170 
WISCONSIN HORTICULTURE 
August, 1918 
Farm and Home Drying of Fruits 
and Vegetables 
POSSIBILITIES AND LIMITATIONS 
OF DRYING 
For preserving perishable food- 
stuffs, one of two general methods 
may be pursued. One of these 
consists of heating to destroy de- 
cay-producing organisms, and seal- 
ing. This is what we do in can- 
ning. The other removes so much 
moisture from the material that 
organisms are not able to grow 
and multiply in it. This is evap- 
oration or dehydration. In the 
case of any particular material, 
judgment must he exercised in de- 
termining whether it shall be dried 
or canned. 
Drying has the very great ad- 
vantages that the product has a 
weight only one-fourth to one- 
ninth that of the fresh material ; 
that there is a very considerable re- 
duction in bulk, due both to actual 
shrinkage and to the fact that all 
portions not actually fit for food 
are removed ; and that the dry ma- 
terial may be stored almost indefi- 
nitely without danger of deteriora- 
tion and without the use of expen- 
sive special containers. At the 
same time it must be understood 
that evaporation has very definite 
limitations and that it is not appli- 
cable by any means to all fruits 
and vegetables. There are a con- 
siderable number of fruits and 
vegetables which it is not advisable 
to attempt to dry, either because 
they undergo changes in drying 
which render them unpalatable or 
because they deteriorate rapidly af- 
ter drying ; also there are a eonsid- 
able number of vegetables which 
are so readily kept for long per- 
iods in storage, either in out-of- 
odoor storage pits or in an ordinary 
cellar, that any labor expended in 
drying them under any ordinary 
conditions would he wasted. 
FUNDAMENTAL PRINCIPLES OF 
DRYING 
Most failures in drying are due 
not so much to imperfections in 
the equipment used as to the fail- 
ure of the operator to understand 
a few fundamental principles 
which must be kept clearly in mind 
if the work is to be successful. The 
purpose in view in drying any food 
material is not merely the removal 
of sufficient water to insure good 
keeping; it is equally important to 
preserve all the food value of the 
product with as much as possible 
of the natural flavor and cooking 
quality characteristic of the raw 
material. This double purpose 
cannot be successfully accomplish- 
ed unless certain guiding prin- 
ciples are kept in mind. 
The air at the earth’s surface is 
capable of taking up and holding 
as water vapor considerably larger 
quantities of moisture than are 
ever present in it; that is to say, 
the free atmospheric air never be- 
comes completely saturated. Con- 
sequently, any wet material expos- 
ed freely to the air will ultimately 
become dry, since the liquid water 
covering its surface will he con- 
verted into water vapor and taken 
up by the air. The rate at which 
this will occur will depend upon 
the temperature of the air and up- 
on the percentage of moisture al- 
ready present in it when brought 
into contact with the material. If 
the air remains at constant tem- 
perature and is undisturbed by 
currents, the loss of water from the 
material will go on very slowly, as 
the air nearest the wet surface will 
soon, become almost saturated and 
can take up more water vapor only 
as that which it already holds is 
lost by diffusion outward and up- 
ward into layers of drier air. If 
the air be kept constantly in mo- 
tion, however, the drying will be 
greatly hastened, as the moving air 
current will displace the blanket 
of moist air surrounding the ma- 
terial as rapidly as it is formed 
and bring in drier air to replace 
it. If the temperature and mois- 
ture content of the air used are 
both constant, the rate of drying 
will increase nropoitionally as the 
rate of movement of the air is in- 
creased, until a point is reached at 
which water can not pass from the 
interior to the surface of the ma- 
terial as rapidly as the air is able to 
take it up, when the surface will 
become dry even though the inter- 
ior is still nearly saturated. The 
effect of a brisk breeze in hastening 
the drying of the surfaces of mud- 
dy fields after a rain is a familiar 
illustration of this principle. 
Drying is also hastened by rais- 
ing the temperature of the air. The 
amount of water vapor which a 
given volume of air can absorb be- 
fore reaching saturation depends 
upon the temperature and is prac- 
tically doubled by every increase 
of 27 degrees in temperature. In 
other words, if a quantity of air be 
warmed from 60° to 87° F., its 
moisture-carrying capacity is doub- 
led ; if the heating be continued 
until a temperature of 114° is 
reached tin* moisture-carrying ca- 
pacity is again doubled, becoming 
four times what it was at 60° F. 
Further heating produces further 
increases in the same proportion, 
until a point is again reached at 
which water is vaporized at the 
surface more rapidly than it is re- 
placed by movement outward from 
