APPLICATION OF REFRIGERATION TO HANDLING OF MILK. 11 



milk is preserved in a frozen state. For this reason the freezing of 

 milk, for the purpose of transportation, has hitherto been little used. 

 If the milk is held at 32° F. for a few days, some types of bacteria 

 may grow and multiply slowly. With a good quality of milk, i. e., 

 that containing few bacteria, it may take weeks or even months for 

 them to gain great headway. What few bacteria develop at low tem- 

 peratures are of different species from those ordinarily found at the 

 higher temperatures, and they may produce marked changes in the 

 chemical composition of the milk without especially changing its ap- 

 pearance. Consequently, it is unsafe to assume that milk which has 

 been held for several days at a low temperature is in good condition. 

 According to Pennington 1 milk exposed continually to a temperature of 

 29° to 32° F. causes, after a lapse of from 7 to 21 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 

 vessel. The milk does not freeze solid. In spite of the fact that the 

 milk was a semisolid mass of ice crystals, an enormous increase in 

 bacterial content took place. Though the bacterial content was 

 numerically in the hundreds of millions per cubic centimeter, there 

 was neither taste nor odor to indicate that such was the case. Neither 

 did the milk curdle when heated, and the unfitness of the milk for 

 household purposes would not ordinarily be detected until the lactic 

 acid bacteria decreased in numbers and the putrefactive bacteria 

 began to develop. 



THE INFLUENCE OF TEMPERATURE ON THE BACTERIOLOGICAL FLORA OF MILK. 



Each species of bacterium found in milk and each particular variety 

 has an upper and lower temperature limit beyond which it does not 

 grow, and a certain temperature, called the optimum, at which it 

 grows best. 



The optimum temperature of most forms occurring in milk is 

 between 70° and 100° F. As the temperature of milk is lowered the 

 rate of growth is diminished until at 40° F. the multiplication is very 

 slow and at a temperature just above the freezing point the develop- 

 ment practically ceases; in fact, there is an apparent decrease in the 

 number, at least for a short time. The action of cold at this tem- 

 perature, however, does not totally destroy life in the bacteria, but 

 causes them to lie dormant. When the temperature of the milk is 

 raised they again begin to multiply. As an illustration of the relative 

 variation in the growth of bacteria in milk held at different temper- 

 atures, one writer gives the data found in Table IV, in which 1 is 

 assumed to represent the number of bacteria in the fresh milk, and 

 the relative numbers which will be found at the end of 6, 12, 24, and 

 48 hours, at the two temperatures, are shown in the succeeding col- 



1 Pennington, Mary E. Bacterial growth and chemical changes in milk kept at low temperatures. 

 Journal of Biological Chemistry, vol. 4. nos. 4 and 5, pp. 353-393. Baltimore, 1908. 



