
CANNING SARDINES 199 
Effect of change in water-vapor pressure of the air on the drying rate of raw and 
steamed fish—KExcept where modified by the nature of the material being dehy- 
drated, the rate of drying depends upon the difference in vapor pressure of the 
water vapor at the surface of the substance undergoing drying and that of the 
water in the air.“ The larger this difference the greater the rate of drying 
should be. If all other conditions of the experiment are kept constant and the 
humidity is inereased, this difference in vapor pressure will be decreased, and by 
comparing the amounts of water removed from the fish an idea will be obtained 
as to the effect of this variation. 
The results obtained from experiments of this nature are given in Table 36. 
They show that there is a tendency for the amount of water removed from raw 
fish to decrease as the vapor pressure of the moisture in the drying air is increased. 
The difference in the total amount of moisture removed, however, is quite small, 
even under the rigorous conditions maintained in some of the tests. Only a small 
part of the moisture removed from the fish was free moisture. Evidently the 
diffusion of the combined moisture to the surface is so slow that the increased 
vapor pressure of the drying air has little effect upon the escape of this moisture 
toit. The experimental evidence is not as complete for steamed fish, but a similar 
tendency is shown. 
Effect of change in air temperature upon the drying rate of raw and steamed fish.— 
Other conditions being equal, an increase in the temperature of the drying air 
brings about an increase in the rate of moisture removal from the fish. This 
conclusion can be drawn from the data given in Table 37. The warmer the air 
the greater will be the heat transfer from it to the fish in a given time. The 
higher temperature of the fish then causes a more rapid diffusion of water to the 
surface, where it can be removed by the drying air.” Increases in the tempera- 
ture of the fish are taken up in the following section. 
TaBLE 36.—Effect of change in water-vapor pressure of the air upon the drying rate 
of raw and steamed California pilchards 4 












RAW FISH 
Vapor . Sat : : P P 
pressure Per cent loss in weight (time in minutes) 
Experiment No. in - _— 
inches of = 
mereury 15 30 60 9C 120 
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i 
STEAMED FISH 
i] 
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¢ All other variables were constant in each experiment. There may be great variation, however, between 
various experiments. 
» Experiments 104b and 104c were begun at the same vapor pressure, which was increased gradually within 
30 minutes to given value. 
© Experiment 105¢c was begun at 0.33 vapor pressure, which was increased gradually within 30 minutes to 
the value indicated. 
4 Vapor pressure for 108b was as follows: Beginning, 0.44; 12 minutes, 0.89; 27 minutes, 1.1; 37 minutes, 
1.50. The increase was gradual. 
_ ¢ Experiments 109d and 109b were begun with a vapor pressure of 0.52 and were then increased to the values 
indicated. Condensation upon the fish was prevented by keeping the dew-point temperature of the air 
always a little lower than the temperature of the fish. 

71 This is shown to be true in the papers referred to in the footnote on p. 120. 
™ Increased diffusion, due to temperature changes in the object being dried, is discussed by Lewis and 
Carrier in the papers referred to in the footnote on p. 120. 
