UNITED STATES DEPARTMENT OF AGRICULTURE 



BULLETIN No. 744 i 



Contribution from the Bureau of Animal Industry 

 JOHN R. MOHLER, Chief. 



Washington, D. C. 



PROFESSIONAL PAPER 



January 17, 1919 



COOLING MILK AND STORING AND SHIPPING 

 IT AT LOW TEMPERATURES. 



By James A. Gamble, Market Milk Specialist, and John T. Bowen, 

 Technologist, Dairy Division. 



CONTENTS. 



Scope of experimental work 



The principle of cooling 



Cooling milk on the farm 



Effect of low temperatures on bacterial count 

 of milk 



Cooling efficiency of various kinds of tanks. . 



Page. 

 1 

 1 



4 



The construction of milk tanks 15 



How to cool milk quickly 18 



Efficiency of cufforent kinds of cans for hold- 

 ing milk 21 



Transporting milk at low temperatures 24 



Summary 28 



SCOPE OF EXPERIMENTAL WORK. 



The experimental work reported in this bulletin covers (1) the 

 relative efficiency of cooling tanks of different construction handled 

 under varying conditions; (2) the most efficient methods of cooling 

 and storing milk on the farm; and (3) the transportation of milk 

 at low temperatures to market. 



THE PRINCIPLE OF COOLING. 



If a warm body is placed in contact with a cold one, heat will flow 

 from the warmer to the colder until both have reached the same 

 temperature. The rate of this flow depends upon the difference of 

 temperature between the two bodies. Heat flows most rapidly when 

 there is the greatest difference in temperature, and the rate grad- 

 ually decreases as the temperatures become equal. 



The temperature of a body does not give a true indication of the 

 amount of heat it contains. Heat is commonly measured by a unit 

 called the British thermal unit, usually abbreviated B. t. u., which is 

 the heat necessary to raise the temperature of 1 pound of water 1° F. 

 The reason that the heat required to raise 1 pound of water 1° F. is 

 taken as the unit is because water is one of the most difficult of all 

 substances to heat. Practically all other substances require less heat 

 to raise a unit quantity 1° F. For example, the heat necessary to 

 raise 10 pounds of iron 100° F. is only 10 X 0.113 X 100 — 113 B. t. u. 

 In other words, while it takes 1 B. t. u. to raise 1 pound of water 



81238°— Bull. 744—19— — 1 (1) 



