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



heat is used to destroy the bacteria, or at least to reduce their num- 

 ber to such an extent as to prevent their producing disease; but pas- 

 teurized milk as well as unpasteurized market milk should be cooled 

 to a temperature of 50° F. or below and held at this lower tempera- 

 ture until used. At a temperature below 50° F. bacteria multiply 

 less rapidly, but between 50° and 100° F. the increase is very fast; 

 hence the necessity for thorough cooling and the maintenance of low 

 temperatures until used. 



DEFINITIONS OF TERMS. 



A knowledge of the terms used in refrigeration is necessary in order 

 to better understand the matter given in the following pages. There- 

 fore definitions of the principal terms and units employed are given 

 for the benefit of those not already familiar with them. 



British thermal, unit. — A British thermal unit (B. T. U.) is the quan- 

 tity of heat required to raise 1 pound of pure water 1 degree Fahren- 

 heit, at or near its maximum density, 39.1° F. Some authorities 

 consider a British thermal unit as the heat required to raise 1 pound 

 of pure water from 61° to 62° F. For practical purposes, however, 

 it may be considered the heat required to raise the temperature of 

 1 pound of water 1 degree Fahrenheit. 



Sensible heat. — Sensible heat is the heat that may be felt by the 

 hand or measured by a thermometer. 



Latent heat. — Latent or ' ' hidden " heat is the heat which is expended 

 in molecular work of separating the molecules of the substance and 

 can not be measured by a thermometer. Every substance has a 

 latent heat of fusion, required to convert it from a solid to a liquid, and 

 another, latent heat of vaporization, required to convert it from a 

 liquid to a gas or vapor. Thus, if heat is applied to a pound of ice 

 at 32° F. it will begin to melt, and no matter how much heat is applied 

 the ice will not get any hotter. After every particle of ice has melted, 

 we will have 1 pound of water at 32° F., the same temperature as the 

 ice before heat was applied. Experiments have shown that it requires 

 144 British thermal units to melt 1 pound of ice at 32° F. into water 

 at 32° F.: hence the latent heat of fusion of ice is said to be 144. 



If heat is applied to 1 pound of water at 212° F., the water will 

 remain at 212° F. under atmospheric pressure until all of it has been 

 evaporated into steam at 212° F. This has been found to require 

 970.4 British thermal units; hence the latent heat of vaporization of 

 steam at atmospheric pressure is said to be 970.4 B. T. U. 



Specific heat. — The specific heat of a substance may be denned as 

 the ability of that substance to absorb heat compared to that of water. 

 Water being one of the hardest of all substances to heat, its specific 

 heat is taken at unity. Therefore the specific heat of other substances 

 is usually less than unity. A better understanding of latent and spe- 



