthermore, it is quite evident that if milks used for pasteuri¬ 
zation contain a large number of spores it would be impossible to 
produce a pasteurized product from such milks with a low bac¬ 
terial content. 
The efficiency of pasteurization is now determined by ascer¬ 
taining the percentage of original bacteria which are killed dur¬ 
ing the process. This method is, however, unsatisfactory, since 
it places a premium on highly contaminated raw milks, as it is 
easier to get a higher percentage of efficiency with them] than with 
good milks. Bearing on this point, the following quotation from 
Ayres and Johnson* is made: “Percentage bacterial reduction 
is of no value in determining the efficiency of the process of 
pasteurization. As a general rule when the bacterial content of 
raw milk is high there will be a high percentage reduction. When 
the bacterial content is low, then the percentage reduction is of¬ 
ten low. The percentage reduction may be 99.9 per cent and 
* Ayers and Johnson: A Study of the Bacteria Which Survive Pas¬ 
teurization. Gov’t. Printing Office, Wash., p. 58. 
