PRESS BULLETIN 184
UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATION
DRYING CITRUS FRUITS BY HEATED AIR
By J. R. Benton
In drying citrus fruits after washing them, the time required has been
found to be very variable when unheated air is used.
Air at a given temperature can carry no more than a certain amount of
water vapor per cubic foot. The air in Florida is usually found to have
much moisture in it already-often almost as much as it is possible for it
to contain. If such air is warmed, its capacity for taking up moisture is
increased. A drier that depends on heating air before it has passed over
the fruit will dry more quickly in any weather than one that uses unheated
In order to evaporate moisture, a certain amount of heat is necessary.
In taking up moisture, accordingly, air becomes cooler. Thus the air coming
out from a drier using hot air may be barely warm to the touch. It has
been cooled by taking up moisture.
The quickest effect in drying citrus fruit would be produced by having
air as hot as the fruit will stand. Suppose the air is taken in at a tempera-
ture of 60 F., and is saturated with moisture. Allow this air to be heated
to 192 F. (such high temperature would be likely to ruin the fruit, and is
here used for illustration only) and then to pass over the wet fruit, taking
up moisture and cooling in doing so until its temperature is reduced to 95 F.
By this time each cubic foot of air will have taken up 0.74 gram (about 1-38
of an ounce) of moisture. The quantity of moisture likely to stick to an
orange is no doubt variable. By first weighing an orange dry and afterwards
weighing it wet, but with the moisture partly wiped off, it was found in one
trial that there remained sticking to the orange 0.2 gram (about 1-140 of an
ounce) of water. To dry oranges with this amount of moisture should ac-
cordingly require only about 1/ of a cubic foot of air for each orange, if the
air is used as described above.
March 2, 1912
Heating the air for use as described above should require only about
600 calories of heat for each foot of air, and accordingly one pound of wood
should be sufficient to heat 3000 cubic feet of air (about half the air in an
ordinary living room).
The above calculations are based on what is theoretically possible and
what could be obtained under laboratory conditions; but actual working con-
ditions are not likely to be so favorable. However, the figures make it
clear that those processes of drying which depend on heating the air in ad-
vance are much more likely to be reliable and thorough than those which
use' the air without heating it.
State papers please copy.