CURING TOBACCO

T. David Reed, Extension Agronomist, Tobacco

Flue-Cured Tobacco Curing

Curing flue-cured tobacco should be considered both an art and a science
due subtle differences between cures as a result the tobacco itself (body,
stalk position, moisture content, etc.), curing facilities, and weather
conditions. It is difficult to use a set curing schedule because each barn of
tobacco is different. Specific curing schedule are general guidelines to be
used and individual cureds modified based on specific conditions.
The harvested leaves must be kept alive during the yellowing period so
that desirable chemical and color changes can occur. At the same time,
sufficient drying must take place so that when yellowing is completed the
leaves will be thoroughly wilted. After the leaves reach the desired yellow
color, temperature should be raised to kill the tissue and stop further
chemical and color changes. If the leaves are killed too early by drying too
fast or high temperatures, the color will remain green. After the desired
color (lemon-orange) is achieved, the remainder of the cure is merely a
matter of drying the leaf and stems to preserve the color.
Tobacco producers may follow different temperature and humidity
schedules and still obtain a satisfactory cure. The exact temperature
schedule is not critical as long as it is within reasonable limits. Mr. S. N.
Hawks, retired Tobacco Extension Specialist at N. C. State University
developed a `Simplified Curing Schedule' designed to reduce the complex
curing procedure to its simplest terms. The three dry-bulb temperatures
(100°F for yellowing, 130°F for leaf drying, and 160°F for stem drying)
are well within safe ranges for each curing phase. Wet-bulb temperature
for yellowing should be adjusted to fit the needs of the tobacco. The upper
limits for leaf drying (105°F) and stem drying (110°F) are conservative.
The following points need to be remembered in following the Simplified
Curing Schedule:
1. Remove all surface moisture from the leaves before beginning
to yellow them. This may take up to 12 hours, depending on
weather and tobacco conditions when the barn is filled. Lower
leaves are often more difficult to yellow without developing
soft rot.
2. Yellowing - Start heat at outside temperature and advance
temperature 2°F per hour to 100°F. It may be necessary to
open vents slightly during yellowing, but care must be taken
to avoid setting green color by lowering relative humidity too
much or drying too fast.
104 Curing Tobacco
3. Maintain a dry bulb temperature of 100°F until all leaves are
yellow. Provide enough ventilation so that when the leaves
become yellow, those on the bottom tier will be completely
wilted. Generally, a difference of 2 to 3°F between the wet-
and dry-bulb reading should be maintained.
4. Leaf drying - When leaves are yellow and sufficiently wilted,
the dry-bulb temperature should be advanced 2°F per hour to
130°F. Increase ventilation enough so that the wet bulb does
not exceed 105°F. Toward the end of the leaf drying period it
will usually be possible to reduce the amount of ventilation
without exceeding 105°F on the wet bulb. A 130°F dry-bulb
temperature should be maintained until all of the leaves on the
lower two tiers are dry.
5. Stem drying - Dry-bulb temperature advanced 2°F per hour to
160°F and maintained until stems are dry. As long as the wet-
bulb does not exceed 110°F, ventilation can be reduced.
Toward the end of the cure the ventilators can be essentially
closed to conserve fuel while drying stems.
A graphical representation of a bulk tobacco curing schedule provided by
Drs. Boyette and Watkins of NCSU is shown on the following page. This
differs only slightly from what is described above, except that there is a
momentary holding of the dry bulb temperature at 1200 F during leaf
drying. This would provide for adequate removal of water from the tissue
to avoid scalding or sweating of the tobacco.

Bulk Tobacco Curing Schedule

170

160
Leaf yellowing Leaf drying Stem drying
150

140
Temperature (F)

130
/hr
F
2o

120
/hr
F

110
2o

Dry Bulb
100

Wet Bulb
90

80
0 24 48 72 96 120 144

Curing time (hours)

