APPLICATION OP REFRIGERATION TO HANDLING OF MILK. 67 



way it is possible to get the temperature down to within 2° or 4° of 

 the cooling medium. 



As the development of bacteria begins as soon as the milk is drawn 

 from the cow, it is of the utmost importance that the cooling be 

 done as quickly as possible after milking, in order to keep the in- 

 itial number of organisms down to a minimum. The rapidity at 

 which the development of bacteria goes on in milk at a given tem- 

 perature depends, of course, on the initial count, hence the impor- 

 tance of keeping the initial count as low as possible. 



Tests were made to determine the time required to cool milk by 

 placing a 10-gallon can in a box and running cooling water around 

 the can, as shown in figure 31 (p. 68). The average temperature of the 

 water was 62.6° F. and the flow of water was regulated so that there 

 was practically no difference between the inlet and outlet water. 

 Thermometers were placed in the can, as shown in the attached 

 sketch, and readings were taken every 15 minutes until the tem- 

 perature of the milk was approximately that of the cooling water. 

 The results of these readings are plotted in the form of curves, which 

 are numbered from 1 to 7, inclusive. Curve No. 8 is plotted from 

 thermometer readings taken in the milk at top of can and shows 

 that that part of the milk above the water level remains from 5° to 

 6° warmer than the portion below the water level; consequently, 

 bacteria will develop at a higher rate in that portion of the milk 

 above the water level and when mixed will hasten the souring of the 

 milk, both by raising the temperature of the whole and by the in- 

 creased number of bacteria contained in the warmer portion. 



The curve showing the comparatively rapid decrease in tempera- 

 ture when the milk was thoroughly stirred at intervals of 15 minutes 

 demonstrates the advantage of agitating the milk while cooling. 



The time taken to cool the milk in either case, however, is too 

 great for good results, and the tests serve best to demonstrate the 

 necessity of employing some efficient form of milk cooler suitable for 

 farm use. 



Figure 32 (p. 69) shows the method of cooling milk employed on the 

 United States experimental dairy farm located at Beltsville, Md. 

 The equipment consists of a one-fifth ton refrigerating machine 

 operated by a one-half horsepower motor, a small rotary circulating 

 pump driven from the shaft of the refrigerating machine, and a corru- 

 gated milk cooler. Water, instead of brine, is used for circulating 

 through the cooler as the night's milk is cooled, placed in cans, and 

 set into the tank until the next morning; if brine were used [it 

 would corrode the cans. The tank holds about 120 gallons of water, 

 which is cooled down to approximately 35° F. and held at this tem- 

 perature until time for cooling the milk, when it is pumped through 



