EXPERIMENT STATION BULLETINS. 197 



water and a little steam are little effective; what is needed is bard brush- 

 ing until all the dirt is removed, and the addition of plenty of hot water 

 (212° F.) for several minutes, and if steam, let it run for a few minutes. 

 This is necessary if the life in the pail is destroyed. Too much care 

 cannot be exercised in this direction. 



Since milk in the udder is free from bacteria and the only likely con- 

 taminations are through the sources named above, the importance of 

 cleanliness is manifest. 



VARIETIES OF BACTERIA IN MILK. 



It would be unsafe to place any definite limit to the number of species 

 found in milk since it harbors bacteria which may come from almost any 

 source. The conclusion is forced that the number of species is unlimited. 



Those bacteria which find their home in the filth of the barnyard, the 

 dust of the barn or dairy, in the soil and water, are the species usually 

 found in milk. To consider the possibilities of their actions is beyond our 

 present scope. Suffice it to say that the bacteria usually finding a resi- 

 dence in milk belong to the saprophytic and non-pathogenic class. 



There is a class of bacteria which, having once established themselves 

 in a dairy, become a part of it and exert their influence daily. These are 

 the lactic acid bacteria, and they are constant factors in the ordinary milk 

 changes. 



MULTIPLICATION OF BACTERIA. 



The bacteria having gained access to the milk and finding a suitable 

 temperature soon begin to multiply. How rapidly they may increase has 

 been considered in Bulletin 139, p. 67. If we start with several millions 

 and increase in a geometrical ratio using every thirty minutes as the unit 

 of time, the enormity of the number after a few hours growth is scarcely 

 imaginable. Frequently after the milk has stood several hours it is possi- 

 ble to find hundreds of thousands in a single drop. Too rapid souring of 

 the milk is not desirable, and if obnoxious bacteria grow in this way and 

 gain the ascendency over the lactic acid bacteria, bad and even harmful 

 results are obtained. 



To secure the best milk products, it should be the plan of every dairy- 

 man to delay unnecessary fermentation by whatever legitimate method 

 is at his command. Perhaps the most economical and inexpensive is the 

 cold water method of reducing the temperature below that required for 

 the growth of bacteria. 



INFLUENCE OF TEMPERATURE UPON BACTERIA IN THE DAIRY. 



Cold prevents growth, heat checks growth or kills the bacteria. Out 

 of fifteen lactic acid producing bacteria, isolated by Kayser (Cent. f. Bact. 

 II. Bd. I. p. 436) none formed lactic acid at 10° C, even when allowed to 

 act for thirty-five days. Five would not produce curdling at 15° C. This 

 shows that a low temperature is able to keep milk from souring for a long 

 period. It must not, however, be understood that cold kills the bacteria 

 or renders them inactive when they are transferred to a warm ^temper- 



