62 FERMENTS IN MILK 



undesirable flavors and aromas. There are a number of species 

 belonging to this group. The pathogenic germs, or disease- 

 producing bacteria, must also be classed with the harmful bac- 

 teria. It is not the intention in this work to give an extended 

 discussion of this subject. For such discussion see special works 

 on Dairy Bacteriology. 



Number of Bacteria in Milk. The number of bacteria found 

 in milk varies so much that it is practically impossible to state 

 accurately the average number. The number of germs found 

 varies according to several conditions, such as degree of cleanli- 

 ness of cows, utensils, and milker; degree of purity of the atmos- 

 phere when the cows are milked; the temperature at which the 

 milk is kept and the time it is held. When the milk is being 

 produced under the best practical sanitary conditions, the number 

 of germs need not exceed 10,000 per cubic centimeter. Such 

 results cannot be obtained unless extreme precautions are taken. 

 Milk produced under average farm conditions seldom contains 

 less than 50,000 germs per cubic centimeter shortly after the 

 milking. Milk which is produced under filthy conditions, and 

 which is several hours old, may contain several millions of bac- 

 teria per cubic centimeter. 



Sources of Bacteria in Milk. Bacteria are widely distributed 

 in nature. They float in the atmosphere and adhere to particles 

 of dust. Especially is this so in the dusty cow-stable. They 

 are present in all well water to a greater or less extent, and are 

 very abundant in streams and rivers. They are present in the 

 soil to a depth of several feet, the number decreasing with the 

 depth. As these germs are practically present everywhere, the 

 sources of germs in milk may be said to be all around us. The 

 principal sources of germs in milk are, however, unclean dairy 

 utensils, unclean cows, and unclean surroundings. As these 

 germs multiply chiefly by fission, or by one cell dividing into two, 

 it is plain that the number of germs will increase very rapidly 

 under favorable conditions. Under the most favorable condi- 

 tions it requires approximately twenty minutes for this process of 

 fission to take place. 



Some germs develop small bodies within the cell, called 



