4 BULLETIN 744, U. S. DEPARTMENT OF AGRICULTURE. 



extraction of 144 B. t. u. In the process of freezing 1 pound of water 

 144 B. t. u. are extracted without change in temperature. In the 

 melting of ice the reverse is true. This phenomenon is called the 

 latent heat of fusion of ice. The heat required to melt 1 pound of ice, 

 therefore, is sufficient to raise 1 pound of water 144° F. or 144 pounds 

 of water 1° F. 



COOLING MILK ON THE FARM. 



Milk may be cooled most efficiently on the farm by running it 

 over a surface cooler in which the available water supply is used at 

 its coldest temperature and the cooling completed by storing the cans 

 of milk in a tank of ice water. It is possible in that way within 

 a short time to lower the temperature of milk to below 50° F. Fre- 

 quently the water used for cooling milk is not used to the best ad- 

 vantage. Spring water is sometimes allowed to flow over the sur- 

 face of the ground and is warmed several degrees before reaching 

 the cooling apparatus. During the summer water from a storage 

 tank aboveground is usually much warmer than that drawn directly 

 from the well. It is best, therefore, to arrange the cooling system so 

 that the water which flows through the surface cooler or cooling tank 

 comes directly from the well or, if from a spring, it is conveyed in 

 a pipe well below the surface of the ground. If ice is used in a cooling 

 tank the quantity of water surrounding the cans should be as small as 

 possible to give satisfactory results. Space enough should be pro- 

 vided between the sides of the tank and the cans of milk to allow 

 for a sufficient quantity of ice and water to cool the milk properly. 

 If a large volume of water has to be cooled much more ice will be 

 necessary. If it is desired to cool milk quickly from an initial tem- 

 perature of about 85° F. to one of 50° F. by setting the cans in 

 a tank of ice water, the ice water in the tank should have a tempera- 

 ture of about 37° F. Under these conditions about 4 gallons of water 

 will suffice for each gallon of milk. 



EFFECT OF LOW TEMPERATURES ON BACTERIAL COUNT 



OF MILK. 



It is commonly recognized that high bacterial counts in market 

 milk are due largely to multiplication of bacteria rather than to in- 

 oculation. It is interesting, therefore, to note the effect of low tem- 

 peratures in reducing the bacterial content of market milk. A sur- 

 vey of dairies in New England where high bacterial counts in the 

 milk had been obtained was followed by instructional work emphasiz- 

 ing the importance of rapid and efficient cooling and demonstrating 

 the best method of cooling at the least expense. A later survey 

 illustrates the effect that prompt cooling and holding the milk at low 

 temperatures had upon the bacterial content (fig. 2). 



