THE RELATION OF MICROORGANISMS TO MILK. 321 



use has been opposed by health authorities and is contrary to the Pure 

 Food Laws. If milk is handled with any degree of care, there should be 

 no need for the use of chemical preservatives. They are simply a means 

 of counteracting the unsanitary conditions of the production and handling. 

 The same results can be obtained by cleanliness in the production of the 

 milk and the use of low temperatures for preventing the contamination 

 and subsequent growth of the bacteria in the milk. The developments 

 in the production of clean milk of the past few years have illustrated very 

 clearly that the use of chemical preservatives is not necessary. 



NORMAL DEVELOPMENT OF MICROORGANISMS IN MILK. 



The flora of any particular sample of fresh milk is determined by the 

 conditions under which it is produced. In stables where extreme clean- 

 liness is practised the flora may be practically limited to those species 

 which occur in the udder of the cows, but under ordinary conditions there 

 will be in addition to the normal udder types such others as may occur 

 in the dust and atmosphere of the stables. Market milk, therefore, 

 when first obtained from the cow ordinarily contains a mixed flora, the 

 different types present depending upon the sanitary conditions under 

 which the milk is produced. 



The future development of this initial flora is largely dependent upon 

 the temperature at which the milk is kept. If the milk is held at tempera- 

 tures between 10 and 21 there will result what may be considered as the 

 normal development of milk fermentations. These changes may be 

 divided for convenience into four periods or stages. 



FIRST STAGE. GERMICIDAL PERIOD. It has been shown by a number 

 of investigators that instead of an increase in the numbers of bacteria in 

 fresh milk there is normally a decrease in the number during the first few 

 hours after its production. The rapidity of this decrease and the length 

 of time over which it extends seem to be determined largely by the tem- 

 perature at which the milk is kept. The higher the temperature the more 

 rapidly the number of organisms decreases and the more quickly the end 

 of the germicidal period is reached. If the temperatures are kept fairly 

 low the rate of decrease is much slower but the decline will extend over a 

 considerably longer period. This is shown by the following examples 

 given by Hunziker. 



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