66 BULLETIN 98, U. S. DEPARTMENT OF AGRICULTURE. 



able hygienic conditions, and not to pathogenic (that is, disease- 

 producing) bacteria. It is impracticable to reduce the temperature 

 of milk much below 50° F. in summer without employing a refrig- 

 erating machine or ice, and as the former is too expensive for the 

 ordinary farmer, we are limited to the use of ice or well water. 

 Where ice is plentiful and may be had at a nominal cost it is an easy 

 matter to reduce the temperature to, say, 40° F., and by referring 

 to Tables IV, V, and VI under "Influence of temperature and time 

 on the development of bacteria in milk" it will be noted that the 

 multiplication of bacteria at this temperature is very small. 



In those locations where natural ice is available it is compara- 

 tively an easy matter to cool milk or cream on the farm before carry- 

 ing it to the receiving station or creamery. This may be done by 

 running the milk or cream over some form of cooler in which cracked 

 ice or a mixture of ice and salt is placed, or through which cold 

 water is circulated. 



Where the milk or cream is placed in cans and set in cool water, 

 or even in a tank filled with ice and water, the cooling goes on very 

 slowly, especially if the cans are large. The outside portion, how- 

 ever, may be cooled in a comparatively short time, but unless it is 

 stirred repeatedly it will take considerable time before the interior 

 is cooled down to a point where the development of bacteria is re- 

 tarded to such an extent that the milk or cream may be safely car- 

 ried to the receiving station or creamery, as the case may be. It is 

 often the case that a can of milk is set into a cooling vat in which the 

 cooling medium is lower in level than the milk in the can, in which 

 case the milk in the lower part of the can may be cooled down to 

 approximately the temperature of the cooling medium, while that 

 above the level will remain at the higher temperature of the atmos- 

 phere; consequently, when the milk is stirred the whole will turn 

 sour and spoil. The cold milk, being heavier than the warm, will 

 naturally remain at the bottom of the can, while the warmer and 

 therefore lighter portion will remain at the top, and practically no 

 circulation will take place and the transfer of heat by conduction in 

 this case is very slow. 



If proper care is exercised, however, milk and cream may be cooled 

 down to a temperature sufficiently low to get to the receiving sta- 

 tion or central creamery in good condition by running spring or well 

 water through the cooler. In the winter months the lower atmos- 

 pheric temperatures assist in the cooling, but in the hot summer 

 months the higher temperatures of the atmosphere retard the cool- 

 ing; consequently, during the hot weather the milk or cream should 

 be run over the cooler very slowly, and if its temperature is not 

 sufficiently lowered it should be run over the second time. In this 



