GENERAL REVIEW OF THE INVESTIGATION. 101 



there will be included here only the conclusions reached, which are 

 as follows: 



SUMMARY OF RESULTS. 



Bacteria in milk increase in numbers when the temperature is maintained at or a 

 little below 0° C. This temperature is below that ordinarily assigned as the limit of 

 bacterial multiplication . 



Milk has been kept for periods ranging from a few days to almost two years at a 

 temperature of 29° to 31° F., and also at 32° F. It has been kept in a packages of ten 

 quarts and one quart. It has been the cleanest milk obtainable, by the most carefully 

 enforced refinements of modern dairying; and it has also been market milk produced 

 in the ordinary dirty stable and subjected in transit to the usual careless handling. 



Bacterial growth at the end of a week, even in the cleanest milk, which contained as 

 low as 300 organisms to the cubic centimeter, was pronounced. There was a steady 

 increase in the number of organisms for five or six weeks, and at their maximum 

 they numbered hundreds of millions. Occasionally they passed the billion mark per 

 cubic centimeter. 



Continued exposure to a temperature of 29° to 31° F. causes, after a lapse of from 

 seven to twenty-one days, the formation of small ice crystals which gradually increase 

 until the milk is filled with them and there may be an adherent layer on the walls of 

 the vessels. The milk does not freeze solidly. In spite of the fact that the milk was 

 a semisolid mass of ice crystals, the enormous increase in bacteria which this study 

 shows took place. Though the bacterial content was numerically in the hundreds 

 of millions per cubic centimeter there was neither odor nor taste to indicate that 

 such was the case. Neither did the milk curd even on heating, and it was not until 

 the bacterial content began to fall, and organisms of putrefaction were underway, that 

 the use of the milk for household purposes would, to the ordinary observer, become 

 contra-indicated. 



A classification on a chemical basis of the organisms occurring at these low tem- 

 peratures shows that there were constantly present bacteria which formed acid and 

 bacteria which acted upon proteid. There were also neutral organisms, which formed 

 neither acid nor alkali and did not act upon gelatin. The acid-forming organisms 

 were generally in relatively smaller numbers than are found when milk is kept at 

 higher temperatures, and the liquefying organisms were more numerous. Certain spe- 

 cies, such as B. formosus, B. solitarius, and B. raveneli, were especially resistant to 

 cold and frequently were the predominating species, or almost in pure culture at the 

 close of the experiment. 



A very marked difference in both the number and kind of organisms which devel- 

 oped on the plates was noticed, depending upon the temperature at which the plate 

 was incubated. In certain experiments the maximum number grew at 37° C. In 

 others the temperature at which the milk was stored served best for colony formation. 

 The relative number of organisms growing at 37° C, 20° C, and 0° C, or a little below, 

 varied greatly also with the length of time that the milk had been kept in storage, 

 the organisms developing at body temperature being ordinarily greatly in excess at 

 the beginning of the experiment and diminishing until near its close, when a sharp rise 

 was apt to take place. 



The determination of the acidity showed that after a few weeks a much higher acid 

 content was reached than is ordinarily required for the spontaneous separation of 

 curd, which, however, seldom happened. Milk having this high acidity, when 

 placed in an ordinary ice chest, increased in acid content but did not curd for days 

 after exposure to the higher temperature. 



The chemical study of the proteid of milk in cold storage showed that the casein 

 was rapidly digested until finally more than 50 per cent of it was changed to soluble 

 compounds. Caseoses, amido acids, and probably peptones, increase apparently at 



