CIRCULAR NO. 252
guides for curing
AGRICULTURAL EXTENSION SERVICE
University of Florida, Gainesville
Guides For Curing
by S. L. Brothers
Many flue-cured tobacco growers have made
changes in spacing, fertilization, irrigation and
varieties without making adjustments in curing
procedures. Numerous piles of tobacco placed on
the warehouse floor in recent years have exhibited
curing faults. Most curing faults can be cor-
rected by proper control of ventilation and tem-
Physical as well as chemical changes take place
in the curing process. The correct curing proced-
ure must be followed to influence these changes in
the right direction. Uniformly ripe tobacco should
be harvested if a quality product is to be cured.
To get a proper cure, moisture must be removed
from the leaf without damaging the quality in
the leaf. Large amounts of water must be re-
moved in the curing process. A barn of 600 sticks
with a potential of 1,000 pounds of cured leaf will
give off from 4,000 to 7,000 pounds of water dur-
ing the curing operation. If this water is not re-
moved properly the tobacco "sweats" and often
brown scald or sponge results.
Brown Scald.-This condition occurs between
the yellowing and leaf drying stage. If the to-
bacco dries too slowly, "sweating" occurs; then
when the temperature is increased for leaf dry-
ing, brown scald results. Proper loading of a barn
and sufficient ventilation at both top and bottom
ventilators will correct this problem.
Sponge.-This condition is caused by too high
humidity and/or too low temperature for the con-
dition of the tobacco. This often occurs with
cloudy days or when soil moisture is excessive.
Sponge may be controlled by coloring at a tem-
perature of about 100 F., and providing ventila-
tion for faster drying.
Early tobacco barns were constructed of logs
with many cracks in the walls. Most roofs were
of wood shingles. Ventilation under these condi-
tions was sufficient. More fuel was required, but
wood from the farm was plentiful, and since
labor was cheap this was not a problem.
Today growers are using building materials that
make tight barns. Ventilators must be built into
these barns to control the amount of air. For a
16-foot barn, ventilators with about 12 square
feet of opening at the ridge, and at least four
square feet of opening well distributed around
the foundation should do a good job of providing
sufficient air to prevent brown scald or sponge.
Loading The Barn
Many curing problems result from overloading
the barn. To avoid overloading, each hand should
be limited to four leaves. If the leaves are extra
large, two or three leaves per hand should be the
limit. Thirty-four hands per stick should also be a
limit with the sticks spaced 8 to 12 inches apart
on the tier poles. Tier poles should be 24 to 26
inches apart vertically in the barn.
Cold Spots and Downdrafts
Cold spots and downdrafts must be eliminated
since a good cure requires uniform heat distri-
bution. Under ideal conditions, cool and relatively
dry air is drawn in through the bottom ventilators
and warmed as it passes over the heating units.
The warm air picks up moisture from the surface
of the leaves as it rises through the tobacco. The
air exhausts through the top ventilators carrying
moisture with it. Often the way a barn is con-
structed, or the way ventilators are operated,
causes interference with normal air movements,
and the cured leaf is adversely affected.
Downdrafts, which cause cool spots and a lack
of uniform drying, are a major source of trouble.
These occur more often in barns that have in-
sufficient bottom ventilation and holes around
the eaves or gable ends. As the air in the barn
is heated, it rises and goes out the ridge ventil-
ator at the top of the barn. Cool air from outside
pushes into the barn through bottom vents to re-
place the warm air. If cool air cannot get in at
the bottom of the barn, it will come in at the
eaves or gable ends and drop down the side or
middle to the heating units. Cold spots result and
the tobacco will not dry properly. Downdrafts
may be prevented by proper control of both top
and bottom ventilators.
On windy days, extremely large vents at the
bottom of a barn may allow air to move directly
across the heating units and out the other side.
A baffle on the bottom vents to reduce the wind
velocity will prevent this.