 Curing Information 105
The retrofitting of curing barns to indirect-fired heating focuses attention
on heating efficiency and fuel consumption and this has only intensified
with rising fuel prices. One measure of curing efficiency is calculation of
pounds of cured tobacco per gallon of fuel. Although there will be
varieties dependent on the senor, the barn, and the tobacco. A reasonable
value would be 10 pounds of tobacco per gallon LPG or 13 pounds per
gallon of fuel oil. Higher weights of cured tobacco per gallon of fuel
would indicate greater curing efficiency.
Simply increasing the amount of tobacco loaded into the barn may not
necessarily result in increased curing efficiency. The uniformity of how
the barn is filled has a substantial impact on air movement throughout the
barn. To obtain optimum curing efficiency, barn filling rates must be
compatible with the airflow capacity on the barn. With development of
box loader systems and load cells to weigh tobacco, growers have been
able to realize improved curing efficiency resulting from more uniformly
filled barns.
Tobacco has traditionally been cured solely with the use of a dry-bulb
temperature or the thermostat setting controlling the burner. A relatively
few growers have made use of a wet-bulb thermometer to cure by. This is
possible due to the wealth of knowledge that growers have developed for
curing tobacco, experience with barns that have been used for many years,
and a feel for the ripeness characteristics of their tobacco. However, the
use of a wet-bulb thermometer if likely to be the single most important
practice that can be used to reduce fuel consumption when curing tobacco.
With older barns, some amount of added insulation and repair will reduce
heat loss and most new barns have improved insulation. Use of a wet-bulb
thermometer will help reduce the amount of over ventilation of the barn.
Over ventilation or opening dampers wider than necessary increase the
drying rate of the tobacco and the burner fires more to heat the inflow of
outside air. Various wet-bulb thermometers or hygrometers (wet-bulb and
dry-bulb thermometers) are available and many designs or homemade
units are also available.
The dry-bulb temperature is a measure of the air temperature within the
barn and is controlled by the thermostat on the burner. In contrast, the
wet-bulb thermometer measures the temperature of the leaf tissue and is
controlled by the amount of ventilation or the size of the damper opening.
The difference between the dry-bulb and wet-bulb temperatures determines
the relative humidity within the barn and therefore the amount of drying
that occurs. Maintaining a high wet-bulb temperature within each stage of
curing will reduce ventilation and thus increase curing efficiency. (See
graph of curing schedule on previous page).
106 Curing Tobacco
Energy Efficient Curing Practices
More than 90 percent of the energy used for the production of flue-cured
tobacco is used in the curing process. The following energy efficient
curing practices should be followed to help reduce the cost of curing.
1. Regulate the barn using a wet-bulb thermometer. Ventilate
only enough to hold humidity down (wet-bulb temperature);
the wider the vent opening, the more fuel that is consumed.
Automatic curing controllers utilize the added convenience of
automatically controlling the damper opening.
2. Harvest only ripe tobacco; shorter curing times mean less heat
loss and more efficient curing.
3. Load racks and boxes uniformly; uniform loading with no
"tight spots" assures even drying and less energy use.
Uniform barn loading reduces the length of the total cure.
4. Have burner set for optimum efficiency; periodic maintenance
and adjustment is required for efficient operation.
5. Stop hot air leaks; check door gaskets and structure for cracks.
6. Assure an air seal around each rack or box; small cracks
between boxes or racks reduces ventilation efficiency to a
large degree.
7. Add insulation; well-insulated walls, roof and floor can save
10 to 20% of fuel consumed per cure. Insulate new barn pads
with 1-inch thick insulation board.
Tobacco Specific Nitrosamines

Tobacco specific nitrosamines (TSNAs) are a principle group of

carcinogens present in tobacco. Formation of these compounds is by two
different pathways. In burley tobacco and fire-cured tobacco, TSNA are
produced by naturally occurring microorganisms present on the leaves
during curing. They feed upon natural compounds found in the tobacco
leaf and produce TSNAs. Although curing conditions may be manipulated
to modify TSNA levels, the curing season has a substantial input on TSNA
levels found in stalk cut tobaccos. The higher temperatures and
accelerated drying of the leaf greatly reduces the activity of
microorganisms responsible for TSNA formation. However, the pathway
for TSNA formation in flue-cured tobacco primarily involves nitrous
oxides (NOx), produced as a by-product of combustion of LP or fuel oil,
with specific alkaloids present in the tobacco. The use of heat exchangers
or indirect-fired heating of curing barns has eliminated the introduction of
NOx into the curing air space. However, heat exchangers must tested
 Curing Information 107
periodically to ensure their physical integrity and repaired if a leak is
detected.

Barn Testing. Although NOx is the actual concern with a leaking heat
exchanger, carbon dioxide (CO 2 ) will also be present in the curing air
space. Carbon dioxide is measured because it is present in much higher
amounts than NOx and measuring devices for CO 2 are much cheaper and
portable than those for NOx. The procedure involves measuring the
ambient CO 2 level (typically 350 to 500 ppm) in the barn with the burner
off and then recording the increase in CO 2 above ambient in the barn after
the burner runs for a sufficient time. Dampers are to be closed and the
barn cannot contain green tobacco.
Interpreting CO 2 Meter Test Results:
• No increase in CO 2 above the ambient indicates that the entire
system is intact at the time of testing.
• An increase in CO 2 less than 100 ppm indicates the present of a
minimal leak somewhere in the furnace system.
• An increase in CO 2 between 100 and 200 ppm warrants further
inspection of the furnace since a crack may be forming in the heat
exchanger or a gap may be present in the exhaust stack.
• A doubling of the ambient CO 2 level indicates that a crack in the
heat exchanger is likely.
Removal and examination of a heat exchanger for a crack can be a difficult
procedure. High temperature (2500oF) caulking is available for minor
repairs. Fortunately, the source of many leaks has been the exhaust stack.
Any gap between the flue pipe and the heat exchanger or opening in the
stack pipe may potentially allow exhaust gases to enter the curing chamber
of the barn.
Although the use of indirect-fired curing removes NOx from the
curing chamber, it is critically important to remember that microbial
production of TSNAs may occur in flue-cured tobacco. It is important
to remove any oxidized or barn rotted leaves from tobacco before
baling and do not bale tobacco with excessive moisture or
compression. Each of these factors will impact the TSNA level of
tobacco.
108 Curing Tobacco