“ QUALITY ” IN MILK 379 
bacteria predominated. At the end of forty hours, when the milk 
had curdled, Bacillus acidi lactict constituted about 90 per cent of the 
total bacteria. At 37° C., B. lactis aerogenes predominated over B. 
acidi lactict as did B. coli when it was present. At 10° C. all types of 
bacteria developed uniformly. Neutral bacteria grew more rapidly 
and liquefiers became abundant. Conn and Esten found little differ- 
ence in the flora at 10° C. and 1° C. Growth, however, was more 
rapid at 10°C. Ravenel et al. (1910) have studied the changes in milk 
at —9°C.and0°C. At —9° C. there was no increase in the number of 
bacteria as determined on agar and gelatin plates. At 0° C. there was a 
decided increase. This was accompanied by an increase in acidity 
and other changes incidort to marked bacterial development. The 
lactic acid bacteria were inhibited oi this temperature but putrefactive 
bacteria were not. 
Pennington et al. (1908, 1913)in their studies on cold storage reported 
that milk stored at 0° C. underwent marked proteolysis which was 
noticeable at the end of two weeks. In a later paper it was stated that 
proteolysis of casein was due primarily to bacteria while the lact- 
albumin was destroyed by the enzymes in the milk. The numbers of 
bacteria greatly increased and this Increase was most striking in the 
raw, untreated milk. The freezing-point was gradually lowered with 
the decomposition. Market milk held below 0° C. increased from 15,956 
to 376,000,000 at the end of five weeks. These investigations tend to 
indicate that milk and other foods may not be held indefinitely even at 
low temperatures. Changes may eventually take place to render the 
food unfit for consumption. 
‘¢What is Meant by Quality in Milk?” Quality in any food sub- 
stance may be defined in different ways. The definition often depends 
entirely on the viewpoint of the definer. The farmer may have quite 
another conception of quality in milk than the sanitarian; the chemist 
may define milk quality from the standpoint of food value while the 
bacteriologist is concerned with disease. It is a far easier propositior 
to define milk quality than to devise methods for measuring it. One 
could scarcely find a field in hygiene which is more complicated than 
that of dairy and milk sanitation. The difficulty of finding practical 
methods for measv®ing quality im milk has caused much contention 
among health authorities. Probably little difficulty would be experi- 
enced in reaching a definition for safe milk. A milk to be safe must be 
clean. In fact cleanliness is the slogan of modern sanitation. 
A number of different methods have been used or proposed for 
measuring quality in milk. Each one has its own special advantages and 