If ventilation is insufficient sweating may oc-
cur, especially if the barn is overcrowded. But
sweating also will occur if too much moisture
remains in the leaf, and the temperature is raised
for leaf drying-even in a barn that isn't over-
Moisture is given off by the leaves when the air
is heated. This heated air will pick up moisture
until it is saturated. If good ventilation is pro-
vided, the warm saturated air is removed from
the barn. If adequate ventilation is not provided,
moisture collects on the leaf surface and sweating
occurs. Often the bottom tiers of tobacco do not
exhibit signs of sweating but the middle and up-
per tiers show pronounced sweating. This indi-
cates poor top ventilation because of overloading
or insufficient venting.
To avoid sweating, provide sufficient and effec-
tive ventilation during the leaf yellowing and
drying period. To get effective ventilation do not
close the bottom vents while the top vents are
open. Do not overload the barn.
Sometimes it may be necessary to close the
ventilators while yellowing tobacco grown and
harvested under extremely dry conditions, or
where tobacco is very thin. Early ventilation in
such cases may dry the leaf too fast and set green
color. Adding moisture by sprinkling the barn
floor will often help.
Two growers may use different curing sched-
ules of temperature and ventilation, yet cure to-
bacco of equal quality. One grower may start ad-
vancing the temperature early but move it up
slowly. Another may start the temperature ad-
vance late and move it up rapidly. The curing
schedule should be adapted to the degree of ripe-
ness, the body of the tobacco and the conditions
under which it was grown and harvested.
The curing schedule can compensate to some
extent for slight variations from normal ripeness.
Thin tobacco that dries easily, and moderately
ripe tobacco will require slower temperature ad-
vances during the coloring and leaf drying periods
than will heavy bodied or over-ripe tobacco. To-
bacco that is harvested when it is wilted in very
hot, dry weather may need to be yellowed at
slightly higher temperatures than tobacco har-
vested under normal conditions.
At all stages of the cure, temperatures should
be advanced gradually as conditions of the to-
bacco warrant. Sudden advances in temperature
may set green color, or cause green scald in the
cure. Improper advance in temperature will in-
crease the chance of sweating. Avoid tempera-
tures above 170* F. in the stem drying stage since
this may cause a red color in the cured leaf.
Good temperature control, effective ventilation
and proper spacing of tobacco in the barn will
eliminate most curing problems.
Easily adjustable ventilators at top and bottom
of the barn will assure flexibility for any given
tobacco crop and weather conditions.
During the coloring stage of the cure, all ventil-
ators should be open or partially open (depend-
ing on conditions) to remove as much moisture
as possible (without setting a green color) and
to prevent brown scald. Thin tobacco and tobacco
harvested in hot dry weather may need to be
colored with the vents closed to slow the drying
rate. Coloring temperatures should begin at 90-
1000 depending on condition of tobacco. A grad-
ual increase in temperature as coloring progresses
is desirable until leaf drying stage begins.
Leaf drying temperatures should be held at
135-1400 with the vents open. When the leaf is
about dry, bottom and top vents should be closed
and temperature advanced to 1700 F. The tem-
perature should be held at 1700 during stem dry-
ing with vents closed.
It is harder to get out of trouble with tobacco
than it is to avoid the trouble. Anticipating venti-
lation needs, providing for adjustment in ventila-
tion and loading the barn properly will minimize
most curing problems.
Tentative Curing Schedule for
Have top and 12-24 hours
bottom vents 4 hours
open or partial-
ly opened de- 4 hours
pending on 4 hours
120" 8 hours
125 Have top and 3 hours
Leaf bottom vents
Drying open. 8 hours
Stage 135 Until leaf dries
150* 2 hours
Drying 1600 Close vents 2 hours
170" Until stems are dry
* All temperatures are F.
Adjustments should be based on consideration of
crop and weather conditions as well as the barn
and heating unit involved.
COOPERATIVE EXTENSION WORK IN AGRICULTURE
AND HOME ECONOMICS
(Acts of May 8 and June 30, 1914)
Agricultural Extension Service, University of Florida, Florida State
University and United States Department of Agriculture, Cooperating
M. O. Watkins, Director