14 
BULLETIN 
U. S. DEPARTMENT OF AGRICULTURE. 
M&.U 
hundred and fifty fold. The figure shows graphically what a decided 
influence the dairyman can exert upon the keeping of his milk by a 
lowering of temperature only 20°. 
From the foregoing it is obvious that proper refrigeration is of the 
utmost importance in the preservation 
of milk. In fact, without thorough cool- 
ing it is impracticable to keep milk for 
any considerable length of time in a con- 
dition that would justify its use for house- 
hold purposes. It should be cooled to 
50° F. or below as quickly as possible 
after it is drawn from the cow, as such 
cooling will at once check the increase in 
bacteria. 
m 
mmm 
Fig. 2.— Graphical representation of 
the relation of temperature to bac- 
terial growth in milk. 
COOLING BY MEANS OF SALT AND ICE MLTTURES. 
Where a small amount of refrigeration is required it is often produced 
by a mixture of common salt and ice. The action of the mixture in 
lowering the temperature below 32° F. is as follows : When two solid 
bodies, as salt and ice, 
mix to form a liquid a cer- 
tain amount of heat be- 
comes latent, called the 
latent heat of solution. 
Since this latent heat is 
taken from the mixture 
itself the temperature falls 
correspondingly. The tem- 
perature obtained by a salt 
and ice mixture depends 
principally on the relative 
proportions of the mixture, 
and to a less extent on the 
rate at which the heat is 
supplied from the outside, 
the size of the ice lumps 
and salt particles, and the 
amount and density of the 
resulting brine. Hence it 
is impracticable to give 
other than approximate temperatures with fixed ratios of salt and 
ice. From the curve in figure 3 the approximate temperature 
resulting from different proportions, by weight, of salt and ice may 
be obtained. If, for example, we desire to know what tempera- 
ture may be obtained by using a mixture of salt and ice containing 
15 per cent of salt, follow the vertical line marked a 15 per cent of 
v5 
\ 
\ 
si 
\ 
\ 
V 
\ 
S* 
V 
\* 
\ 
\ 
S* 
\ 
\ 
Ju 
\ 
i: 
\ 
\ 
\ 
\ 
i 
. 
P 
4 

rcei 
/ 
s 
sir 
or 
\ 
H.T. 
5 
•3 

-i 
\ 
\ 
-a 
\ 
Fig. 3.— Approximate temperatures obtained with different 
proportions of salt. 
